Abstract
Starting from the observation that the reduced state of a system strongly coupled to a bath is, in general, an athermal state, we introduce and study a cyclic battery–charger quantum device that is in thermal equilibrium, or in a ground state, during the charge storing stage. The cycle has four stages: the equilibrium storage stage is interrupted by disconnecting the battery from the charger, then work is extracted from the battery, and then the battery is reconnected with the charger; finally, the system is brought back to equilibrium. At no point during the cycle are the battery–charger correlations artificially erased. We study the case where the battery and charger together comprise a spin-1/2 Ising chain, and show that the main characteristics—the extracted energy and the thermodynamic efficiency—can be enhanced by operating the cycle close to the quantum phase transition point. When the battery is just a single spin, we find that the output work and efficiency show a scaling behavior at criticality and derive the corresponding critical exponents. Due to always present correlations between the battery and the charger, operations that are equivalent from the perspective of the battery can entail different energetic costs for switching the battery–charger coupling. This happens only when the coupling term does not commute with the battery’s bare Hamiltonian, and we use this purely quantum leverage to further optimize the performance of the device.
Highlights
Quantum batteries store and deliver energy to a quantum system coherently
Starting from the observation that the reduced state of a system strongly coupled to a bath is, in general, an athermal state, we introduce and study a cyclic battery–charger quantum device that is in thermal equilibrium, or in a ground state, during the charge storing stage
The cycle has four stages: the equilibrium storage stage is interrupted by disconnecting the battery from the charger, work is extracted from the battery, and the battery is reconnected with the charger; the system is brought back to equilibrium
Summary
Quantum batteries store and deliver energy to a quantum system coherently. For such a device, energy leaking during the storing phase is a key issue [1,2,3,4,5,6,7,8,9,10] that is absent if kept at thermodynamic equilibrium [1, 3, 9]. We consider the thermodynamic cycle depicted, where a subset of spins (the battery) is disconnected from the chain initially in the ground or in a thermal state; energy is extracted from it; and the exhausted battery is reconnected to the rest of the chain playing the role of the charger. We observe that a set of phases of the unitary operator that extracts the battery’s ergotropy, which are irrelevant for the (reduced) state of the battery, play a significant role in the reconnecting energy when the battery– charger correlations are taken into account This provides us with an additional set of parameters that, as we will see in the following, can be tuned so as to further increase the cycle efficiency.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
