Novel decoding techniques for time-bin encoded Quantum Communication

Costantino Agnesi
Università degli Studi di Padova

The temporal photonic degree of freedom is vastly exploited for quantum communications, both in prepare-and-measure (P&M) or entanglement-based protocols. However, decoding the quantum information encoded in time-bin (TB) can be a cumbersome task. In the case of TB entanglement a major issue is related to the discovery of the Post-selection loophole (PSL), a local hidden-variable models which explain the violation due to the post-selection of detection events, thus requiring extra assumptions to trust the Bell test result and rendering it vulnerable to quantum hacking attacks. Instead for P&M protocols, the need of interferometric measurements requires the development complex phase stabilization strategies, which increase the overall complexity and cost of the quantum communication system. In this presentation we discuss novel experimental techniques for time-bin decoding. Regarding the TB entanglement and the PSL two techniques, pioneered by our research group are presented. Firstly, PSL can be removed through the use of fast and stable optical switches to route the photons in the measurement interferometers to avoid the need for post-selection [Vedovato et al., Phys. Rev. Lett. 121, 190401 (2018)]. PSL can also be closed by exploiting a topologically different interferometric arrangement called the hug interferometer. This scheme, first developed for energy-time entanglement, can be extended to the TB case and allows a reduction in the frequency stability requirements of the pump laser and benefits from having specific photon generation times instead of a uniform distribution [Santagiustina et al., Optica 11, 498-511 (2024)]. Both of these methods certify genuine TB entanglement generation, i.e., allowing the violation of a Bell's inequality free of the Post-Selection Loophole. In this presentation we also present the experimental results that validate these methods. In particular, we implement the optical switches using standard off-the-shelf fiber components and the extinction ratio was observed to be over 99% (27 dB) [Vijayadharan et al., Proc. SPIE, 1291119 (2024)]. Instead, we implement the hug interferometer in a Silicon Nitride chip. For both methods, experiments were performed obtaining Bell-violations and demonstrating the validity of our techniques. Instead, regarding P&M protocols, we present a novel fully passive receiver for time-bin encoded Reference- Frame-Independent (RFI) QKD [Giacomin et al., arXiv:2408.17304 (2024)] . RFI-QKD aims to simplify QKD implementations by allowing to reduce the requirements of alignment on a shared reference frame. Conversion of time-bin to polarization is employed to perform the required quantum measurement in a fully passive manner. Furthermore, to overcome experimental errors, we retrieved a complete description of our measurement apparatus by employing a recently introduced Quantum Detector Self-Characterization technique, without performing tomographic studies on the detection stage. In fact, the security analysis carried out in this work uses experimentally retrieved Positive Operator Valued Measurements, which consider our receiver defects, substituting the ideal expected operators and thus increasing the overall level of secrecy. Lastly, we conducted a proof-of-principle experiment that validated the feasibility of our method and its applicability to QKD applications.

The first public 24-qubit superconducting quantum computing platform in Italy: a roadmap towards quantum utility

Halima Giovanna Ahmad
University of Napoli "Federico II", Department of Physics "Ettore Pancini"

The roadmap towards quantum utility requires hardware platforms with a sufficient number of qubits, single- and two-qubit gate fidelities, and low readout errors. Among the most promising hardware, superconducting quantum platforms have earned considerable attention [1,2]. Their artificial nature favors the engineering of coherence [3], novel control and readout [4-5], and tunability [4,6-7], as well as coupling schemes for enhanced scalability [8,9]. In the frame of the Spoke 10 of the National Center for High-Performance Computing, Big Data and Quantum Computing Center (ICSC), we here report on preliminary results on the performances of a modular and scalable infrastructure up to a 24-transmon qubits Quantum Processing Unit (QPU). We will discuss the fundamental role played by calibration and gate pulses optimization procedures [10], as well as the impact of decoherence and noise, in the Noisy and Intermediate Scale Quantum (NISQ) era [11]. Within the final goal to provide a functional scalable infrastructure with up to 40 transmon qubits by the end of 2024, we here also report on the experimental validation of a novel Quantum Error Mitigation (QEM) algorithm on a 5-qubit superconducting QPU, which uses Fuzzy C-Means (FCM) clustering to identify and mitigate measurement error patterns in NISQ devices [11]. Such a technique may provide fundamental improvements on the output readout of quantum algorithms on devices with a larger number of qubits, thus allowing the field to implement more complex quantum algorithms. [1] Arute, F., et al., Nature 574, 505–510 (2019) [2] Kim, Y., et al., Nature 618, 500–505 (2023) [3] Siddiqi, I., Nat. Rev. Mater. 6, 875–891 (2021) [4] P. Krantz, et al., Appl. Phys. Rev. 1 June 2019; 6 (2): 021318 [5] L. Di Palma, Phys. Rev. Applied 19, 064025 (2023) [6] Casparis, L., et al., Nature Nanotech. 13, 915–919 (2018) [7] Ahmad H.G., et al., Phys. Rev. B 105, 214522 (2022) [8] Majer, J., et al., Nature 449, 443–447 (2007) [9] Yan F. et al, Phys. Rev. Applied 10, 054062 (2018) [10] Ahmad H.G., et al., Condensed Matter. 2023; 8(1):29. [11] Ahmad H.G., et al., Adv Quantum Technol. 2024, 2300400.

Spectroscopy of quantum emitters from the point of view of quantum metrology

Francesco Albarelli
Scuola Normale Superiore

Spectroscopy of a single quantum emitter (e.g. atom, molecule) using pulses of quantized radiation can be regarded as a fundamental problem in quantum science. Abstractly, it can be seen as the task of learning information about the emitter parameters by measuring traveling quantum probes that interact with it. We present recent work that initiates a quantum informationtheoretic approach to quantum light spectroscopy, based on quantum estimation theory. First, we consider the paradigmatic example of estimating the dipole moment of a two-level atom [1]. In the simplest case of single-photon pulses we are able to observe from a unified perspective the interplay between the information gained from photon absorption by the atom, as measured in absorption spectroscopy, and the perturbation to the temporal mode due to spontaneous emission, akin to fluorescence-based approaches. Going beyond the single-photon regime, we show that the limit of short pulses can be understood using a simple approximate model. Second, we briefly discuss the role of entanglement in biphoton probes, also for more general quantum emitters and parameters [2]. Finally, we report on ongoing progress towards obtaining a general methodology based on tensor networks methods, able to analyze more challenging scenarios, in which additional sources of noise affect the quantum emitter. We draw connections and potential applications to quantum sensing with continuous measurements and collisional quantum metrology. We illustrate the power of this general framework with the simplest example of resonance fluorescence of a two-level system and evaluate the quantum Fisher information of the portion of the emission field that can be detected, e.g. in a free-space geometry. [1] F. Albarelli, E. Bisketzi, A. Khan, and A. Datta, Phys. Rev. A 107, 062601 (2023) [2] A. Khan, F. Albarelli, and A. Datta, Quantum Sci. Technol. 9 035004 (2024)

Optimality and noise resilience of critical quantum sensing

Uesli Alushi
Aalto University

We compare critical quantum sensing to passive quantum strategies to perform frequency estimation, in the case of single-mode quadratic Hamiltonians. We show that, while in the unitary case both strategies achieve precision scaling quadratic with the number of photons, in the presence of dissipation this is true only for critical strategies. We also establish that working at the exceptional point or beyond threshold provides sub-optimal performance. This critical enhancement is due to the emergence of a transient regime in the open critical dynamics, and is invariant to temperature changes. When considering both time and system size as resources, for both strategies the precision scales linearly with the product of the total time and the number of photons, in accordance with fundamental bounds. However, we show that critical protocols outperform optimal passive strategies if preparation and measurement times are not negligible. Our results are applicable to a broad variety of critical sensors whose phenomenology can be reduced to that of a single-mode quadratic Hamiltonian, including systems described by finite-component and fully-connected models.

U. Alushi, W. Górecki, S. Felicetti, R. Di Candia: “Optimality and Noise-Resilience of Critical Quantum Sensing”, https://doi.org/10.48550/arXiv.2402.15559.

Number of steady and asymptotic states of quantum evolutions

Daniele Amato
University of Bari

In this talk, we provide sharp universal upper bounds on the number of linearly independent steady and asymptotic states of discrete- and continuous-time Markovian evolutions of finite-dimensional open quantum systems. We show that the bounds depend only on the dimension of the system and not on the details of the dynamics. The connection with a recent spectral conjecture for Markovian evolutions is also discussed.

Fully machine learning-driven control and characterisation of quantum devices

Natalia Ares

Machine learning is rapidly proving indispensable in tuning and characterising quantum devices. By facilitating the exploration of complex high-dimensional parameter spaces, these algorithms not only allow for the identification of optimal operational conditions but also surpass human experts in the characterisation of different operational regimes. I will present the first fully autonomous tuning of a spin qubit. This is a major advancement for the scalability of semiconductor quantum technologies. I will also discuss the robustness of machine learning algorithms across various semiconductor devices, emphasising their role in the comparative analysis of quantum device architectures. I will conclude by demonstrating the potential of machine learning to understand variability in nominally identical devices. By using physics-informed machine learning approaches, we revealed the disorder potential in a quantum dot device, providing insights into device characteristics that were previously inaccessible. I will thus discuss how we can bridge the gap between quantum device simulation and reality.

Certifying spectral gaps of 1D many-body systems using semidefinite programming

Flavio Baccari
UniPD

Certifying the presence of a spectral gap of a many-body quantum systems is a fundamental challenge that finds numerous applications across very different fields. In many-body physics, a non-zero gap in the thermodynamic limit indicates properties such as area law scaling of entanglement and exponential decay of correlations, both in 1D [1] and (partially) in 2D [2]. In quantum computing, the size of the spectral gap along the path is in one-to-one correspondence with the efficiency of the adiabatic algorithm [3]. Furthermore, classical Monte-Carlo algorithms that are rapidly-mixing can be directly mapped to gapped many-body quantum hamiltonians [4]. However, estimating the spectral gap of a many-body quantum systems is a very challenging task, and an undecidable problem in general [5]. Nonetheless, given the relevance of the problem, several methods are known to lower bound the spectral gap in the thermodynamic limit. Two main exponents are the martingale method [6] and finite-size criteria [7], which have allowed to prove the existence of a spectral gap in a plethora of physically-relevant cases. Yet, each of those results required carefully tailoring the method to the specific model of interest. I will present a novel and general approach to certify the gap of local Hamiltonians in the thermodynamic limit. We leverage the fact that the gap can be estimated as a minimisation problem under the constraint that a degree-two polynomial of the hamiltonian is positive semidefinite. By taking ideas from sum-of-squares proof of positivity, we introduce a relaxation of the minimisation problem that provides lower bounds to the spectral gap. The quality of the lower bound can be systematically improved by controlling a single parameter in our method. Being based on semidefinite programming, our technique provides an efficient and reliable numerical algorithm that can be applied flexibly to any local, frustration-free hamiltonian. Lastly, our method recovers many previous approaches as a special case. We benchmark our method on several 1D models and show a clear improvement with respect to previous approaches. In all the observed cases, our technique allows to estimate a noticeably larger gap and to prove the existence of a gap in parameter regimes that are inaccessible to other methods. When combined with variational estimations of the gap, our lower bounds often match the corresponding upper bound, providing an exact calculation of the gap. Finally, we discuss extensions to 2D systems and future prospects of the method. [1] M. B. Hastings, JSTAT, P08024 (2007)[2] A. Anshu et al, Proc. of the 54th Annual ACM SIGACT Symp. on Theory of Computing (2022) [3] S. Jansen et al, Journal of Mathematical Physics 48, 102111 (2007) [4] D. Aharonov et al, quant-ph/0301023 (2003) [5] T. Cubitt et al, Nature 528, 207–211 (2015) [6] B. Nachtergaele, Communications in Mathematical Physics 175(3) (1996) [7] S. Knabe, Journal of statistical physics 52, 627 (1988)

Theory of fractional quantum Hall liquids coupled to quantum light and emergent graviton-polaritons

zeno Bacciconi
SISSA

Recent breakthrough experiments [1, 2] have demonstrated how it is now possible to explore the dynamics of quantum Hall states interacting with quantum electromagnetic cavity fields. While the impact of strongly coupled non-local cavity modes on integer quantum Hall physics has been recently addressed, the effects on fractional quantum Hall (FQH) liquids – and, more generally, fractionalized states of matter – remain largely unexplored. In this talk I will discuss our recent efforts [3] in understanding the fate of FQH states under the action of quantum light. Combining analytical arguments with tensor network simulations, we study the dynamics of a ν = 1/3 Laughlin state in a single-mode cavity with finite electric field gradients. The existence of cavity field gradients is crucial to achieve a finite coupling with intra-Landau Level excitations as these have a vanishing dipole moment. We inspect the resilience of topological order by calculating the bulk Hall resistivity and by studying bipartite entanglement spectra. The latter carry direct fingerprints of light-matter entanglement and topology, revealing peculiar polaritonic replicas of the U(1) counting. As a further response to cavity fluctuations, we also find a squeezed anisotropic FQH geometry, encoded in long-wavelength correlations. Only at very strong cavity field gradients finite systems become unstable towards the formation a sliding Tomonaga-Luttinger liquid phase, featuring a strong density modulation in the gradient direction. Finally, by exploring the low-energy excited spectrum inside the FQH phase, we identify a new quasiparticle, the graviton-polariton, arising from the hybridization between quadrupolar FQH collective excitations (known as gravitons [4]) and light. I will then discuss the experimental implications of our findings and possible extension of our results to more complex scenarios. [1] F. Appugliese, J. Faist et. al. , Science 375, 1030 (2022). [2] J. Enkner, J. Faist et. al. , arXiv:2311.10462 (2023). [3] Z. Bacciconi, H. Xavier, I. Carusotto, T. Chanda, and M. Dalmonte , arXiv:2405.12292 (2024). [4] J. Liang, Z. Liu, Z. Yang, Y. Huang, U. Wurstbauer, C. R. Dean, K. W. West, L. N. Pfeiffer, L. Du, and A. Pinczuk, Nature 628, 78 (2024).

Generation and characterization of polarization-entangled states using quantum dot single- photon sources

Paolo Barigelli
Sapienza - Rome University

The generation of entangled states is the fundamental step in most of the quantum information applications. The sources based on spontaneous parametric down-conversion can be employed to obtain high-fidelity entangled photons, but have an intrinsic trade-off between the brightness and quality of the state. In this work, we implement and characterize a method for the creation of entangled states based on probabilistic interference of identical photons emitted from the same single-photon source; Semiconductor Quantum Dots are the ideal candidates due to their high single-photon indistinguishability, on-demand generation and low multiphoton emission. The generation scheme has been implemented via a simple, compact design that produces entangled photon pairs in the polarization degree of freedom. The proposed platform has been tested and analysed with single photons produced through two different pumping schemes, the resonant excited one and the longitudinal-acoustic phonon-assisted configuration. A novel theoretical model has been developed to characterize the entangled two-photon states and determine the experimental variables limiting Bell’s inequality’s maximum violation. The source shows long-term stability in terms of fidelity and Bell’s parameter, thus constituting a reliable building block for optical quantum technologies and communications protocols.

Dynamic Cooling on Contemporary Quantum Computers

Lindsay Bassman Oftelie
CNR Pisa

Quantum computers require qubits to be initialized in a pure (i.e., cold) state for successful computation. Dynamic cooling offers a route to effectively lower qubit temperatures beyond what is possible with direct, physical cooling techniques. It works by cooling a subset of qubits, at the expense of heating others, by applying certain logic gates to the entire system. While it was initially dismissed as impractical for the high-temperature NMR-based quantum computers available at the time of its inception, we show how dynamic cooling is substantially more effective and efficient on the low-temperature quantum computers available today. In this talk, we will examine how optimal dynamic cooling scales with total system size, in terms of the minimum achievable final temperature, the work cost, and the complexity of the associated quantum circuits. We will observe the effect of hardware noise on cooling and share results of a successful demonstration of dynamic cooling with a 3-qubit system on a real quantum processor. Finally, we will propose a sub-optimal dynamic cooling scheme with fixed (low) complexity to improve the feasibility of implementation on noisy quantum hardware.

Two-photon-interaction effects in the bad-cavity limit

Bruno Bellomo
Institut UTINAM, Université de Franche-Comté, Besançon, France

Atomic systems interacting with confined photonic or phononic modes represent one of the most studied classes of quantum-optical systems. It has been recently predicted that using solid-state or atomic systems it is possible to implement nondipolar light-matter couplings, where the linear interaction is inhibited and quantum emitters and localized bosonic modes interact via the exchange of two-excitation quanta. In particular, such two-photon couplings can be observed by engineering superconducting atom-resonator systems or by applying analog quantum simulation schemes in trapped-ion or ultracold-atom systems. Here, we consider a damped quantum harmonic oscillator interacting with N two-level systems via a two-photon coupling in the so called bad-cavity limit, aiming to compare this configuration with the dipolar case characterized by a one-photon coupling. We suppose that the harmonic oscillator and each two-level system are each in contact with an independent thermal bath at finite temperature T (equal for all baths) and that a resonant coherent pumping on the harmonic oscillator and an incoherent local pumping on the two-level systems are available [1]. We have succeeded in applying a recently developed adiabatic elimination technique to derive an effective master equation for the two-level systems, presenting two fundamental differences compared to the case of a dipolar interaction: an enhancement of the two-level systems spontaneouslike emission rate, including a thermal contribution and a quadratic term in the coherent driving, and an increment of the effective temperature perceived by the two-level systems [1]. Our analytical and numerical analysis of the time evolution and steady-state behavior unveils indeed an unexpected phenomenology induced by nondipolar light-matter interactions. Compared to the dipolar case, the differences in the effective master equation give rise to striking effects in the two-level systems dynamics, including a faster generation of steady-state coherence and a richer dependence on the temperature T of the collective effects, which can be made stronger by increasing T [1]. We finally remark that the models here investigated can be feasibly implemented with both solid-state and atomic existing quantum technologies. [1] N. Piccione, S. Felicetti, and B. Bellomo, Phys. Rev. A 105, L011702 (2022).

Exact Ansatz of Fermion-Boson Systems for a Quantum Device

Carlos Benavides-Riveros
University of Trento

We present an exact ansatz for the eigenstate problem of mixed fermion-boson systems that can be implemented on quantum devices. Based on a generalization of the electronic contracted Schrödinger equation (CSE), our approach guides a trial wave function to the ground state of any arbitrary mixed Hamiltonian by directly measuring residuals of the mixed CSE on a quantum device. Unlike density-functional and coupled-cluster theories applied to electron-phonon or electron-photon systems, the accuracy of our approach is not limited by the unknown exchange-correlation functional or the uncontrolled form of the exponential ansatz. To test the performance of the method, we study the Tavis-Cummings model, commonly used in polaritonic quantum chemistry. Our results demonstrate that the CSE is a powerful tool in the development of quantum algorithms for solving general fermion-boson many-body problems.

Quantum Sensing of Magnetic Fields with Molecular Spins

Claudio Bonizzoni
Università di Modena e Reggio Emilia

Molecular spins hold potential for quantum technologies when embedded into planar superconducting microwave (MW) resonators [1,2]. The coherent manipulation of molecular spins by means of suitable sequences of MW pulses, eventually down to sub-nanoliter sample volumes [3], allows for encoding molecular spin qubits [2] or for implementing temporary memories for information [2]. The readout of the amplitude or of the phase of molecular spin qubits can be further assisted and improved by combining supervised and postselection machine learning methods [4]. Molecular spins have been proposed also for quantum sensing experiments [5] but, however, quantum sensing schemes involving such spins still need to be experimentally demonstrated. Here we report the quantum sensing of radiofrequency (RF) magnetic fields by means of molecular spin qubits [6]. To this end, we first consider a diluted VO(TPP) molecular spin ensemble embedded into a planar MW superconducting resonator. The sensing protocol consists in a MW pulse sequence which is used to coherently drive the spins and to obtain a Hahn’s echo, while the RF signal to be detected is sent during the free spin precession time through an additional RF coil. We show that it is possible to detect changes in both echo amplitude and phase and to relate them to the presence and the parameters (amplitude, phase, symmetry) of the RF field applied [6]. We then extend our sensing protocol to the case in which MW Dynamical Decoupling sequences, such as Carr-Purcell-Meiboom-Gill [2] and Period Dynamical Decoupling are used to drive spins. Here we now test an ensemble of diluted BDPA organic radical [6,3]. The effect of the RF field on the echo is found to increase with the number of π pulses used in the MW sequence. The resulting magnetic field sensitivity can reach values as high as nT/√Hz with a relatively low number (4-5) of π pulses applied, which is comparable with the typical values reported for Nitrogen Vacancy centers magnetometry performed through Optically Detected Magnetic Resonance spectroscopy [6]. These results show, for the first time, quantum sensing protocols successfully implemented on molecular spins. The minimum detectable field resulting from data analysis and from Allan variance estimation is on the order of μT, comparable with the one experienced at nanometric distance from a spin with magnetic moment equal to Bohr’s magneton. This result suggests the possibility to use ensemble of molecular spins as local field sensors attached on surfaces or to functional molecules [6]. [1] C. Bonizzoni et al. Advances in Physics: X 3, 1435305 (2018). [2] C. Bonizzoni et. Al. npj Quantum Inf. 6, 68 (2020). [3] C. Bonizzoni et al. Appl. Magn. Reson 54, 143 (2023). [4] C. Bonizzoni et al. Phys. Rev. Applied. 18, 064074 (2022). [5] F. Troiani et al. J. Magn. Mag. Mat.491, 165534 (2019). [6] C. Bonizzoni et al. npj Quantum Inf. 10, 41 (2024).

Inference of interference – from single to many particles

Andreas Buchleitner

Interference and its relation to the coherence properties of the system under study can be considered well understood, at least on the level of single particle interference. This is less true when many identical particles get involved, since their – at least partial – indistinguishability gives rise to additional – many-body – interference contributions. (De-) Coherence of the many-body dynamics then hinges on the available many-body which-way information, and garnishes the many-body counting statistics. The talk will convey some of the essential structures underlying many-body interferences, and touch upon strategies to discriminate bona fide many-body interference contributions, in the absence as well as in the presence of many-body interactions.

von Neumann’s "other" entropy: properties, interpretations, and applications

Francesco Buscemi
Nagoya University

In addition to the quantity now eponymously known as von Neumann entropy, in his 1932 book von Neumann also discusses another entropic quantity, which he calls "macroscopic", and argues that it is the latter, and not the former, that is the relevant quantity to use in the analysis of thermodynamic systems. For a long time, however, von Neumann's "other" entropy was largely forgotten, appearing only sporadically in the literature, overshadowed by its more famous sibling. In this talk I will discuss a recent generalization of von Neumann's macroscopic entropy, called "observational entropy", focusing on its mathematical properties (leading to a strong version of the Petz recovery theorem), its statistical interpretation (as statistical deficiency on the one hand, and as "irretrodictability" on the other), and its application in explaining the emergence of the Second Law and an "H-like Theorem" for closed systems evolving unitarily.

Verso una strategia nazionale per le scienze e le tecnologie quantistiche

Tommaso Calarco
Coordinatore gruppo di lavoro strategia nazionale per le tecnologie quantistiche, Università di Bologna

Pulse variational quantum eigensolver algorithm on cross-resonance-based hardware

Chiara Capecci
Università degli studi di Trento

State-of-the-art noisy digital quantum computers are currently limited to executing short-depth quantum circuits. Variational algorithms, such as the Variational Quantum Eigensolver (VQE), offer a promising solution to unlock the potential of these noisy devices by operating within hardware-imposed depth limits [1,2]. Typically, variational parameters are represented by virtual RZ gate angles, which are implemented through phase changes of calibrated pulses. In the proposed approach, we encode these parameters directly as hardware pulse amplitudes and durations [3]. This method significantly shortens the pulse schedule and overall circuit duration, thereby reducing the impact of qubit decoherence and gate noise [4]. To demonstrate the effectiveness of our Pulse-VQE [5], we calculated the ground state of hydrogen-based systems (H2, H3, and H4) using IBM's cross-resonance hardware. Our approach achieved up to a 5× reduction in schedule duration and lower measured energy compared to CNOT-based Ansätze, with a notable improvement in the minimal energy configuration of H3. REFERENCES [1] Nikolaj Moll et al, Quantum Sci. Technol. 3, 030503 (2018) [2] L. Ratini et al, J. Chem. Theory Comput. 18, 2, 899–909, (2022) [3] T. Alexander et al, Quantum Sci. Technol. 5, 044006, (2020) [4] N. Earnest et al, Phys. Rev. Res. 3, 043088, (2021) [5] D. J. Egger, Phys. Rev. Res. 5, 033159 (2023)

Critical non-Hermitian topology induced quantum sensing

Angelo Carollo
University of Palermo

Non-Hermitian physics predicts open quantum system dynamics with unique topological features such as exceptional points and the non-Hermitian skin effect. We show that this new paradigm of topological systems can serve as probes for bulk Hamiltonian parameters with quantum-enhanced sensitivity reaching Heisenberg scaling. Such enhancement occurs close to a spectral topological phase transition, where the entire spectrum undergoes a delocalization transition. We provide an explanation for this enhanced sensitivity based on the closing of point gap, which is a genuinely non-Hermitian energy gap with no Hermitian counterpart. This establishes a direct connection between energy-gap closing and quantum enhancement in the non-Hermitian realm. Our findings are demonstrated through several paradigmatic non-Hermitian topological models in various dimensions and potential experimental implementations.

Multiphoton experiments with semiconductor quantum-dot sources

Lorenzo Carosini
University of Vienna

Over the past two decades, semiconductor quantum dots have emerged as a leading platform for realizing highly efficient sources of near-identical single-photon. At the Christian Doppler Laboratory at the University of Vienna, we leverage commercially available quantum dots (from Quandela and Sparrow) to push the boundaries of photonic quantum information processing and explore fundamental quantum optics phenomena. In this talk, I will present the latest breakthroughs from our laboratory, with a particular focus on applications in photonic quantum computing. Highlights include the development of a resource-efficient programmable time-bin processor for multi-photon interference [1], the heralded generation of three-photon GHZ states [2], and the pioneering experimental demonstration of single-photon indistinguishability purification using a linear optical circuit [3]. References: [1] L. Carosini et al. , "Programmable multiphoton quantum interference in a single spatial mode", Sci. Adv.10, eadj0993(2024) [2] H. Cao et al., "Photonic source of heralded Greenberger-Horne-Zeilinger states", Phys. Rev. Lett. 132, 130604 (2024). [3] C.F.D. Faurby et al., “Purifying photon indistinguishability through quantum interference” (2024). arXiv:2403.12866

Noisy Quantum Diffusion: from transport phenomena to quantum generative AI

Filippo Caruso
Department of Physics and Astronomy, Florence University

Transport problems are very popular in several fields of science, such as biology, chemistry, sociology, information science, physics, and even in everyday life. Our theoretical and experimental past efforts showed the remarkable and counter-intuitive role of noise in enhancing the transport efficiency in complex quantum networks, e.g. light-harvesting proteins and quantum communication networks. This behavior was also experimentally simulated by exploiting other physical systems such as telecom optical fibers, cold atoms in optical lattices and bio-engineered viruses. Inspired by our previous works, we have investigated new algorithms of quantum machine learning where noise plays a crucial and even beneficial role in optimizing their performances. They have allowed us to successfully address machine learning tasks as image classification, pattern recognition, and also data generation, e.g. via quantum generative adversarial networks. In this context, we have proposed and discussed the first quantum generalization of diffusion models such as the very popular GPT. In particular, we have suggested how to exploit quantum noise not as an issue to be detected and corrected but instead as a very remarkably beneficial key ingredient to generate much more complex probability distributions that would be difficult or even impossible to express classically, and from which a quantum processor might sample more efficiently than a classical one. Therefore, we believe that our results might have widespread real-world future applications ranging from climate forecasting to healthcare, from traffic flow analysis to financial forecasting.

Assessing a binary quantum channel exploiting a silicon photomultiplier based hybrid receiver

silvia Cassina
università degli studi dell'insubria

Quantum communication has been a topic of strong interest in the last few decades and the improvements in the technological field have given us all the tools to implement several communications protocols. These may be divided in two macro categories: continuous variables (CV) protocols and discrete variables (DV) protocols. Focusing on CV protocols, the most exploited detection scheme is the well-known homodyne detection. This work is meant to be a preliminary study to investigate the feasibility of a new CV detection scheme based on photon-number resolving detectors as a quantum communication channel. The mentioned scheme has many similarities with the standard homodyne, such as the mixing at a balanced beam splitter of the state of interest (in our case a coherent state) and a more intense coherent state, called local oscillator. In our case the exploited detectors are silicon photomultipliers (SiPM), that have already been demonstrated to be suitable for the characterization of several states of light. This study involves a first step of classical characterization of the channel in terms of mutual information and a further investigation of the channel its performance in the case of QKD protocol, under the wiretap channel assumption, in terms of key generation rate . To evaluate the performance of the channel we took into account the possible source of information. In fact, according to how the detection is performed we can access the direct outputs of the detectors or their difference. Here are the possibilities we considered in our work: 1) the sign of the output difference that represents a binary discrimination strategy (BDS); 2) the statistical distribution of the difference of the two outputs (namely the so-called Skellam distribution), which is similar to the standard homodyne detection and therefore referred to as homodyne-like (HL); 3) the statistical distribution of the direct output of each PNR detector, considering the scheme as a weak field receiver (WF). We prove that the experimental data agree with the expected numerical models. In particular, we demonstrate that both in a state-discrimination scenario and in a QKD protocol based on a binary alphabet the WF and HL approaches coincide, while the BDS method leads to worse results.

Resource-Theoretic Hierarchy of Contextuality for General Probabilistic Theories

Lorenzo Catani
International Iberian Nanotechnology Laboratory

In this work we present a hierarchy of generalized contextuality. It refines the traditional binary distinction between contextual and noncontextual theories, and facilitates their comparison based on how contextual they are. Our approach focuses on the contextuality of prepareand-measure scenarios, described by general probabilistic theories (GPTs). To motivate the hierarchy, we define it as the resource ordering of a novel resource theory of GPT-contextuality. The building blocks of its free operations are classical systems and GPT-embeddings. The latter are simulations of one GPT by another, which preserve the operational equivalences and thus cannot generate contextuality. Noncontextual theories can be recovered as least elements in the hierarchy. We then define a new contextuality monotone, called classical excess, given by the minimal error of embedding a GPT within an infinite classical system. In addition, we show that the optimal success probability in the parity oblivious multiplexing game also defines a monotone in our resource theory. We end with a discussion of a potential interpretation of the non-free operations of the resource theory of GPT-contextuality as expressing a kind of information erasure.

Thermodynamic and Protection of Discrete-Time Crystal

Gabriele Cenedese
Università degli Studi dell'Insubria

Discrete time crystals (DTCs) represent a fascinating frontier in the realm of quantum systems, characterized by non-equilibrium dynamics and robust periodicity. Despite their potential applications in various fields such as quantum computing and precision sensing, the inherent challenge lies in their susceptibility to decoherence and short lifetimes. In this study, we delve into the thermodynamic properties of these open quantum systems, proposing an approach to extend their lifespans through repeated measurement schemes. We investigate the dynamics of DTCs under the influence of environmental coupling using the Lindblad master equation, and we meticulously and comprehensively examine their thermodynamic properties, surpassing the existing literature. Through numerical simulations and analytical modeling, we reveal an increased lifetime of the DTC dynamics. Moreover, we introduce a novel methodology for detecting time crystal signatures utilizing quantum trajectories. By tracking the quantum evolution of the system through trajectory analysis, we identify distinct signatures associated with the presence of DTCs, providing a promising avenue for experimental verification and characterization. Overall, our study contributes to advancing the understanding of DTCs and offers a promising avenue for mitigating decoherence effects in such quantum systems. The proposed methodology not only extends the lifetimes of DTCs but also unveils intriguing connections between quantum thermodynamics and non-equilibrium dynamics. Through further exploration and experiments, we envision unlocking new avenues for harnessing DTCs in quantum technologies and fundamental physics research.

Superconducting Quantum Systems for Axion Search and Other Fundamental Physics Applications

Fabio Chiarello
CNR-IFN

The detection of single photons in the microwave range is important for various applications, from quantum computing to the search for exotic particles such as Axions, hypothetical weakly interacting particles that could partially explain dark matter. A promising approach for this detection is based on the use of systems exploiting the Josephson effect. Josephson systems can be used as single microwave photon detectors in different configurations, including current-biased single junctions in coplanar waveguides or qubits coupled to resonators. These systems not only show great potential in detecting weakly interacting particles but are also of considerable interest for other fundamental physics applications, for example as quantum simulators for complex systems. We present various approaches developed within the QubIt INFN collaboration, discussing the obtained results, challenges, and future perspectives.

Molecular Nanomagnets: a promising tool for quantum information processing

Simone Chicco
University of Parma

The potential to solve problems with large impact on science, society and economy makes the realization of quantum computers one of the hottest topics in current research. Molecular nanomagnets (MNMs) offer a promising route towards a scalable quantum computer [1]. Being controllable quantum objects, they have in fact attracted a considerable attention as molecular qubits, thanks to their highly engineerable spin Hamiltonians and long coherence times. In addition, the feature that makes them potentially disruptive for quantum technologies is that they naturally provide a multi-level energy spectrum suitable to encode qudits and thus increasing the power of quantum logic applications. In particular, MNMs can be exploited to define qubits with embedded QEC in single molecules [2], circumventing the large overhead in the number of physical units required by standard QEC codes. Here I will show the first working proof-of-concept quantum simulator realized with an ensemble of molecular qudits and a radiofrequency broadband spectrometer [3]. We exploit an ensemble of 173Yb(trensal) qudits [4] to implement the quantum simulation of two different classes of problems: an integer spin subject to quantum tunnelling of the magnetization and a pair of spins 1/2 coupled by Ising interaction in a transverse field. Having correctly reproduced the time evolution of both physical models, we proved the versatility of this prototypical quantum simulator, also making an important step toward the actual use of molecular spin qudits in quantum technologies. This work received financial support from European Union-NextGenerationEU, PNRR MUR project PE0000023-NQSTI, from the European Union’s Horizon 2020 program under Grant Agreement No. 862893 (FET-OPEN project FATMOLS), from the Novo Nordisk foundation under grant NNF21OC0070832 in the call ”Exploratory Interdisciplinary Synergy Programme 2021” and from Fondazione Cariparma. [1] A. Chiesa, P. Santini, E. Garlatti, F. Luis, S. Carretta, “Molecular spins: a viable path toward quantum information processing?”, Rep. Progr. Phys., 87, 034501 (2024). [2] A. Chiesa, E. Macaluso, F. Petiziol, S. Wimberger, P. Santini, S. Carretta, “Molecular Nanomagnets as Qubits with Embedded Quantum-Error Correction”, J. Phys. Chem. Lett. 11, 8610–8615 (2020). [3] S. Chicco, G. Allodi, A. Chiesa, E. Garlatti, C. D. Buch, P. Santini, R. De Renzi, S. Piligkos, S. Carretta, “Proof-of-concept Quantum Simulator based on Molecular Spin Qudits”, J. Am. Chem. Soc., [4] R. Hussain, G. Allodi, A. Chiesa, E. Garlatti, D. Mitcov, A. Konstantatos, K. S. Pedersen, R. De Renzi, S. Piligkos, S. Carretta, J. Am. Chem. Soc. 140, 9814-9818 (2018).

Dissipative dynamics and thermal Purcell effect in cryogenic cavities

Giuliano Chiriacò
Università di Catania

In recent years, there has been significant interest in using optical cavities to manipulate the properties and phases of embedded quantum materials. Through the Purcell effect, a cavity alters the photon phase space and thus the rate of electromagnetic transitions within the material, affecting the exchange rate of heat radiation with the photon environment. This effect is compared with typical energy dissipation processes,showing that it may dramatically impact the temperature of a material coupled to the cavity. Additionally, the temperature of the photons inside the cavities is studied using a Lindblad formalism that accounts for different temperatures of the environment and of the cavity mirrors.

Noise-mitigated VQE algorithm for topologically-non trivial systems

Carola Ciaramelletti
Università degli Studi dell'Aquila

In quantum computing the Variational Quantum Eigensolver (VQE) algorithm is a versatile method for the estimation of the ground state energies of quantum systems. Its significance lies in its potential to utilize quantum hardware for solving complex problems more efficiently than classical methods. However, VQE faces significant challenges, particularly in converging to the ground state when dealing with degenerate solutions. This study focuses on two many-body physical systems: the Su-Schrieffer-Heeger (SSH) model and the Kitaev model, proposing strategies to address convergence issues within topologically non-trivial phases. Our investigation extends to the impact of simulated noise and the efficacy of noise mitigation techniques. Comparative analyses of noise-affected and noise-mitigated results are presented.

Dynamically Emergent Quantum Thermodynamics of Open Systems Under non-Markovian Evolution

Alessandra Colla
University of Milan

The theoretical description of thermodynamic quantities for open quantum systems is particularly challenging for systems under strong or structured interactions with the environment, which in general lead to memory effects and non-negligible energy contributions. We propose a framework based on open quantum systems techniques by identifying an effective energy operator within a time-local master equation. This emergent Hamiltonian contains information about the influence of the coupling on the energy of the system and guides the definition of thermodynamic quantities, leading to a generalization of the laws of quantum thermodynamics. In particular, we find under this framework how negative entropy production rates are related to the backflow of information from the environment to the system.

A Variational Approach to the Quantum Separability Problem

Mirko Consiglio
University of Malta

We present the variational separability verifier (VSV), which is a novel variational quantum algorithm (VQA) that determines the closest separable state (CSS) of an arbitrary quantum state with respect to the Hilbert-Schmidt distance (HSD). We first assess the performance of the VSV by investigating the convergence of the optimization procedure for Greenberger-Horne-Zeilinger (GHZ) states of up to seven qubits, using both statevector and shot-based simulations. We also numerically determine the (CSS) of maximally entangled multipartite X-states (X-MEMS), and subsequently use the results of the algorithm to surmise the analytical form of the aforementioned (CSS). Our results indicate that current noisy intermediate-scale quantum (NISQ) devices may be useful in addressing the NP-hard full separability problem using the VSV, due to the shallow quantum circuit imposed by employing the destructive SWAP test to evaluate the (HSD). The (VSV) may also possibly lead to the characterization of multipartite quantum states, once the algorithm is adapted and improved to obtain the closest k-separable state (k-CSS) of a multipartite entangled state.

Topological phase induced by Kerr non-linearities in spontaneously symmetry broken quantum resonators

Alessandro Coppo
CNR Istituto di Sistemi Complessi (ISC)

In this talk we show that cross-Kerr non-linearities can induce topological phases and edge states in an otherwise topologically trivial system. In particular, we consider a chain of parametrically-driven quantum resonators, with local Kerr and nearest-neighbour cross-Kerr non-linearities. We study the limit in which the non-linearities are vanishingly small and the system undergoes a second-order critical phase transition. When the parametric drive is below threshold, all Kerr terms can be neglected, the resonators are effectively decoupled, and the low-energy spectrum is given by local squeezed Fock states. On the contrary, when the drive overcomes a critical value, the system enters a spontaneously symmetry broken regime, the growth of excitations makes non-linear contributions unavoidably relevant, and the Kerr terms play a key role for the stabilization of the system. Using Gaussian expansions around semi-classical equilibrium points, we find different effective models for periodic (bulk) and open (edge) boundary conditions. We then analytically derive approximate solutions for the low-energy spectrum beyond threshold and show how staggered cross-Kerr terms induce a non-trivial Zak phase associated with topological edge states at the boundary.

Bayesian mitigation of measurement errors in multiqubit experiments

Francesco Cosco
VTT Technical Research Centre of Finland

We introduce a Bayesian error mitigation implementation tailored for multiqubit experiments conducted on near-term quantum devices. Our approach leverages complete information from the readout signal, available prior to any binary state assignment of the qubits. We provide a detailed workflow of the algorithm, starting from the calibration of detector response functions to the post-processing of measurement outcomes, offering a computationally efficient solution suitable for the output size typical of current quantum computing devices. We benchmark our protocol on actual quantum computers with superconducting qubits where the readout signal encodes the measurement information in the IQ clouds before the qubit state assignment. Finally, we compare our algorithm performances against other measurement error methods.

Engineering quantum states from a spatially structured quantum eraser

Vincenzo D'Ambrosio
Università di Napoli Federico II

Quantum interference is a central resource in many quantum-enhanced tasks, from computation to communication protocols. While it usually occurs between identical input photons, it can also be enabled by performing projective measurements that render the photons indistinguishable, a process known as quantum erasing. Structured light, on the other hand, is another hallmark of photonics: it is achieved by manipulating the degrees of freedom of light at the most basic level and enables a multitude of applications in both classical and quantum regimes. By combining these ideas, we design and experimentally demonstrate a simple and robust scheme that tailors quantum interference to engineer photonic states with spatially structured coalescence along the transverse profile. To achieve this, we locally tune the distinguishability of a photon pair via spatial structuring of their polarisation, creating a structured quantum eraser. We believe these spatially-engineered multi-photon quantum states may be of significance in fields such as quantum metrology, microscopy, and quantum information.

Permutation-equivariant quantum convolutional neural networks

Sreetama Das
CNR-INO Florence, Italy

Quantum convolutional neural networks (QCNNs) are parametrized quantum circuits composed of quantum analogs of convolutional and pooling layers. They have shown promising efficiency in classifying topological phases of quantum many-body systems as well as classical images. When the class labels of the underlying dataset remain invariant under action of certain symmetry groups, one can build Equivariant QCNN (EQCNNs) which respects that label symmetry. EQCNNs have less number of trainable parameters, improved generalization and sometimes better accuracy compared to a non-equivariant QCNN. The Symmetric group S_{n} manifests itself in large classes of quantum systems as the invariance of certain characteristics of a quantum state with respect to permuting the qubits. The subgroups of S_{n} arise, among many other contexts, to describe label symmetry of classical images with respect to spatial transformations, e.g. reflection or rotation. In our work, we propose the architectures of EQCNNs adherent to S_{n} and its subgroups. We demonstrate that a careful choice of pixel-to-qubit embedding order can facilitate easy construction of EQCNNs for small subgroups of S_{n}. In this case, our numerical results using MNIST datasets show better classification accuracy than non-equivariant QCNNs. Our novel EQCNN architecture corresponding to the full permutation group S_{n} is built by applying all possible QCNNs with equal probability, which can also be conceptualized as a dropout strategy in quantum neural networks. The S_{n}-equivariant QCNN architecture shows significantly improved training and test performance than non-equivariant QCNN for classification of connected and non-connected graphs. When trained with sufficiently large number of data, the S_{n}-equivariant QCNN shows better average performance compared to S_{n}-equivariant QNN. These results contribute towards building powerful quantum machine learning architectures in permutation-symmetric systems.

Quantum optics with organic molecules in solid-state matrices

Daniele De Bernardis
INO-CNR

Besides emerging as excellent single photon emitters, organic molecules trapped in solid-state matrices are also promising candidates to be a new fertile and versatile complete quantum optics platform. Unlike atoms in optical tweezers, these quantum emitters face the complexity of existing in a structured solid-state environment, which strongly affects their behavior. Particularly striking is the effect of the matrix’s phonons and intrinsic molecular vibrations, which are known to produce strong decoherence and dephasing in the optical emission properties. Additionally, positional disorder and matrix defects can generate significant inhomogeneous broadening and spectral diffusion. Through the most recent experimental and theoretical advancements, I will show in this talk that these features are not necessarily detrimental but can actually be a resource that brings technical advantages or even the possibility to address new physical phenomena.

Shapiro steps in a strongly-interacting atomic Josephson junction under AC drive

Giulia Del Pace
University of Florence

The Josephson effect is one of the most striking manifestations of a macroscopic system phase coherence. Besides representing a powerful probe of phase coherence, Josephson junctions (JJ) are also fundamental building blocks for atomtronics circuits, thanks to their well defined current-chemical potential and current-phase characteristics. In this talk, I will present our recent research on the response of an atomic JJ with Fermi superfluids of lithium-6 under an AC driving. To inject in the junction an alternate current, we modulate the position of the tunneling barrier at a given frequency and probe the chemical potential imbalance developed across the junction after a few modulation periods. The AC drive introduces in the current-chemical potential characteristic a number of Shapiro steps at a chemical potential value that is an integer multiple of the driving frequency, similarly to superconducting JJ with an external radiofrequency drive. We connect the presence of the steps to the synchronization of the relative phase at the junction with the external drive, which leads to n phase slips events in the n-th Shapiro steps, which we could directly access by counting the number of emitted vortices. Our work not only demonstrates the existence of Shapiro steps in fermionic JJ, but also highlights the microscopic mechanism behind such a phenomenon.

Density Classification with Non-Unitary Quantum Cellular Automata

Federico Dell'Anna
University of Bologna

The density classification (DC) task, a computation which maps global density information to local density, is studied using one-dimensional non-unitary quantum cellular automata (QCAs). Two approaches are considered: one that preserves the number density and one that performs majority voting. For number preserving DC, two QCAs are introduced that reach the fixed point solution in a time scaling quadratically with the system size. One of the QCAs is based on a known classical probabilistic cellular automaton which has been studied in the context of DC. The second is a new quantum model that is designed to demonstrate additional quantum features and is restricted to only two-body interactions. Both can be generated by continuous-time Lindblad dynamics. A third QCA is a hybrid rule defined by both discrete-time and continuous-time threebody interactions that is shown to solve the majority voting problem within a time that scales linearly with the system size.

Exploring dynamical quantum phase transitions and work extraction in an open two-qubit Rabi model

Grazia Di Bello
University of Naples "Federico II"

We investigate both the dynamical and thermodynamic properties of an open two-qubit Rabi model using state-of-the-art numerical methods [1]. By inducing a quench on the coupling between the qubits and the oscillator, the global system, including bath degrees of freedom, undergoes dynamical quantum phase transitions [2]. We identify two types of transitions, characterized by different critical exponents depending on the interactions and entanglement in the system. These transitions are marked by kinks in the rate function of the Loschmidt echo, occurring in the same range of parameters where a thermodynamic Berezinskii-Kosterlitz-Thouless transition occurs. Notably, the onset of these transitions is signaled not only by the bimodal character of the magnetization distribution but also by changes in the entanglement of the two qubits. These findings shed light on the complex dynamics of quantum phase transitions [3]. Additionally, given the insights obtained from this model and its relationship with quantum phase transitions, we aim to leverage them to extract work from the subsystem. Ergotropy, which quantifies the maximum extractable work from a quantum system, has recently garnered increasing attention for its relationship with many-body phenomena such as quantum phase transitions. In this context, we scrutinize the recent concept of local ergotropy [4], which measures work extraction ability in the presence of an uncontrollable surrounding environment. Specifically, we investigate the behavior of local ergotropy and its relative fluctuations across the previously discussed equilibrium quantum phase transition. We propose a realistic protocol encompassing charging, quasi-decoherence-free storage, and work extraction [5]. Our findings reveal that that high couplings to an external bath approximately double the local ergotropy immediately post-charging. Finally, we demonstrate that local ergotropy and its fluctuations can detect the quantum phase transition within the model. References: 1. G. De Filippis, A. de Candia, G. Di Bello, C. A. Perroni, L. M. Cangemi, A. Nocera, M. Sassetti, R. Fazio, and V. Cataudella, “Signatures of Dissipation Driven Quantum Phase Transition in Rabi Model”, Phys. Rev. Lett. 130, 210404 (2023). 2. M. Heyl, “Dynamical quantum phase transitions: A brief survey”, Europhys. Lett. 125, 26001 (2019). 3. G. Di Bello, A. Ponticelli, F. Pavan, V. Cataudella, G. De Filippis, A. de Candia, and C. A. Perroni, “Environment induced dynamical quantum phase transitions in two-qubit Rabi model”, arXiv preprint arXiv:2312.05697 (2023). 4. R. Salvia, G. De Palma, and V. Giovannetti, “Optimal local work extraction from bipartite quantum systems in the presence of Hamiltonian couplings”, Phys. Rev. A 107, 012405 (2023). 5. G. Di Bello, et al., “Local Ergotropy and its fluctuations across a dissipative quantum phase transition”, in prep.

Dipole-dipole interactions mediated by a photonic flat band

Enrico Di Benedetto
Università degli Studi di Palermo

Flat bands (FBs) are energy bands with zero group velocity, which in electronic systems were shown to favor strongly correlated phenomena. Indeed, a FB can be spanned with a basis of strictly localized states, the so called "compact localized states" (CLSs), which are yet generally non-orthogonal. Here, we study emergent dipole-dipole interactions between emitters dispersively coupled to the photonic analogue of a FB, a setup within reach in state-of the-art experimental platforms. We show that the strength of such photon-mediated interactions decays exponentially with distance with a characteristic localization length which, unlike typical behaviours with standard bands, saturates to a finite value as the emitter's energy approaches the FB. Remarkably, we find that the localization length grows with the overlap between CLSs according to an analytically-derived universal scaling law valid for a large class of FBs both in 1D and 2D. Using giant atoms (non-local atom-field coupling) allows to tailor interaction potentials having the same shape of a CLS or a superposition of a few of these.

Direct measurement of the quantum geometric tensor

Francesco Di Colandrea
University of Ottawa

The quantum dynamics of two-band wavepackets is controlled by the quantum geometric tensor. The Berry curvature (the imaginary part of the tensor) accounts for adiabatic trajectories, while the quantum metric (the real part of the tensor) describes non-adiabatic corrections. The quantum metric also features a fundamental geometrical interpretation, expressing the distance between the system eigenstates. It has measurable effects on flat-band superfluidity, exciton Lamb shift, and orbital magnetic susceptibility. We show that, in chiral-symmetric systems, the quantum geometric tensor can be retrieved by measuring the mean chiral displacement of delocalized wavefunctions. Interestingly, an appropriate gauge choice allows linking the tensor to the Berry connection. These findings are experimentally demonstrated in a topological quantum walk of structured light [1]. [1] F. Di Colandrea et al., arXiv: 2401.07946

Measurement-Induced Phase Transitions: From No-Click Limits to Full Lindbladian

Giovanni Di Fresco
Palermo University

Out-of-equilibrium closed systems undergo very interesting physical phenomena such as thermalization, transport, and entanglement properties. Recently, a lot of attention has been given to these systems in situations where unitary dynamics are alternated with measurements. This is particularly due to measurement-induced phase transitions (MIPT), where the interplay between measurement and unitary dynamics gives rise to different non-equilibrium phases characterized by distinct entanglement structures. In this work, we investigate whether signatures of MIPT persist under partial post-selection, i.e., averaging over a subset of trajectories as opposed to the full ensemble. Conceptually, this corresponds to retaining only partial information about the measurement outcome. Here, we introduce a Liouvillian model that interpolates between the full post-selected scenario, i.e., the so-called no-click limit, and the Lindbladian model. We examine the steady state of a continuously monitored fermionic Kitaev chain, characterizing its correlation and entanglement properties across the whole phase diagram.

Ultracold bosons in the flat band of an optical kagome lattice

Luca Donini
University of Cambridge

The kagome lattice hosts a flat band resulting from extensive geometric frustration. For fermions, this makes the kagome antiferromagnet a candidate for studying the quantum spin liquid phase. But even for weakly-interacting scalar bosons, the many-body physics in a flat band is complex and not fully understood. Stable loading of ultracold atoms into the flat band of an optical kagome is challenging, as it is not the ground band. For the first time, we load bosonic 39K atoms into the flat band of the kagome lattice using a negative absolute temperature scheme and achieve long lifetimes. I will report on our observations of the melting of the negative-temperature Mott Insulator into the flat band and on measurements characterising the resulting flat-band state.

Hybrid Variational Algorithms on a neutral atom platform

elisa ercolessi
University of bologna

Quantum Computing is seen as a potential breakthrough for the study of hard classical problems as well as for quantum many body systems. However, we are in the era of NISQ devices and still far away from fault-tolerant machines. This leads us to consider the possibility of hybrid classical-quantum protocols of variational type: they exploit quantum resources to efficiently prepare states that depend on a suitable chosen set of variational parameters, which can then be determined by means of optimization algorithms to be run on a classical computer. The choice of such classical optimizer schemes is to be guided by compatibility requirements with respect to current available quantum platforms. To evaluate the feasibility of such an approach, we present an application of the Quantum Approximate Optimization Algorithm to a typical classical hard combinatorial problem, that has been emulated and tested on the Rydberg atom quantum machine Fresnel of Pasqal.

Quantum Resources with constraints

Gianluca Esposito
Scuola Superiore Meridionale

Quantum Resource Theory is a framework for the understanding of what is useful and costly in quantum operations and quantum information processing. In this framework, there are free operations and free states that one can generate and use at no cost. For example, in the resource theory of entanglement, LOCC operations and separable states are free. In the resource theory of stabilizer properties, stabilizer states and Clifford operations are free. These two examples are paramount for obtaining quantum advantage in quantum computation. In the presence of constraints, one is only able to access states and operations within a subspace. One therefore wonders what is the average resource in this subspace. Moreover, to understand how much the constraints have either implemented or depleted resources, one defines the notion of resource gap associated to a subspace. We study the resource gap of stabilizer entropy associated to a certain class of projectors. Our results have a large range of applications from quantum error correcting codes to gauge theories.

Adiabatic passage in solid state: from ultrastrong coupling to noise sensing

Giuseppe A. Falci
Università di Catania - INFN - NQSTI - ICSC

Adiabatic passage is a powerful control technique atomic physics which is gaining interest also in the solid-state realm since it implements quantum operations weri robust against parametric fluctuations. We exploit the application of coherent techniques as coherent transport by adiabatic passage (CTAP) or stimulated Raman adiabatic passage (STIRAP) in quantum architectures where the robustness of the protocols may determine key advantages for selected tasks[1,2]. As an example we discuss quantum operation for modular computing in ultrastrongly coupled structures of artificial atoms [3] showing that CTAP-like manipulation ensure the suppression of unrecoverable errors due to the dynamical Casimir effect. A second example is noise classification in multilevel quantum structures where we propose a STIRAP-based supervised learning procedure to recognize energy-correlations of noise and their relation to the Markovianity of the environment [4]. This work is supported by the PNRR MUR projects ICSC – Centro Nazionale di Ricerca in High-Performance Computing, Big Data and Quantum Computing and PE0000023-NQSTI and the QuantERA grant SiUCs (Grant No. 731473), the University of Catania, Piano Incentivi Ricerca di Ateneo 2020-22, project Q-ICT and the COST Action CA 21144 superqumap. [1] Jonathon Brown et al 2021 New J. Phys. 23 093035 [2] L. Giannelli, et al. 2024 Phys. Rev. Research 6, 013008 [3] L. Giannelli et al. Il Nuovo Cimento C 45-6, 171 (2022); G. Falci et al, draft. [4] S. Mukherjee, et al., arXiv:2405.01987

Collective Quantum Enhancement in Critical Quantum Sensing

Simone Felicetti
Istituto dei Sistemi Complessi CNR-ISC

Critical quantum sensing (CQS) is by now a well-established approach, based on the exploitation of quantum properties spontaneously developed in proximity of phase transitions. Numerous theoretical studies and first experimental demonstrations show that a quantum-enhanced sensing precision can be achieved by exploiting static or dynamical properties of many-body systems in proximity of the critical point. In recent seminal contributions, it has been shown that CQS protocols can be implemented using finite-component phase transitions (FCPTs), where the thermodynamic limit is replaced with a rescaling of the system parameters. This class of phase transitions emerges in quantum resonators with atomic or Kerr-like nonlinearities, and it is of high theoretical and experimental relevance. Designing sensing protocols based on FCPT presents three main advantages: (1) Theoretically, FCPT allows us to analytically (or semi-analytically) investigate achievable precision, and compare it with fundamental bounds of quantum metrology. (2) Experimentally, FCPT provides a feasible approach to implementing optimal critical quantum sensors using small-scale, controllable devices. Finally, (3) physical devices undergoing FCPTs can be easily coupled and embedded in linear chains or bi-dimensional lattices. It is then feasible to implement arrays of coupled critical systems and generalize the CQS approach to the multipartite case.  Here, we show for the first time that achieving collective quantum enhancement in CQS is possible using an array of critical quantum sensors. We demonstrate both a fundamental advantage, in terms of scaling with fundamental resources, and a practical advantage, in terms of accessible physical parameters. In particular, we consider a model composed of a chain of nonlinear quantum resonators coupled via parametric nearest-neighbour coupling. We focus on the weak nonlinearity limit, where each node of the array becomes locally critical. We assess the metrological performance of a static CQS protocol by analytically computing the quantum Fisher information (QFI) over the ground state manifold. In proximity to the critical point, we find closed-form analytical solutions for the asymptotic scaling of the QFI with respect to fundamental resources, i.e. the number of resonators, the number of excitations, and the protocol duration time. This analysis allows us to benchmark the estimation precision achievable with the coupled-resonator array, with that achievable with the same number of independent critical sensors. We find that the interacting array achieves a quadratic enhancement in the scaling of the QFI with respect to the number of resonators, an unambiguous demonstration of collective quantum enhancement. Beyond asymptotic scalings, we find a technologically relevant advantage for finite values of the physical parameters. This work paves the way for designing and implementing multipartite critical quantum sensors.

Controlling superconducting qubits for quantum computing

Stefan Filipp

Quantum computers have the potential to solve complex problems efficiently. However, to unleash their full capability, complex quantum systems have to be manufactured, manipulated and measured with unprecedented accuracy and precision. In this presentation I will focus on superconducting qubits as one of the most promising platforms for quantum computing. To enhance their quantum processing capabilities we have systematically optimized the material parameters and reached several hundred microseconds coherence times. Furthermore, we have investigated optimal control methods and demonstrated high-fidelity single and two-qubit gates. Finally, by simultaneously coupling multiple qubits we could realize multi-qubit operations to efficiently create many-body entangled state. As a specific example I will demonstrate a fractional state transfer protocol on a chain of superconducting qubits and discuss its potential use case for quantum simulations and parity readout.

La strategia nazionale per le tecnologie quantistiche

Francesca Galli
Dirigente, Ufficio di Gabinetto – Segreteria Tecnica del Ministro dell'Università e della Ricerca

Daemonic ergotropy in continuously monitored quantum batteries

Marco Genoni
Università degli Studi di Milano

The amount of work that can be extracted from a quantum system can be increased by exploiting the information obtained from a measurement performed on a correlated ancillary system. The concept of daemonic ergotropy has been introduced to properly describe and quantify this work extraction enhancement in the quantum regime. We explore the application of this idea in the context of continuously monitored open quantum systems, where information is gained by measuring the environment interacting with the energy-storing quantum device. We show that the corresponding daemonic ergotropy takes values between the ergotropy and the energy of the corresponding unconditional state. The upper bound is achieved by assuming an initial pure state and a perfectly efficient projective measurement on the environment, independently of the kind of measurement performed. On the other hand, if the measurement is inefficient or the initial state is mixed, the daemonic ergotropy is generally dependent on the measurement strategy. We will first theoretically investigate this scenario via a paradigmatic example of an open quantum battery: a two-level atom driven by a classical field and whose spontaneously emitted photons are continuously monitored via either homodyne, heterodyne, or photodetection. We will then present a work-of-principle experimental demonstration of daemonic work extraction by simulating a continuously monitored collision model on an IBM quantum computer.

In-Field Comparison between G.652 and G.655 Optical Fibers for Polarization-Based Quantum Key Distribution

Massimo Giacomin
Università degli Studi di Padova

Quantum Key Distribution (QKD) is a quantum communication protocol that allows users to distill a secret key with unconditional security. The importance of this technology is highlighted by recent breakthroughs in quantum computing, that is becoming a threat for classical cryptographic infrastructures. Furthermore, QKD is the first quantum communication protocol to have reached industrialization and commercialization. This has resulted in various national and international efforts that have encouraged the implementation of QKD systems in our telecommunications networks. Two of the most commonly exploited optical fibers widely implemented in telecommunications are the standards ITU-T G.652 and ITU-T G.655. Both fiber types support single-mode operations for the 1310 nm and 1550 nm bands, making them suitable for long-haul links. The main difference between the two lies in their dispersion properties. G.652 fibers are optimized for the 1310 nm band as they have zero dispersion at this wavelength. In contrast, G.655 fibers are more appropriate for the 1550 nm wavelength region due to their reduced dispersion value in the C-band (1530-1660 nm). G.655 fibers are better suited for scenarios where controlling the launch conditions is challenging, such as in long-distance or undersea communications and can also be easily implemented with Erbium Doped Fiber Amplifiers (EDFA), making them adequate for Wavelength Division Multiplexing (WDM) communication systems. To our knowledge, no direct comparison between the performance of G.652 and G.655 fibers for QKD applications with polarization encoding has yet been made. Our study is of interest to the QKD community, as both standards are widely employed in telecommunications networks. We report the results of a QKD field-trial exploiting both standards deployed between two urban centers in the Veneto region of Italy. The QKD system used in the trial was made of commercial devices that implement the BB84 protocol exploiting polarization encoding. We performed a 24-hour trial in both ``dark-fiber'' and coexistance configurations. We tested the co-propagation and counter-propagation schemes, demonstrating the feasibility of these configurations when real optical fibers in urban environment are used. The overall QKD performance in both fiber standards can be assumed to be excellent, especially when considering that they were obtained from deployed fibers used for day-to-day telecommunications between the Venezia - Mestre and Treviso POPs. References: 1. C. Agnesi, M. Giacomin, et al., “In-Field Comparison between G.652 and G.655 Optical Fibers for Polarization-Based Quantum Key Distribution”, https://doi.org/10.48550/arXiv.2312.04203, (2023). 2. C. Bennett and G. Brassard, “Quantum cryptography: Public key distribution and coin tossing”, Theor. Comput. Sci., 560, 20355177 (1984) Funding: M.G.’s PhD scholarship was co-funded by Telebit S.p.A. and MIUR (Ministerial Decree 352/2022), partially by QUID GA 101091408.

Magic phase transition and non-local complexity in generalized W State

Salvatore Marco Giampaolo
Ruđer Bošković Institute

In the framework of topologically frustrated quantum systems, we employ the Stabilizer Rényi Entropy (SRE) to characterize a quantum phase transition that has so far eluded any standard description and can thus now be explained in terms of the interplay between its non-stabilizer properties and entanglement. This transition separates a region with a unique ground state from one with a degenerate ground state manifold spanned by states with opposite non-vanishing momenta. We show that SRE has a jump at the crossing points, while the entanglement entropy remains continuous. Moreover, by leveraging on a Clifford circuit mapping, we connect the observed jump in SRE to that occurring between standard and generalized W-states with finite momenta. This mapping allows us to quantify the SRE discontinuity analytically.

Mutual Information Bounded by Fisher Information

Wojciech Górecki
INFN Sez. Pavia

We derive a general upper bound to mutual information in terms of the Fisher information. The bound may be further used to derive a lower bound for Bayesian quadratic cost. These two provide alternatives to the Efroimovich and to the van Trees inequality that are useful also for classes of prior distributions where the latter ones give trivial bounds. We illustrate the usefulness of our bounds with a case study in quantum phase estimation. Here, they allow us to adapt to mutual information the known and highly nontrivial bounds for Fisher information in the presence of noise. This nicely complements quantum metrology, since Fisher information is useful to gauge local estimation strategies, whereas mutual information is useful for global strategies. https://arxiv.org/abs/2403.10248

Quantum signatures in
close-to-equilibrium Stochastic Thermodynamics

GIACOMO GUARNIERI
UNIVERSITÀ DI PAVIA

Out-of-equilibrium processes describe a wide plethora of phenomena, from bio-molecular motors in our cells to financial markets and quantum computers. A first universal property they all share is that they necessarily entail a certain amount of dissipation, i.e. thermodynamic resources in the form of entropy production are irreversibly generated. Moreover, they display significant fluctuations, that ideally must be minimized for reliable outcomes but which stop being negligible at the microscopic scale. These fluctuations can be of thermal classical nature and also of genuinely quantum origin. This raises the question: How do we identify genuinely quantum contributions to stochastic fluctuations and what are their thermodynamic implications? In this talk I will present a general answer to this question for processes that keep a system close to equilibrium, either by slow-driving or by external perturbation in linear response regime. Finally, I will present a recent experimental measurement of these quantum signatures in a trapped-ion experiment.

Shadow estimation via quantum extreme learning machines

luca innocenti
university of palermo

Shadow tomography is a general methodology used to estimate properties of input states while avoiding the resource scaling with the state dimension that is intrinsic to traditional tomographic approaches. In contrast, quantum extreme learning machines and quantum reservoir computing are quantum machine learning methods aimed at learning from a training dataset how to optimally post-process measurement data to retrieve target functions of input data. While these two methods appear quite distinct—one relying on machine learning training methods and the other firmly rooted in standard quantum estimation theory—we will highlight and discuss the deep connections between them. Specifically, we will examine how the framework of shadow tomography for general POVMs (Positive Operator-Valued Measures) allows for a precise understanding of the post-processing learned using quantum extreme learning machines through the concept of dual measurement. Additionally, we will discuss a formal approach to quantum extreme learning machines that demonstrates how both methodologies can be viewed as quantum estimation techniques differing only in their assumptions about prior knowledge of the measurement apparatus.

Minimizing resources for quantum devices with control theory

Christiane Koch

Control is the prerequisite to exploit the two essential elements of quantum physics, non-locality and coherence, for practical applications. Key challenges are to preserve the relevant non-classical features at the level of device operation and to scale the devices up in size. Control theory provides tools for tackling both challenges. On the one hand, controllability analysis aims at answering the question which control targets are accessible. On the other hand, control theory provides methods to derive the actual control fields that implement the desired dynamics. I will discuss how to leverage control theory to minimize resources for quantum devices and thus ease requirements towards scaling up their size. In particular, I will show how controllability analysis allows us to identify the minimum number of local controls required to implement universal quantum computing in an array of coupled qubits. Moreover, I will provide examples for the control of open quantum systems where the environment leads to decoherence but also opens new prospects for control. I will discuss examples for both strategies, with practical applications in Rydberg atoms, trapped ions, and superconducting circuits.

Asymptotics of quantum channels

Arturo Konderak
Center for Theoretical Physics Polish Academy Of Sciences

Quantum hardware, including quantum computers, faces challenges due to noise and decoherence. Asymptotic dynamics in open quantum systems offer opportunities for implementing noise-protected unitary operations. By explicitly defining the asymptotic map, we show how permutations affect this evolution. These permutations typically make the process non-unitary and depend on how divisible the quantum channel is.

Stabilizer Entropies are monotones for magic-state resource theory

Lorenzo Leone
Freie Universität Berlin

The core principle of magic-state resource theory hinges on the following dichotomy: while stabilizer operations are easily implemented in a fault-tolerant fashion, achieving fault-tolerance for nonstabilizer operations proves to be challenging. Its task is to utilize nonstabilizer resource states and stabilizer operations to distill non-Clifford gates fault-tolerantly. Being stabilizer operations efficiently classically simulatable and nonstabilizer states hard to simulate classically, beyond the scope of distillation, magic-state resource theory is also employed to quantify the hardness of classically simulating quantum states using stabilizer formalism. In resource theories, the main challenge is selecting a resource monotone, similar to a 'thermometer,' to accurately quantify the resource content in a given state. While universal resource monotones do exist, they are impractical to compute and measure experimentally. For practical applicability, such as in quantum many-body physics and quantum computation, identifying a resource-specific measure is crucial yet highly nontrivial. Stabilizer entropies, introduced with $\alpha$-Rényi indices, have been introduced to probe nonstabilizerness in multiqubit quantum states. A number of works has shown that they offer both analytical and numerical computability, along with experimental measurability. Furthermore, they have bridged magic-state resource theory with various quantum physics topics like quantum chaos, scrambling, and cryptography, as well as finding utility in fidelity estimation, Pauli sampling, and classical shadows in quantum information. However, a counterexample to the monotonicity of stabilizer entropies with Rényi index $\alpha$ strictly less than 2 under stabilizer protocols was recently provided. Since then, the question ``are stabilizer entropies with Rényi index $\alpha$ larger than 2 good monotones for magic-state resource theory?'' has sparked growing interest, given the significance of stabilizer entropies across the field of quantum information. In this work, we establish the monotonicity of stabilizer entropies with Rényi index larger than 2 in various flavors. We first show that they are pure-state magic monotones, i.e. for those stabilizer operations that send deterministically pure states to pure states. In fact, we prove a even stronger result: we show that their linearized versions -- linear stabilizer entropies -- exhibit strong monotonicity, meaning they do not, on average, increase following a non-deterministic protocol. Additionally, as a result of our investigation, we provide a convex-roof extension of stabilizer entropies to mixed states, ensuring monotonicity under stabilizer protocols and paving the way for further exploration of magic-state resource theory beyond pure states.

Dephasing-tolerant quantum sensing for magnetic fields

Luca Lepori
Università di Parma

We describe a quantum protocol for single effective spins, generally higher than 1/2, to measure magnetic fields with high precision and spatial resolution, compared to the so far available schemes. This relevant improvement is allowed by a parallel qudit-based quantum error-correction procedure against dephasing. Finally, we discuss the feasibility of the proposed approach in present experimental platforms, as molecular nanomagnets, trapped ions and ultra-cold atoms.

Quantum Extreme Learning Machine for quantum chemistry and biological tasks

Gabriele Lo Monaco
University of Palermo

Among the paradigms of quantum supervised machine learning, quantum reservoir computing, and specifically quantum extreme learning machine (QELM), have emerged as some of the most promising approaches. The notable advantages of QELM lie in the simplicity of its optimization routine which require minimal resources. Classical data are encoded in the quantum state of input qubits, with the choice of encoding determining model’s expressivity. The information is distributed across a reservoir interacting with the encoding qubits through a fixed scrambling dynamics that does not require any optimization during training. The reservoir is then measured and the outcomes are collected on a classical computer for post-processing, the optimization step, involving a simple linear regression to fit the target data. We present the first implementation of QELM on NISQ devices with applications to quantum chemistry and biological tasks of practical interest. We use QELM to reconstruct the potential energy surface of various molecules, mapping the geometry of a molecular species to its energy. Our setup outperforms classical neural networks and other quantum routines such as VQE, while minimizing quantum resource costs. It is scalable and has controlled depth, making it suitable for practical implementation on real hardware. The implementation performed on IBM Quantum platforms yielded encouraging results, with the average error close to chemical accuracy and no need for error mitigation. In the biological domain, we employ QELM as a support vector machine for protein classification on large experimental datasets. A practical example is classifying proteins based on their interaction with angiotensin enzyme, predicting peptides with anti-hypertensive effects that could contribute to cardiovascular health. The proteins are described in terms of 40 descriptors of biological relevance, such as aromaticity or hydrophobicity, a large number of input features presenting a challenge and benchmark for the current status of QELM. This second application confirms the feasibility of QELM and manifests an interesting resistance to statistical noise and decoherence, suggesting its potential for imminent applications in industrial pharmacology.

Multi-parameter estimation of the state of two interfering photonic qubits

Luca Maggio
University of Portsmouth

Achieving high-precision estimation of the polarization parameters of photonic qubits is key in both quantum sensing and quantum information processing. Multiphoton-photon interference serves as a powerful tool to achieve such a goal. However, current techniques are unable to retrieve the full photonic quantum metrology information to reach the ultimate precision in the estimation even for a single parameter [1]. Furthermore, they are unable to probe the relative phase of photonic qubits. We present a two-photon interfering technique enabling for the first time the estimation of multiple polarisation parameters associated with the photonic quantum states of two polarization qubits, including their relative phase, with the ultimate quantum sensitivity and within a relatively low number of experimental iterations [2]. In particular, our sensing protocol allows us to estimate simultaneously the polar angle of the two photons and the difference in their relative phase by polarisation-resolved sampling measurements. We evaluate its precision and accuracy by means of multiparameter quantum metrology [3]. Remarkably, we prove that the estimation reaches asymptotically the ultimate precision given by the Quantum Cramer-Rao bound for both parameters. Furthermore, we discuss the precision and accuracy in the non-asymptotic regime, showing that already for a number of sampling measurements of order 100 the sensing scheme approaches the ultimate precision for most of the values of the parameters we aim to estimate. Remarkably, our scheme allows for the first time the estimation of difference in relative phase of the polarisation of two photons with ultimate precision for arbitrary unknown values of the polar angle. These remarkable results at the interface between quantum metrology, quantum optical interference, and boson sampling can pave the way to a new platform of high-precision sensing techniques based on polarization measurements with ultimate quantum precision. It also lays the foundation for future schemes based on multi-parameter interferometry techniques for the estimation of polarisation parameters that involve more than two qubits in arbitrary quantum information networks for quantum communication and quantum information processing. [1] N. Harnchaiwat, F. Zhu, N. Westerberg, E. Gauger, and J. Leach, Opt. Express 28, 2210 (2020). [2] L. Maggio, D. Triggiani, P. Facchi,and V. Tamma, (2024), arXiv:2405.12870. [3] H. CRAMER, Mathematical Methods of Statistics (PMS-9) (Princeton University Press, 1999).

Tensor Network methods for quantum circuit emulation and quantum optical systems in non-Markovian regimes

Giuseppe Magnifico
University of Bari & INFN

Tensor network methods have emerged as powerful tools for tackling complex problems in quantum science and technology. In this talk, I will present developments in tensor network techniques for two important areas. First, I will present the integration of hyper-optimized contraction protocols into tensor network algorithms for emulating quantum circuits, highlighting substantial improvements in computational efficiency and accuracy. Second, I will describe techniques for simulating quantum optical systems in the non-Markovian regime, focusing on waveguide quantum electrodynamics. I will present results that show how properly engineered single-photon scattering can generate entangled bound states, outperforming traditional entanglement by relaxation procedures.

Entanglement Order Parameters and Transport in Topological Superconductors

Alfonso Maiellaro
Università degli Studi di Salerno

Topological superconductivity (TSC) is an exotic phase of matter characterized by a fully gapped superconducting bulk that hosts Majorana bound states protected by non-Abelian statistics and symmetries. Traditionally, TSC has been explored in semiconducting nanowires proximized by conventional superconductors. Recently, evidence for TSC has also been observed at LaAlO3/SrTiO3 (LAO/STO) interfaces. In the first part of this talk, we introduce an entanglement-based non-local order parameter for topological superconductors. Through various toy-lattice models, we demonstrate that this parameter is ideal for characterizing topological superconductors in both one-dimensional and quasi-one-dimensional systems. Specifically, for the Kitaev chain, this order parameter is quantized to ln(2)/2 in the topological phase and vanishes in the trivial phase. It accurately scales at quantum phase transitions, remains stable under interactions, and is robust against disorder and local perturbations. In the second part, we explore the transport properties of an oxide-based Josephson junction formed by constraining the 2DEG at the LAO/STO (001) interface into a quasi-1D system. We find that the strong enhancement of the critical current with an applied magnetic field can be linked to the emergence of Majorana bound states with an orbital-flavored internal structure, a feature absent in single-band models. These results provide strong evidence that the anomalous Josephson patterns observed experimentally are indicative of topological properties.

Relativistic quantum communication

Stefano Mancini
Università di Camerino

We consider quantum communication between two particle detectors interacting through a scalar field in a (3 + 1)-dimensional spacetime. Specializing to Minkowski spacetime, we show that the possibility to have a quantum capacity greater than zero stems from a relative acceleration between the detectors. However, this only occurs with infinite-level detector models (i.e. harmonic oscillators), not with finite-level detector models (i.e. qubits).

Addressing the Non-perturbative Regime of the Quantum Anharmonic Oscillator by Physics-Informed Neural Networks

Antonio Mandarino
Department of Physics Aldo Pontremoli, University of Milan

The use of deep learning in physical sciences has recently boosted the ability of researchers to tackle physical systems where little or no analytical insight is available. Recently, the Physics-Informed Neural Networks (PINNs) have been introduced as one of the most promising tools to solve systems of differential equations guided by some physically grounded constraints. In the quantum realm, such an approach paves the way for a novel approach to solving the Schroedinger equation for non-integrable systems. Following an unsupervised learning approach, we apply the PINNs to the anharmonic oscillator in which an interaction term proportional to the fourth power of the position coordinate is present. We compute the eigenenergies and the corresponding eigenfunctions while varying the weight of the quartic interaction. We bridge our solutions to the regime where both the perturbative and the strong coupling theory work, including the pure quartic oscillator. We investigate systems with real and imaginary frequency, laying the foundation for novel numerical methods to tackle problems emerging in quantum field theory.

A Tailor-made Quantum State Tomography Approach

Diego Maragnano
University of Pavia

Quantum state tomography (QST) aims at reconstructing the state of a quantum system. However in conventional QST the number of measurements scales exponentially with the number of qubits. Here we propose a QST protocol, in which the introduction of a threshold allows one to drastically reduce the number of measurements required for the reconstruction of the state density matrix without compromising the result accuracy. In addition, one can also use the same approach to reconstruct an approximated density matrix depending on the available resources. We implemented our protocol on the IBMQ system lagos to characterize random states from 4 up to 7 qubits, thus providing an accurate analysis of the efficacy of our strategy in a real environment. In all the considered cases the fidelity achieved with tQST was as large as 95% and compatible, within the experimental uncertainty, with the fidelity obtained by using conventional QST. However, our method allowed us to significantly reduce the amount of measurements, in some cases even 300 times less than conventional QST. This result is associated with the capability of our approach to efficiently extract all the available information about the state depending on the level of noise in the system. By using synthetic data, we also pushed the approach to our computational limit and performed the characterization of W states up to 14 qubits. In this case we reached a fidelity larger than 90% with only ∼ 16, 000 expectation values, four orders of magnitudes less than the 268,435,456 measurements that would be required by conventional QST. Finally, we developed a Python library that is freely available to implement tQST with an arbitrary large number of qubits, depending on the system under consideration and the available resources. Our protocol is a flexible and practical approach for the full characterization of large quantum systems, including those based on atoms or photons. For this reason, we believe that it will be particularly useful to the whole quantum community.

Generating bound-entangled states using multicore fibres

Carlo Marconi
CNR-INO

Multicore fibres are recently gaining considerable attention in the context of quantum communication tasks, where their capability to transmit multiple quantum states along different cores of the same channel make them a promising candidate for the implementation of scalable quantum networks. In this work, we show that multicore fibres can be effectively used not only for the scope of communication but also for the generation of entangled states. By exploiting the formalism of CPTP maps, we describe the action of a multicore fibre as a quantum channel and propose a protocol to implement bound entangled states of two qudits. Notably, the presence of crosstalk among the cores of the fibre is fundamental for the generation of the desired families of entangled states.

Precision magnetometry exploiting excited state quantum phase transitions

Ugo Marzolino
Università di Trieste

Critical behaviour in phase transitions is a resource for enhanced precision metrology. The reason is that the so-called Fisher information is superextensive at critical points, and, at the same time, quantifies performances of metrological protocols. Therefore, preparing metrological probes at phase transitions provides enhanced precision in measuring the transition control parameter. I will focus on the Lipkin-Meshkov-Glick model that exhibits excited state quantum phase transitions at different magnetic fields. I will show numerical evidence of broad peaks for the Fisher information with superextensive scaling, which are explained by the Hamiltonian spectral properties. I will also propose metrological schemes for estimating the magnetic field in the Lipkin-Meshkov-Glick model with precision better than the shot-noise and comparable with the Heisenberg limit achieved by interferometric schemes with entangled states.

Classification of One-dimensional Single Fermionic Quantum Cellular Automata

Paolo Meda
University of Pavia - INFN Sec. Pavia

Fermionic Quantum Cellular Automata (FQCA) define a local and causal discrete evolution of a grid of Fermionic modes located at each cell. The mathematical description of FQCA may be provided by assigning a graded quasi-local algebra of observables for each Fermionic mode, and thus homogeneous automorphisms modelling the evolution of the quantum cells. In this talk, I discuss some new developments in the study of FQCA and their properties of implementability as finite depth quantum circuits (FDQCs). In the first step, it is shown how to relate two FQCA through a FDQC without the addition of ancilla degrees of freedom, by employing the so-called method of ancilla removal. In the second step, I present a complete classification of nearest-neighbours FQCA over one-dimensional cubic lattices, consisting of a single local Fermionic mode per site. Among all possible cases, a novel class of automata called forking automata is discovered, which is unique to the Fermionic theory and has not ungraded counterparts. The talk is based on a joint work with A. Bisio, M. Lugli, P. Perinotti, A. Tosini, and L. Trezzini which is currently in preparation.

Metrology for real-world QKD

Alice Meda
INRiM

Quantum Key Distribution (QKD) is a technology that enables the sharing of secret cryptographic keys between two distant users (Alice and Bob), with intrinsic security guaranteed by the fundamental laws of nature. QKD has become a mature technology, and in Europe, all 27 member states are collaborating on a European Commission initiative (EuroQCI) to design, develop, and deploy a quantum communication infrastructure. In Italy, the QUID project is responsible for implementing the Italian segment of EuroQCI [1]. QKD relies on single photons to secure the distribution of the keys and, to become a viable real-world solution, the metrological characterization of optical components and systems is fundamental. To obtain the appropriate security requirements, test and evaluation methods at single-photon level need to be developed; in particular, since the single-photon detectors represent the most vulnerable part of a QKD system, their characterization in terms of operating parameters (quantum efficiency, dead time, jitter, afterpulsing..) is of the utmost importance. We present the INRIM efforts in the quantum efficiency calibration of single-photon avalanche detectors (SPADs), focusing on QKD application. The detection efficiency is evaluated for a fibre-coupled InGaAs/InP-SPAD [2, 3, 4] and for a free-space Si-SPAD [5]. The calibration is performed using different experimental setups and reference standards with proper traceability chains at the wavelength of 1550 nm and 850 nm respectively. The work is fundamental to align the Italian deployment of QKD, in the framework of QUID, with validation needs, providing test services for the characterization, validation and certification for QKD. [1] https://quid-euroqci-italy.eu/it/ [2] M. López, A. Meda, G. Porrovecchio, R. A. Starkwood (Kirkwood), M. Genovese, G. Brida, M. Šmid, C. J. Chunnilall, I. P. Degiovanni, and S. Kück, “A study to develop a robust method for measuring the detection efficiency of free-running InGaAs/InP single-photon detectors”, EPJ Quantum Technol. 7, 14, (2020). [3] H. Georgieva, A. Meda, H. Hofer, S. M. F. Raupach, M. Gramegna, I. P. Degiovanni, M. Genovese, M. López and S. Kück, “Detection of ultra-weak laser pulses by free-running single-photon detectors: Modeling dead time and dark count effects”, Appl. Phys. Lett. 118, 174002, (2021). [4] S. M. F. Raupach, I. P. Degiovanni, H. Georgieva, A. Meda, H. Hofer, M. Gramegna, M. Genovese, S. Kück, and M. López, “Detection rate dependence of the inherent detection efficiency in single-photon detectors based on avalanche diodes”, Phys. Rev. A 105, 042615 (2022) [5] https://arxiv.org/abs/2407.01120

Superconducting qubits as sensing platforms for Dark Matter search

Roberto Moretti
Università di Milano Bicocca

Quantum Sensing is a research field in rapid expansion and finds one of its applications in Fundamental Physics experiments such as the search for weakly EM-coupled Dark Matter (DM) candidates, namely Axion and Dark Photon. Recent developments in superconducting qubits and fabrication techniques are contributing significantly to driving progress in Quantum Sensing, thanks to their high sensitivity to AC fields and the possibility to leverage detection schemes based on Quantum Non-Demolition (QND) [1] and direct detection [2]. QND consists of establishing an entangled state between a qubit system and a photon trapped in a cavity, allowing us to infer the presence of the photon without absorbing it, thus enabling multiple measurements that exponentially suppress the dark count rate. Conversely, the direct detection scheme relies on resonant, low-power, Dark Matter-induced AC fields exerting a slow Rabi oscillation of the qubit state, which becomes measurable in high-coherence state-of-the-art transmons and fluxoniums. This contribution is part of the INFN Qub-IT collaboration, which aims to advance microwave single-photon detection through quantum superconducting devices. The presentation will illustrate the Qub-IT status towards achieving few-hundreds microseconds coherence time and engineering the DM detection setup. The work investigates the modelling and design optimization of planar single and coupled transmon qubit chips, leveraging the Lumped Oscillator Model (LOM) [3] and Energy Participation Ratio (EPR) [4] for extracting Hamiltonian parameters. Novel EPR-based strategies are developed to enhance the accuracy of Two-Level System (TLS) losses estimation through finite-element simulations. The possibility of boosting DM sensitivity through coupled multi-qubit systems is also discussed, along with the characterization of single-qubit chips fabricated at the National Institute of Standards and Technology (NIST) and a thorough comparison between simulated and measured qubit parameters, like resonant frequencies, anharmonicity and coupling strength to the readout structure. The preliminary results presented in this work hold promise for further enhancing the sensitivity and reliability of quantum sensing platforms, which could surpass the limitations of current light DM search experiments. This work is supported by INFN-CSN5 under the Qub-IT project, and from PNRR MUR projects PE0000023-NQSTI and CN00000013-ICSC. [1] Akash V. Dixit et al. “Searching for Dark Matter with a Superconducting Qubit”. In: Phys. Rev. Lett. (Apr. 2021). [2] Shion Chen et al. “Detecting Hidden Photon Dark Matter Using the Direct Excitation of Transmon Qubits”. In: Phys. Rev. Lett. (Nov. 2023). [3] Zlatko K. Minev et al. Circuit quantum electrodynamics (cQED) with modular quasi-lumped models. 2021. [4] Zlatko Minev et al. “Energy-participation quantization of Josephson circuits”. In: npj Quantum Information (Aug. 2021).

Practical discrete-variable and continuous-variable quantum communication protocols with coherent and squeezed states

Faezeh Mousavi
University of Trieste

Regarding the fundamental concern of security in classical cryptosystems, quantum cryptography (QC) plays a heartsome role in unconditional secure networks based on quantum mechanical laws. Quantum Key Distribution (QKD) and Quantum Secure Direct Communication (QSDC), are two advanced solutions for the approaches of QC, the former based on the key negotiation and the latter via direct transmission of secret messages without setting up a private key. These QC schemes have been developed over the optical fiber and free-space channels, both in discrete-variable (DV) and continuous-variable (CV) encodings. For DV implementations, the key bits are typically encoded on the polarization, phase, or arrival time of pulses, and single-photon detectors are employed; while in CV approaches, encoding is on the quadratures of quantized electromagnetic fields (e.g. coherent and squeezed states) via homodyne detection. We are interested in the entire QC development, where the experiments gradually move from proof-of-principle lab demonstrations to in-field implementations. On proof-of-principle steps, we worked on the demonstration of a CV-QSDC protocol over the optical fiber channel. The CV scheme is preferred due to its lower costs, excellent integrability with existing optical communication systems, easy implementation from state preparation to measurement, and fast experimental realization. It was also theoretically proved that using squeezed states shows higher tolerance in purely lossy channels and enhanced robustness against highly noisy ones. Thus, we utilized both coherent and squeezed states to investigate their performance in our protocol. Based on the applier of squeezing, we analytically and numerically studied the security of two different configurations as symmetric or asymmetric; and considered two scenarios regarding the locked and random phases of coherent states. According to the higher secrecy capacity and more practicability, we implemented the asymmetric protocol with the perpendicular direction of squeezing against phase-locked coherent states. Our measurements and numerical results against beam splitter attack agreed together and revealed the advantage of squeezed states over coherent ones for achieving higher secrecy. On in-field steps, we pursue the DV-QKD protocols along free-space channels between fixed (shore) and moving (ultimately the ship) parties, to achieve secure marine communications. Accordingly, we considered a three-state one-decoy weak-coherent BB84 protocol with time-bin encoding and a finite-key analysis method. We designed a practical configuration to initially perform the protocol between two parties over a lab-scale channel, then enlarge the terrestrial channel length via telescopes. In this configuration, we also investigate the effects of atmospheric turbulence on security, verify the tracking system for long channels between moving parties, and consider the phase noise on the protocol.

Toward a Scalable Diamond-based Spin-Qubit Register

Matthias Müller
Forschungszentrum Jülich

Diamond-based quantum technology is a fast-emerging field with both scientific and technological importance. The nuclear spins in the solid-state diamond crystalline structure allow for a room-temperature qubit register with a star topology around the central electron of the nitrogen-vacancy (NV) center. Entangling gates between such spin registers are the key challenge toward scalable architectures. Entanglement between dipolar-coupled NV spin pairs has been demonstrated, but with a limited entanglement fidelity and its error sources have not been characterized. Here, I present the recent experimental achievement of such a two-qubit gate between two NV centers and a theoretical analysis of the further potential, including with tools for benchmarking and quantum optimal control [2,3,4]. By tuning the relevant parameters and the application of decoupling sequences, we managed to design a robust and simple entangling gate between two NV centers. We quantified the influence of multiple error sources on the gate performance. Experimentally, this resulted in a gate fidelity of (96 pm 2.5)% at room temperature. Our identification of the dominant errors paves the way towards NV-NV gates beyond the error correction threshold. [1] T. Joas et al., arXiv:2406.04199 (2024) [2] P. Vetter et al. arXiv:2403.00616 (2024) [3] P. Rembold et al., AVS Quantum Sci. 2, 024701 (2020) [4] M. Rossignolo et al. Comp. Phys. Comm. 291, 108782 (2023)

Entanglement distillation and algorithmic cooling via indistinguishability effects

Farzam Nosrati
University of Palermo

Indistinguishability, a non-classical phenomenon associated with identical particles, plays a pivotal role in the formation of composite states of light and matter. This contributes to phenomena such as electron orbital occupation and photon bunching. Beyond these fundamental aspects, indistinguishability has emerged as a quantum resource for performing quantum information tasks, such as generating multipartite complex entangled quantum states. Also, it has been shown that particle statistics imprint and indistinguishability can be utilized in a controllable way to protect quantum correlations in a system of identical constituents. In this talk, we discuss an entanglement distillation protocol leveraging indistinguishability effects. We demonstrate the performance of our protocol by showcasing typical noisy states, such as thermal states and Werner states. This motivates us to design an indistinguishability-based algorithmic cooling process that mitigates thermal noise and cools a qubit down. Unlike earlier studies with non-identical subsystems, our mechanism is based on indistinguishable fermions which can be harnessed and exploited through both coherent and incoherent operations. We analyze our indistinguishability-based algorithmic cooling efficiency via the overall success probability and thermodynamic energetic cost. We show that our mechanism allows us to reach a very low qubit temperature (pure state) with experimentally-relevant nonzero success probabilities, starting from a relatively high qubit temperature (highly mixed state), by only consuming indistinguishability as a fuel. The proposed scheme paves the way towards quantum thermal machines made of controllable identical quantum particles that exploit indistinguishability effects as fuel to generate work.

W4Q - Women for Quantum

Elisabetta Paladino
Dipartimento di Fisica e Astronomia "Ettore Majorana", Università di Catania

TBA

Elsevier Sponsorhip

Elisabetta Paladino
Dipartimento di Fisica e Astronomia "Ettore Majorana", Università di Catania

Massimo Palma
Università di Palermo

Quantum magic in permutationally invariant systems

Gianluca Passarelli
Università di Napoli Federico II

Nonstabilizerness, also known as quantum magic, quantifies the deviation of a quantum state from the set of stabilizer states and has emerged in recent years as an information-theoretic quantity to measure the true power of quantum computation beyond entanglement. Simulating and manipulating volume-law entangled quantum states with extensive magic is widely believed to pose considerable challenges for classical algorithms. Thus, numerical experiments have predominantly focused on systems with few qubits. Conventional methods for assessing the nonstabilizerness of an $N$-qubit system necessitate the computation of $4^N$ expectation values of Pauli strings over a state with a dimension of $2^N$. Permutationally invariant systems are the perfect playground to overcome this limitation. For permutationally invariant systems, the exponential computational overhead of computing the quantum magic can be significantly reduced to $O(N^3)$ expectation values on a state of dimension $O(N)$. This reduction allows exploring not only nonstabilizerness phase transitions in the ground states of this class of models but also across the entire many-body spectrum, paving the way to studying excited-state quantum phase transitions and magic dynamics in nonequilibrium (closed and open) settings in systems comprising hundreds of qubits.

Multi-client distributed blind quantum computing over a linear quantum network architecture

Beatrice Polacchi
Sapienza University of Rome

The development of noisy intermediate-scale quantum processors and techniques to distribute entanglement among remote quantum nodes is paving the way for a quantum cloud, allowing users with minimal quantum resources to delegate computations to remote quantum servers. However, along with this exciting perspective, serious privacy issues arise, since users’ data may be exposed to malicious servers or other malicious clients. Therefore, universal blind quantum computing (BQC) was proposed [1] to allow a client, having a single-qubit source, to delegate quantum and classical computations to a remote quantum server while keeping hidden input, output and algorithm details. Despite demonstrations of single-client settings have been performed on different platforms, those of multi-client scenarios remain challenging. This is mainly due to the fact that most multi-client protocols need a large number of additional classical communication rounds. From an experimental point of view, this translates into the need to coherently store quantum states for time intervals incompatible with currently available quantum memory technologies. In the work reported in [2], we introduce a modular lightweight multi-client BQC protocol based on a linear quantum network architecture (Qline), with reduced quantum capabilities on the clients’ side, since they only need to perform single-qubit rotations. The main strength of our protocol lies in the use of a fully classical trusted third party (TTP) that secures classical communication between the clients and the server. Such a solution proves an efficient trade-off between time-latency on the server side and trust assumptions, while guaranteeing data privacy even when generalized to larger and more complex scenarios. We experimentally demonstrate such a protocol in a two-qubit two-client scenario within a versatile adaptive fiber-based photonic platform. In our platform, the TTP is embodied by an electronic circuit enabling active feed-forward of information, while the server is equipped with an electro-optical modulator (EOM) to enable measurement adaptivity, a key requirement to achieve determinism of the computation. Our results show that the computation is faithfully implemented and that the protocol is secure. Moreover, it is optimized from the hardware timing point of view, thus minimizing losses due to long-term storage of quantum states. Our platform is scalable and can be generalized to larger networks involving more than two clients and more complex resource states. For these reasons, we believe that our findings have insightful implications both from a theoretical and an experimental point of view. 1- A. Broadbent, J. Fitzsimons, and E. Kashefi. "Universal blind quantum computation." 2009 50th annual IEEE symposium on foundations of computer science. IEEE, 2009. 2- B. Polacchi et al. "Multi-client distributed blind quantum computation with the Qline architecture." Nature Communications 14.1 (2023): 7743.

Quantum-inspired tensor networks applied to computational-intensive financial problems.

Antonio Policicchio
NTT DATA

In recent years, the financial industry has seen a growing interest in leveraging advanced computational techniques to solve complex problems. Among these, Quantum Information Theory offers promising tools that can significantly enhance our computational capabilities. This talk will explore the application of quantum-inspired tensor networks in addressing financial problems. Tensor networks, originally developed in the context of quantum many-body physics, provide a powerful framework for representing and manipulating large-scale data sets efficiently. By adapting these techniques, we can tackle various financial challenges, such as portfolio optimization, risk management, and option pricing, with improved accuracy and computational efficiency. This presentation will illustrate tensor network practical implementation in financial contexts, and showcase empirical results demonstrating their potential in financial analytics.

Eigenstates Engineering to Mitigate Decoherence in Molecular Nanomagnets

Leonardo Ratini
University of Parma

Molecular nanomagnets are spin quantum systems, potentially serving as the qudits for future quantum computers. In this context, the central electronic spins define our qudit, while the surrounding nuclear spins constitute a bath that interacts with the central ones, leading to decoherence and degrading the information stored in the qudits. To suppress the effect of decoherence, we have investigated the role played by the structure of the eigenstates of the qudits in the decoherence dymanics, highlighting the crucial role played by the spin texture. We first demonstrate this result analytically, then corroborate it with numerical simulations performed on giant spins and spin systems with competing antiferromagnetic (AF) interactions, confirming our hypothesis. To derive the model Hamiltonian for these systems, we employed the Schrieffer-Wolff transformation. Moreover, we implemented the cluster correlation expansion technique to calculate the decoherence induced by baths containing up to 1000 spins. Finally, we propose a realistic molecule with competing AF interaction as a promising platform for long-lived qudits.

Compiling quantum circuits with quantum computers

Davide Rattacaso
University of Padova

Executing quantum algorithms on actual quantum computers requires a compilation process that generates a circuit implementation optimized for minimal infidelity or runtime specific to the target hardware. This circuit optimization problem is crucial for advancing quantum computing, but its NP-complete nature makes finding optimal solutions difficult. We propose a novel paradigm for compiling quantum circuits using quantum computers, analogous to how classical algorithms are compiled with classical computers. We map the compilation problem to the search for the ground state of a suitable Hamiltonian constrained to states representing circuits that implement the same quantum algorithm. This search can be performed via quantum algorithms such as Quantum Annealing, Optimal Control, and the Quantum Approximate Optimization Algorithm, potentially offering a quantum advantage in finding low-infidelity circuit implementations of the target algorithm. We validate our approach by introducing a Quantum Annealing-based quantum compiler and simulating its dynamics using tensor network techniques. Additionally, we implement Simulated Annealing to compile circuits involving up to 64 qubits, demonstrating that the benefits of our many-body approach to quantum compilation increase with circuit volume.

Rechargeable quantum batteries: Thermodynamic characterization, solid-state implementation, and quantum simulation

Luca Razzoli
Università degli Studi dell'Insubria

Compared to conventional electrochemical batteries, quantum batteries represent a new paradigm in quantum technologies for harvesting, storing, and releasing energy in small quantum systems. It is only recently that a thermodynamic approach has been adopted to characterize the maximum work that can be extracted by cyclic unitary processes (ergotropy) and the whole cycle of charging-storage-discharging of a quantum battery. Here, we propose an experimentally feasible scheme of rechargeable quantum battery, based on superconducting circuits, and investigate the associated thermodynamic cycle. The battery, consisting of two qubits whose interaction is externally driven, is weakly coupled with a thermal bath throughout the cycle. The following four-stroke cycle is then performed on an initial thermal state: Qubits are disconnected, ergotropy is extracted, qubits are reconnected and then thermalize, restoring the initial state. The ratio between ergotropy and the energy cost of disconnection/connection defines the efficiency of the cycle. Ergotropy is extracted from either or both the qubits with respect to the local Hamiltonians by local or global unitary processes. Unlike the latter, local unitary processes affect the coherences of the two-qubit state after ergotropy extraction, thus providing a knob to maximize the efficiency without affecting the ergotropy. In this regard, the proposed quantum battery can deliver finite ergotropy at finite efficiency and, remarkably, there exist parameter regions in which local unitary processes are more efficient than the global ones in extracting the same ergotropy. Finally, we have simulated the considered working cycle on IBM superconducting quantum computers. The critical issue of preparing the initial thermal state only by means of unitary operations and measurements has been overcome by using the thermofield-double-state technique within a variational approach. Despite the errors inherently present in real quantum devices, the very good agreement between the ideal and the simulated results corroborates the idea that the proposed quantum battery can be successfully tested in current solid-state experimental platforms.

Quantum simulation of the tricritical Ising model in tunable Josephson junction ladders

Matteo Rizzi
University of Cologne & Forschungszentrum Jülich GmbH (Germany)

Modern hybrid superconductor-semiconductor Josephson junction arrays are a promising platform for analog quantum simulations. Their controllable and non-sinusoidal energy/phase relation opens the path to implement nontrivial interactions and study the emergence of exotic quantum phase transitions. Here, we propose the analysis of an array of hybrid Josephson junctions defining a 2-leg ladder geometry for the quantum simulation of the tricritical Ising phase transition. This transition provides the paradigmatic example of minimal conformal models beyond Ising criticality and its excitations are intimately related with Fibonacci non-Abelian anyons and topological order in two dimensions. We study this superconducting system and its thermodynamic phases based on bosonization and matrix-product-states techniques. Its effective continuous description in terms of a three-frequency sine-Gordon quantum field theory suggests the presence of the targeted tricritical point and the numerical simulations confirm this picture. Our results indicate which experimental observables can be adopted in realistic devices to probe the physics and the phase transitions of the model. Additionally, our proposal provides a useful one-dimensional building block to design exotic topological order in two-dimensional scalable Josephson junction arrays. Ref: Maffi, Tausendpfund, Rizzi, Burrello, PRL 132, 226502 (2024)

Diagnosing non-Hermitian many-body localization and quantum chaos via singular value decomposition

Federico Roccati
Columbia University

Strong local disorder in interacting quantum spin chains can turn delocalized eigenmodes into localized eigenstates, giving rise to many-body localized phases. This is accompanied by distinct spectral statistics: chaotic for the delocalized phase and integrable for the localized phase. In isolated systems, localization and chaos are defined through a web of relations among eigenvalues, eigenvectors, and real-time dynamics. These may change as the system is made open. We show [1] that random dissipation (without random disorder) can induce chaotic or localized behavior in an otherwise integrable system. The dissipation is described using non-Hermitian Hamiltonians, which can effectively be obtained from Markovian dynamics conditioned on null measurement. In this non-Hermitian setting, we argue in favor of the use of the singular value decomposition. We complement the singular value statistics with different diagnostic tools, namely, the singular form factor and the inverse participation ratio and entanglement entropy of singular vectors. We thus identify a crossover of the singular values from chaotic to integrable spectral features and of the singular vectors from delocalization to localization. Our method is illustrated in an XXZ Hamiltonian with random local dissipation. [1] Federico Roccati, Federico Balducci, Ruth Shir, and Aurélia Chenu Phys. Rev. B 109, L140201 (2024)

Quantitative phase and amplitude imaging enhanced by quantum correlation in a non-interferometric scheme

Ivano Ruo-Berchera
INRiM

Recovering the relevant amplitude and phase information on a sample in a quantitative way is fundamental in many sensing and imaging applications. At the same time, it is important to keep the photon dose as low as possible to avoid alteration of the sample, especially in biological studies. Quantum imaging is a valuable pathway to extract more information per photon than classical scheme can do. Quantum entanglement and squeezing have demonstrated to significantly improved phase estimation and imaging in interferometric setups beyond the classical limits. However, only recently a genuine quantum advantage has been demonstrated in a non-interferometric phase imaging scheme [1], where the phase is retrieved only by measuring its effect on the intensity of the free-propagating field. This represents a turning point as non-interferometric phase imaging/retrieval methods, (e.g. diffraction imaging, wave-front sensing, ptychography) are vastly used in the classical domain, opening the way to new applications of quantum technologies. More in detail, in [1] we exploit a quantitative phase retrieval method based on the so-called “transport of intensity equation" (TIE), that provides the absolute value of the phase without prior knowledge of the object and operates in wide-field mode, so it does not need time-consuming raster scanning. The TIE works also with partial coherence light proving to be perfectly compatible with multimode quantum light sources used in sub-shot-noise imaging approaches [2,3]. In fact, the quantum enhancement derives from the reduction of the shot-noise in the measured intensity distribution obtained by exploiting spatially quantum correlated beams. Besides a general improvement of the phase image quality at a fixed number of photons irradiated through the object, resulting in better discrimination of small details, we demonstrate experimentally 40% reduction of the uncertainty in the quantitative phase estimation for each point of the image. And on top of that, here we show that both phase and amplitude full-field quantitative imaging can be attained in the same measurement scheme, and both with a significant practical quantum advantage. Thanks to its versatility, wide-field and real-time properties, this amplitude and phase imaging method represents an important step forward in quantum imaging of delicate samples. By state-of-the-art technology, it can find application in optical microscopy and in perspective to X-ray imaging, reducing the photon dose necessary to achieve a fixed signal-to-noise ratio both in phase and amplitude wide- field measurements. [1] Ortolano, G., Paniate, A., Boucher, P. et al. Quantum enhanced non-interferometric quantitative phase imaging. Light Sci Appl 12, 171 (2023). [2] I. Ruo-Berchera, A. Meda, E. Losero, et al. Improving resolution-sensitivity trade off in sub-shot noise quantum imaging. Applied Physics Letters 116, 214001 (2020)

Quantitative phase and amplitude imaging enhanced by quantum correlation in a non-interferometric scheme

Ivano Ruo-Berchera
INRiM

Recovering the relevant amplitude and phase information on a sample in a quantitative way is fundamental in many sensing and imaging applications. At the same time, it is important to keep the photon dose as low as possible to avoid alteration of the sample, especially in biological studies. Quantum imaging is a valuable pathway to extract more information per photon than classical scheme can do. Quantum entanglement and squeezing have demonstrated to significantly improved phase estimation and imaging in interferometric setups beyond the classical limits. However, only recently a genuine quantum advantage has been demonstrated in a non-interferometric phase imaging scheme [1], where the phase is retrieved only by measuring its effect on the intensity of the free-propagating field. This represents a turning point as non-interferometric phase imaging/retrieval methods, (e.g. diffraction imaging, wave-front sensing, ptychography) are vastly used in the classical domain, opening the way to new applications of quantum technologies. More in detail, in [1] we exploit a quantitative phase retrieval method based on the so-called “transport of intensity equation" (TIE), that provides the absolute value of the phase without prior knowledge of the object and operates in wide-field mode, so it does not need time-consuming raster scanning. The TIE works also with partial coherence light proving to be perfectly compatible with multimode quantum light sources used in sub-shot-noise imaging approaches [2]. In fact, the quantum enhancement derives from the reduction of the shot-noise in the measured intensity distribution obtained by exploiting spatially quantum correlated beams. Besides a general improvement of the phase image quality at a fixed number of photons irradiated through the object, resulting in better discrimination of small details, we demonstrate experimentally 40% reduction of the uncertainty in the quantitative phase estimation for each point of the image. And on top of that, here we show that both phase and amplitude full-field quantitative imaging can be attained in the same measurement scheme, and both with a significant practical quantum advantage. Thanks to its versatility, wide-field and real-time properties, this amplitude and phase imaging method represents an important step forward in quantum imaging of delicate samples. By state-of-the-art technology, it can find application in optical microscopy and in perspective to X-ray imaging, reducing the photon dose necessary to achieve a fixed signal-to-noise ratio both in phase and amplitude wide- field measurements. [1] Ortolano, G., Paniate, A., Boucher, P. et al. Quantum enhanced non-interferometric quantitative phase imaging. Light Sci Appl 12, 171 (2023). [2] I. Ruo-Berchera, A. Meda, E. Losero, et al. Improving resolution-sensitivity trade off in sub-shot noise quantum imaging. Applied Physics Letters 116, 214001 (2020)

Hybrid Stabilizer Tensor Network Algorithms

Alessandro Santini
SISSA

We introduce novel hybrid approaches that integrates tensor network methods with the stabilizer formalism to tackle the challenges of simulating many-body quantum systems. These methods improve the ability to accurately model unitary dynamics and compute expectation values of observables while mitigating the growth of entanglement typically encountered in classical simulations. We will discuss the fundamental concepts of tensor networks and the stabilizer formalism. Then, by leveraging the strengths of both techniques, we discuss Clifford dressed Matrix Product Operators for the dynamics of many-body quantum systems.

Realizing non-Hermitian dynamics via non-unitary photonic quantum walks

Paola Savarese
Università degli Studi di Napoli Federico II

In recent years, non-Hermitian photonics collected significant attention as a rising field in optics due to the emergence of numerous physical concepts and novel effects. Unlike systems described by a Hermitian Hamiltonian, where the Hermitian conjugate ensures system closure to the environment and energy conservation, a non-Hermitian system characterized by complex eigenvalues enables the description of open systems and facilitates understanding of how a system can interact with the environment. Here, we propose an innovative approach for simulating non-Hermitian dynamics by realizing a non-unitary photonic quantum walk based on a light beam propagating in free space and manipulated via step operators acting jointly on its polarization and transverse momentum. Within this framework, we use the latter degrees of freedom to encode the coin and walker systems, respectively, typically characterizing coined quantum walks. To induce spin-rotation, we utilize a uniform liquid-crystal (LC) plate and an LC dichroic polarization grating to obtain a spin-dependent non-unitary translation operation on the walker. Through the combination of liquid crystals and absorbing dyes, we can manipulate polarization, phase and light amplitude, effectively recreating an open system. This development yields a compact and versatile platform that significantly expands the scope of photonic simulations in studying quantum dynamics. It, also, introduces a new dimension for manipulating topological states, enabling the observation of phenomena such as those related to non-Hermitian topological phases.

Tensor Network applications to optimization problems

Ilaria Siloi
Universita' di Padova

Tensor network (TN) methods have been successfully applied in various fields, including quantum many-body systems and machine learning. In this talk, we explore TN applications for optimization problems. Firstly, we address mission planning for satellites in earth observation, optimizing satellite schedules based on user requests within a given timeframe. The original knapsack formulation including several constraints, is mapped into a QUBO formulation. We demonstrate the efficacy of TN methods in finding ground-state solutions that meet these constraints. Additionally, we tackle lattice-based cryptography for RSA prime factorization. We develop a TN approach to improve Schnorr's algorithm, encoding prime factors into optimal solutions of NP-hard instances of the closest vector problem (CVP). Our TN method efficiently extracts optimal solutions from these CVPs spectra and successfully factorizes RSA semiprimes up to over 100 bits, marking the largest RSA number factored using Schnorr’s sieving method to date.

Higher-order maps without causal order: Applications to quantum information, cryptography, and thermodynamics

Kyrylo Simonov
University of Vienna

The nature of causality remains one of the key puzzles in science. In quantum theory, the causal structure is not subject to quantum uncertainty and plays rather a background role. One can ask whether the background causal structure can be dropped, for example, by respecting causality only locally. Such scenarios of local validity of quantum theory while relaxing the global definite causal order of operations can be described via the machinery of higher-order operations, i.e. supermaps. An important example of scenarios of this kind is quantum SWITCH, a process realizing a quantum superposition of causal orders of operations. Looking for the possible applications of quantum SWITCH has been the subject of growing interest in the scientific community as it could provide communication and computational resources not realizable via standard quantum theory. Moreover, very recently, the benefits potentially offered by quantum SWITCH for thermodynamic and cryptographic tasks have appeared in the spotlight. My talk aims at highlighting the benefits and applications of higher-order maps to information processing, cryptography, and thermodynamics.

Quasi-probabilities in quantum mechanics: fundamental ideas and applications

Paolo Solinas
University of Genova

In addition to the usual projective measurements, quantum mechanics allows for alternative ways to extract information from a quantum system. Some of these lead to a quasi-probability distribution for the observable measured which are not positively defined. In analogy with the Wigner quasi-probability distribution, the presence of negative regions in the distribution can be used to spot pure quantum behaviors of the system or the dynamics. I will present a particular scheme, called quantum non-demolition, where quasi-probability distribution arises naturally. It exploits an additional quantum detector coupled to the system to be measured which allows us to gain important information about the wave-function of the system. I will discuss the connection with the violation of the Leggett-Garg inequalities, and how this approach identifies pure quantum effects and quantum-to-classical transition due to the interaction with an environment. I will discuss what are the advantages and disadvantages of this approach with some practical examples: the measure of the work done on a quantum system driven by an external field and the calculation of the derivative of a quantum operator.

Multiphoton Interference: from the physics to the ultimate quantum technological advantage

Vincenzo Tamma
University of Portsmoth

Quantum interference is one of the most intriguing phenomena in quantum physics at the very heart of the development of quantum technology in the current quantum industry era. It underpins fundamental tests of the quantum mechanical nature of our universe as well as applications in quantum computing, quantum sensing and quantum communication. I will focus on some of our recent results demonstrating the ultimate quantum sensitivity in the estimation of physical parameters by exploiting multiphoton interference with experimentally feasible single photon sources. I will first give an overview of multiphoton sensing techniques saturating the quantum Cramér-Rao bound based on sampling measurements which resolve the inner degrees of freedom, such as time, frequency, position, and polarization, of single photons interfering at a beam splitter [1,2]. This includes estimation of the transverse position of a given source for applications in super-resolved single-molecule localization microscopy, by circumventing the requirements in standard direct imaging of camera resolution at the diffraction limit, and of highly magnifying objectives [1]; imaging of nanostructures, including biological samples, and nanomaterial surfaces, with arbitrary values of thickness through estimation of photonic time delays [2]. Finally, we have shown how multiphoton interference is a powerful tool to achieve fluorescence lifetime measurements of 0.1 picosecond timescale with applications in contact-free nanorheology and the study of fast biological processes [3]. Finally, I will also describe how the metrological power of multiphoton quantum interference is intimately connected with an exponential speed-up in quantum optical networks, particularly in the development of scalable boson sampling experiments with “real world” nonidentical single photons [4,5]. This research opens a new paradigm based on the interface between the physics of quantum interference, quantum sensing and quantum exponential speed-up with experimentally feasible “real world” photonic sources. [1] D. Triggiani and V. Tamma, Phys. Rev. Lett. 132, 180802 (2024) [2] D. Triggiani, G. Psaroudis, V. Tamma, Phys. Rev. Applied 19, 044068 (2023) [3] A. Lyons et al., Nature Communications 14, 8005 (2023) [4] V. Tamma and S. Laibacher, Eur. Phys. J. Plus 138, 335 (2023), V. Tamma and S. Laibacher, Phys. Rev. A 104, 032204 (2021); S. Laibacher and V. Tamma, Phys. Rev. A 98, 053829 (2018); V. Tamma and S. Laibacher, Phys. Rev. Lett. 114, 243601 (2015); S. Laibacher and V. Tamma, Phys. Rev. Lett. 115, 243605 (2015) [5] X.-J. Wang et al., Phys. Rev. Lett. 121, 080501 (2018); V. V. Orre, et al. Phys. Rev. Lett. 123, 123603 (2019)

Reinforcement Learning for Quantum Circuit Design

Gloria Turati
Politecnico di Milano

Most research has focused on using Quantum Computing to accelerate tasks such as optimization, simulation, and more recently, machine learning and artificial intelligence (AI). However, there has been less focus on employing machine learning or AI to support quantum computers by tackling tasks that are challenging to perform and to mitigate its current limitations. We focus in particular on Variational Quantum Algorithms to tackle optimization problems. These algorithms rely on a parametric quantum circuit, known as an ansatz, optimized by a classical algorithm. However, how to design efficient ansatzes for specific problems that are resilient to noise and account for the hardware limitations of current generation devices remains a significant challenge. This talk will present our ongoing work on how to use Reinforcement Learning (RL) to design new ansatzes for variational quantum algorithms. The RL agent was trained on various optimization problems, including Maximum Cut, Maximum Clique, and Minimum Vertex Cover, using different graph topologies. Our study shows that the RL agent was able to discover useful quantum circuits, with approximation ratios that favorably compare to commonly used ansatzes . These results highlight the potential of RL techniques in designing efficient quantum circuits and their broad applicability in quantum computing, opening new directions in the generation of efficient ansatzes.

Strategies for quantum thermodynamics treatments in non-Markovian strong-coupling regimes

Bassano Vacchini
UNIMI & INFN

La strategia nazionale per le tecnologie quantistiche

Valeria Vinci
Dirigente, Ministero delle imprese e del made in italy, direzione generale per le nuove tecnologie abilitanti

Sensing and quantum state synthesis and processing in cavity optomechanics

David Vitali
University of Camerino

We show how optomechanical systems can be used to generate stationary arbitrary entangled Gaussian states of N mechanical modes for storage and manipulation of quantum information. This is achieved by tailoring the driven dissipative dynamics of the system, and specific examples are provided for continuous variable cluster states, which represents the universal resource for measurement-based continuous variable computation. N. Yazdi, S. Zippilli, D. Vitali, “Generation of stable Gaussian cluster states in optomechanical systems with multifrequency drives”, Quantum Sci. Technol. 9, 035001 (2024)

Study of Shortcuts to Adiabaticity for QAOA

Mara Vizzuso
Universià degli Studi di Napoli Federico II

Study of Shortcuts to Adiabaticity for QAOA Mara Vizzuso, Gianluca Passarelli, Giovanni Cantele, Procolo Lucignano Dipartimento di Fisica “E. Pancini”, Università degli Studi di Napoli “Federico II”, Complesso Universitario M.S, Angelo, via Cintia 21, 80126, Napoli, Italy CNR-SPIN, c/o Complesso Universitario M.S, Angelo, via Cintia 21, 80126, Napoli, Italy The Quantum Approximate Optimization Algorithm (QAOA) is a promising hybrid quantum-classical algorithm that can solve combinatorial optimization problems [1]. The quantum part of the algorithm involves using parametric unitary operations on a quantum computer to prepare a trial solution state. The parametric QAOA angles are variationally optimized minimizing a cost function using classical methods. We study two generalized QAOA ansatzes that include corrections to the Trotter expansion at the first and second order based on the Baker-Campbell-Hausdorff (BCH) expansion [2], denoted QAOA-CD and QAOA-2CD respectively [3]. In the regime in which QAOA is close to Quantum Annealing (QA) [4], these new unitaries correspond to the countediabatic potential of Shortcuts to Adiabaticity [5]. The latter assists the adiabatic evolution limiting transitions towards excited states. In our work, we show that counterdiabatic QAOA has improved performances with respect to QAOA for the paradigmatic MaxCut optimization problem. [1] E. Farhi, J. Goldstone, and S. Gutmann, arXiv:1411.4028 (2014) [2] X.-P. Li and J. Q. Broughton, The Journal of Chemical Physics, (May 1987), vol. 86, pp. 5094–5100 [3] Vizzuso, Mara, et al. "Convergence of digitized-counterdiabatic QAOA: circuit depth versus free parameters." New Journal of Physics 26.1 (2024): 013002. [4] Morita, Satoshi, and Hidetoshi Nishimori. "Mathematical foundation of quantum annealing." Journal of Mathematical Physics 49.12 (2008). [5] Guéry-Odelin, David, et al. "Shortcuts to adiabaticity: Concepts, methods, and applications." Reviews of Modern Physics 91.4 (2019): 045001.

Reservoir computing with complex quantum systems

Roberta Zambrini

Non-conventional computing inspired by the brain, or neuromorphic computing, is a successful approach in a broad spectrum of applications, and in the last few years proposals of Quantum Reservoir Computing have been explored. Quantum physical reservoirs have the potential to boost the processing performance in temporal tasks by exploiting quantum coherence, not requiring error correction. Furthermore this approach is naturally suited for fully quantum information processing (with quantum inputs). In this talk we will briefly review the state of the art and focus on recent results exploring the potential of different platforms and operation regimes, the role of quantum coherence and entanglement, and how to overcome the challenges of real-time quantum reservoir computing.

Entanglement-based efficient protocol for state discrimination among n alternatives

Gennaro Zanfardino
Università dagli Studi di Salerno

Current schemes for the discrimination of single-qubit states rely upon state measurements in order either to achieve unambiguous state identification or to minimize the error probability. Here we describe an alternative method based on the detection of the entanglement that is established between the qubit and an appropriate ancilla after suitable global unitary operations on the entire system. Our work is motivated by recent developments on entanglement detection with only one or few copies [1,2]. We exploit such feature to introduce a novel and more efficient state discrimination protocol. In our scheme one does not need to know the exact amount of entanglement but only to detect its presence. If one can detect entanglement with only a single copy of the state, then in order to discriminate the state of interest within a pool of N alternatives our protocol requires at most N-1 copies [3]. Our results have some far-reaching consequences in quantum communication and quantum cryptography. For instance, provided that entanglement detection can be realized in a single-copy experiment, an immediate implication is that some QKD protocols turn out to be less secure than previously assumed. Simulating the dynamics of classical spin-glass systems requires a large amount of computational resources especially for the sizes that are needed in order to provide safe estimates on the behavior in the thermodynamic limit. Here we introduce a general scheme for the quantum simulation of the dynamics of a p-spin glass interacting on an arbitrary lattice [4]. Specifically, with a composite n-qubits system, it is possible to simulate the dynamics of 2n interacting spins. Inspired by previous work with classical coherent light [5], we specialize our proposal to a concrete all-optical experimental set-up for simulating a fully connected p-spin-glass. With a state of n photons uniformly distributed over m modes, it is possible to simulate a classical disordered system where the total number of spins is equal to m and the number p of spins in the local interaction terms is 2n. For this case, we provide arguments indicating that since the computational time scales as the total number of spins m, compared to the scaling m3 that can be obtained in state-of-the-art classical Monte Carlo simulation schemes. [1] S. J. van Enk, C. W. and Beenakker, Phys. Rev. Lett. 108 110503 (2012). [2] Y. Zhou, P. Zeng, and Z. Liu, Phys. Rev. Lett 125 200502 (2020). [3] G. Zanfardino and F. Illuminati, Preprint ArXiv:2408.yyyyy (2024). [4] F. Illuminati, M. Leonetti, L. Leuzzi, S. Paesani, G. Ruocco, R. Santagati and G. Zanfardino, Preprint ArXiv:2408.yyyyy (2024). [5] M. Leonetti, E. Hörmann, L. Leuzzi, G. Parisi, and G. Ruocco, Proceedings of the National Academy of Sciences, 118 2015207118 (2021).

False vacuum decay in a ferromagnetic superfluid

Alessandro Zenesini
INO-CNR

In quantum field theory, the decay of an extended metastable state into the real ground state is known as ``false vacuum decay'' and it takes place via the nucleation of spatially localized bubbles. Despite the large theoretical effort to estimate the nucleation rate and intriguing speculations over the fate of our universe, experimental observations were still missing. In our experiment, we observe bubble nucleation in isolated and highly controllable superfluid atomic systems, and we find good agreement between our results, numerical simulations and instanton theory opening the way to the emulation of out-of-equilibrium field phenomena in atomic systems.

Interferometric imaging of amplitude and phase of spatial biphoton states

Danilo Zia
Sapienza - Università di Roma

Characterizing states of high dimensionality is a pivotal task in quantum information due to the wide range of their applications and the benefits brought in terms of security and amount of information transmitted. This task normally requires a number of projective measurements that scale exponentially with the system’s dimensionality. In this work, we address the problem of reconstructing high-dimensional quantum states in spatially correlated photon pairs emitted via a nonlinear crystal by means of a coincidence imaging approach. We implemented an innovative protocol to overcome the limitations of the full spatial mode characterization of photon pairs, that has hitherto suffered from the inefficiency and losses of detection schemes and the extreme sensitivity to the alignment of the measurement setup. In particular, we show how, for spatially correlated two-photon states, these difficulties can be surpassed by a bi-photon interferometric approach, where the unknown quantum state is superimposed with a reference one. Coincidence imaging of the resulting superposition collected with a single photon sensitive time-stamping camera allows us to retrieve the full biphoton wave function. We perform this analysis for different cases of two-photon states generated by a structured pump beam that induces spontaneous parametric down-conversion in the nonlinear crystal. From the resulting retrieved state, we show how to investigate different features of it, like the decomposition in arbitrary sets of spatial modes, orbital angular momentum correlations, certification of high-dimensional Bell states, parity conservation, and radial mode correlations. Moreover, we provide an example of the use of our approach for phase-encoded image reconstruction, which could have beneficial applications in quantum imaging protocols. The results show remarkable advantages with respect to recently developed techniques, based on projective measurements, in terms of fidelity, noise reduction, measurement time (reduced by three orders of magnitude), and accessibility to subspaces of different dimensionality. Our findings introduce a new approach to benchmark complex quantum states and open a new route toward unexplored quantum imaging and metrology techniques.