PhD Claudio Bonizzoni

Quantum Sensing of Magnetic Fields with Molecular Spins

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).