Coherence-Powered Charge and Discharge of a Quantum Battery

A key question in quantum thermodynamics is how coherence impacts energy exchanges between quantum systems. Here we study theoretically and experimentally the role of coherence in work transfer processes of charging and discharging a quantum battery. During charging, work is transferred from a qubit into a reservoir of modes of the electromagnetic field, acting as a quantum battery. We show that the amount of work is proportional to the initial quantum coherence of the qubit, and is reduced by temperature. Using homodyne-type measurements, we discharge the quantum battery into a coherent field, acting as a work receiver. The amount of work transferred to the receiver is controlled by the relative classical optical phase between the two fields, the quantum purity of the charged battery field, and long-term fluctuations in the qubit environment. Complete discharge is reached in the low-energy limit where the battery field mostly overlaps with a coherent field.