Three-level Dicke quantum battery

A quantum battery (QB) is an energy storage and extraction device that is governed by the principles of quantum mechanics. Here we propose a three-level Dicke QB and investigate its charging process by considering three quantum optical states: a Fock state, a coherent state, and a squeezed state. The performance of the QB in a coherent state is significantly superior to that in a Fock or squeezed state. We show that the locked energy is positively related to the entanglement between the charger and the battery, and diminishing the entanglement increases the ergotropy. The role of quantum phase transitions in the charging process of the QB system is analyzed and we find that the behavior of maximum stored energy is jointly determined by ground and excited-state quantum phase transitions. We further demonstrate that, no matter what the initial state is, the stored energy can be completely extracted when $N$ is small ($N=10$), far below the thermodynamic limit, and the charging power follows a behavior consistent with that of the stored energy.