Quantum batteries represent one of the most promising applications of quantum thermodynamics, whose goal is not only to store energy inside small quantum systems but also to potentially leverage genuine quantum effects to outperform classical counterparts. In this context, however, energy fluctuations become extremely relevant and have a significant impact on the charging efficiency. In our work, we consider a simple yet paradigmatic model in which a flying qubit (the battery) coherently interacts with a single-mode optical cavity (the charger) through a number conserving Jaynes-Cummings interaction. By making use of full-counting statistics techniques, we fully characterize the average charging power, its fluctuations, and the associated charging efficiency for several different choices of initial states of the optical cavity, demonstrating that preparing the latter in a genuinely quantum non-Gaussian Fock state (rather than a classical or even nonclassical Gaussian state) leads to a definite and in principle measurable advantage in all these figures of merit.

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• Quantum thermodynamics