Switchable coherent quantum batteries with large ergotropy

The quantum battery (QB) is a conceptually new energy storage and conversion device, which consists usually of a quantum charger and an energy store (usually called the QB for simplicity). The demonstrated advantage of a QB, over its classical counterpart, is that its charging efficiency can be significantly enhanced by using quantum entanglement resources. In this paper, we investigate alternatively how to realize the switchable charging and the lossless power detection of the charged QB. With the proposed QB configuration we show that, by adjusting the eigenfrequency of the qubit-based charger, the cavity-based QB can be switched on between the charging and power-off states and its stored energy can be nondestructively monitored by probing the transmitted spectrum of the external electromagnetic waves scattered by the qubit-based charger. As the qubit-based charger has never been excited, the proposed cavity-based QB could directly store the coherent-state energy (rather than the single-photon one) and thus possess significantly large ergotropy. The physical realization of the proposed switchable QB configuration is demonstrated specifically with the experimental circuit quantum electrodynamical devices.