A method is proposed for obtaining the spectrum for noise that causes the phase decoherence of a qubit directly from experimentally available data. The method is based on a simple relationship between the spectrum and the coherence time of the qubit in the presence of a π pulse sequence. The relationship is found to hold for every system of a qubit interacting with the classical-noise, bosonic, and spin baths.
For an open quantum system, we investigate the pumped current induced by a slow modulation of control parameters on the basis of the quantum master equation and full counting statistics. We find that the average and the cumulant generating function of the pumped quantity are characterized by the geometrical Berry-phase-like quantities in the parameter space, which is associated with the generator of the master equation. From our formulation, we can discuss the geometrical pumping under the control of the chemical potentials and temperatures of reservoirs. We demonstrate the formulation by spinless electrons in coupled quantum dots. We show that the geometrical pumping is prohibited for the case of non-interacting electrons if we modulate only temperatures and chemical potentials of reservoirs, while the geometrical pumping occurs in the presence of an interaction between electrons.
We derive general properties, which hold for both quantum and classical systems, of response functions of nonequilibrium steady states. We clarify differences from those of equilibrium states. In particular, sum rules and asymptotic behaviors are derived, and their implications are discussed. Since almost no assumptions are made, our results are applicable to diverse physical systems. We also demonstrate our results by a molecular dynamics simulation of a many-body interacting system.
For open systems described by the quantum Markovian master equation, we study
a possible extension of the Clausius equality to quasistatic operations between
nonequilibrium steady states (NESSs). We investigate the excess heat divided by
temperature (i.e., excess entropy production) which is transferred into the
system during the operations. We derive a geometrical expression for the excess
entropy production, which is analogous to the Berry phase in unitary evolution.
Our result implies that in general one cannot define a scalar potential whose
difference coincides with the excess entropy production in a thermodynamic
process, and that a vector potential plays a crucial role in the thermodynamics
for NESSs. In the weakly nonequilibrium regime, we show that the geometrical
expression reduces to the extended Clausius equality derived by Saito and
Tasaki (J. Stat. Phys. {\bf 145}, 1275 (2011)). As an example, we investigate a
spinless electron system in quantum dots. We find that one can define a scalar
potential when the parameters of only one of the reservoirs are modified in a
non-interacting system, but this is no longer the case for an interacting
system.Comment: 28 pages, 3 figures. 'Remark on the fluctuation theorem' has been
revised in ver. 2. Brief Summary has been added in Sec. 1 in ver.
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