# Off-resonant Dicke Quantum Battery: Charging by Virtual Photons

## TL;DR

This paper explores an off-resonant Dicke quantum battery where virtual photons enable charging through matter-radiation coupling modulation, revealing new charging mechanisms and performance scaling in the dispersive regime.

## Contribution

It introduces the concept of charging a quantum battery via virtual photons in the off-resonant regime, expanding understanding of quantum energy storage mechanisms.

## Key findings

- Charging occurs even with an empty cavity due to virtual photon interactions.
- Performance scales favorably with the number of two-level systems in strong coupling.
- Efficient energy extraction is possible across various parameter ranges.

## Abstract

We investigate a Dicke quantum battery in the dispersive regime, where the photons trapped into a resonant cavity are way more energetic with respect to the two-level systems embedded into it. Under such off-resonant conditions, even an empty cavity can lead to the charging of the quantum battery through a proper modulation of the matter-radiation coupling. This counterintuitive behaviour has its roots in the effective interaction between two-level systems mediated by virtual photons emerging from the fluctuations of the quantum electromagnetic field. In order to properly characterize it, we address relevant figures of merit such as the stored energy, the time required to reach the maximum charging, and the averaged charging power. Moreover, the possibility of efficiently extracting energy in various ranges of parameters is discussed. The scaling of stored energy and power as a function of the number $N$ of two-level systems and for different values of the matter-radiation coupling is also discussed showing, in the strong coupling regime, performances in line with what reported for the Dicke quantum battery in the resonant regime.