Dr Luca Razzoli

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

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.