TY  - JOUR
A1  - Barra, Felipe
A1  - Hovhannisyan, Karen V.
A1  - Imparato, Alberto
T1  - Quantum batteries at the verge of a phase transition
JF  - New journal of physics : the open-access journal for physics
N2  - 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.
KW  - quantum batteries
KW  - quantum thermodynamics
KW  - quantum phase transition
Y1  - 2022
U6  - https://doi.org/10.1088/1367-2630/ac43ed
SN  - 1367-2630
VL  - 24
IS  - 1
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Hovhannisyan, Karen V.
A1  - Nemati, Somayyeh
A1  - Henkel, Carsten
A1  - Anders, Janet
T1  - Long-time equilibration can determine transient thermality
JF  - PRX Quantum
N2  - When two initially thermal many-body systems start to interact strongly, their transient states quickly become non-Gibbsian, even if the systems eventually equilibrate. To see beyond this apparent lack of structure during the transient regime, we use a refined notion of thermality, which we call g-local. A system is g-locally thermal if the states of all its small subsystems are marginals of global thermal states. We numerically demonstrate for two harmonic lattices that whenever the total system equilibrates in the long run, each lattice remains g-locally thermal at all times, including the transient regime. This is true even when the lattices have long-range interactions within them. In all cases, we find that the equilibrium is described by the generalized Gibbs ensemble, with three-dimensional lattices requiring special treatment due to their extended set of conserved charges. We compare our findings with the well-known two-temperature model. While its standard form is not valid beyond weak coupling, we show that at strong coupling it can be partially salvaged by adopting the concept of a g-local temperature.
Y1  - 2023
U6  - https://doi.org/10.1103/PRXQuantum.4.030321
SN  - 2691-3399
VL  - 4
IS  - 3
PB  - American Physical Society
CY  - College Park
ER  -