TY  - JOUR
A1  - Anders, Janet
A1  - Sait, Connor R. J.
A1  - Horsley, Simon A. R.
T1  - Quantum Brownian motion for magnets
JF  - New journal of physics : the open-access journal for physics
N2  - Spin precession in magnetic materials is commonly modelled with the classical phenomenological Landau-Lifshitz-Gilbert (LLG) equation. Based on a quantized three-dimensional spin + environment Hamiltonian, we here derive a spin operator equation of motion that describes precession and includes a general form of damping that consistently accounts for memory, coloured noise and quantum statistics. The LLG equation is recovered as its classical, Ohmic approximation. We further introduce resonant Lorentzian system-reservoir couplings that allow a systematic comparison of dynamics between Ohmic and non-Ohmic regimes. Finally, we simulate the full non-Markovian dynamics of a spin in the semi-classical limit. At low temperatures, our numerical results demonstrate a characteristic reduction and flattening of the steady state spin alignment with an external field, caused by the quantum statistics of the environment. The results provide a powerful framework to explore general three-dimensional dissipation in quantum thermodynamics.
KW  - open quantum systems
KW  - coloured and quantum noise
KW  - memory effects
KW  - spin
KW  - dynamics
KW  - LLG equation
KW  - magnetisation
KW  - quantum thermodynamics
Y1  - 2022
U6  - https://doi.org/10.1088/1367-2630/ac4ef2
SN  - 1367-2630
VL  - 24
IS  - 3
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Barra, Felipe
A1  - Hovhannisyan, Karen
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  - Henkel, Carsten
T1  - Heat transfer and entanglement
BT  - non-equilibrium correlation spectra of two quantum oscillators
JF  - Annalen der Physik
N2  - The non-equilibrium state of two oscillators with a mutual interaction and coupled to separate heat baths is discussed. Bosonic baths are considered, and an exact spectral representation for the elements of the covariance matrix is provided analytically. A wide class of spectral densities for the relevant bath modes is allowed for. The validity of the fluctuation-dissipation relation is established for global equilibrium (both baths at the same temperature) in the stationary state. Spectral measures of entanglement are suggested by comparing to the equilibrium spectrum of zero-point fluctuations. No rotating-wave approximation is applied, and anomalous heat transport from cold to hot bath, as reported in earlier work, is demonstrated not to occur.
KW  - entanglement
KW  - heat transfer
KW  - non-equilibrium steady state
KW  - master
KW  - equation
KW  - quantum thermodynamics
Y1  - 2021
U6  - https://doi.org/10.1002/andp.202100089
SN  - 0003-3804
SN  - 1521-3889
VL  - 533
IS  - 10
PB  - Wiley-VCH
CY  - Weinheim
ER  -