門内 隆明

It is a fundamental problem to characterize nonequilibrium processes. For a moving one-dimensional potential, we explore the nonequilibrium dynamics of the initial energy eigenstates for a confined quantum system interacting with a large reservoir. For concreteness, we investigate a dragged harmonic oscillator linearly interacting with an assembly of harmonic oscillators, and explore the deviation from adiabatic processes by rigorously calculating the so-called persistent amplitude. In particular, we show that the phase of the persistent amplitude is considered to be common both for the ground and excited states. Also, we can define the quasi adiabatic processes in a welldefined double limit of small perturbation and a sufficiently long time in terms of the phase and absolute value of the persistent amplitude.

It is known that each single typical pure state in an energy shell of a large isolated quantum system well represents a thermal equilibrium state of the system. We show that such typicality holds also for nonequilibrium steady states (NESS's). We consider a small quantum system coupled to multiple infinite reservoirs. In the long run, the total system reaches a unique NESS. We identify a large Hilbert space from which pure states of the system are to be sampled randomly and show that the typical pure states well describe the NESS. We also point out that the irreversible relaxation to the unique NESS is important to the typicality of the pure NESS's.

For isolated quantum many-body systems, we extend the availability of the intrinsic thermal nature of typical pure states to a class of nonequilibrium processes which start from an initial equilibrium. For concreteness, we calculate the spectral distribution of the work done on the system on the basis of a single pure state. It means that we can accurately calculate the entire fluctuation of the energy only from a single pure state instead of the thermodynamic ensembles.

We address the issue of the characterization of the nonequilibrium states for the time-dependent processes of a system interacting with a large reservoir, which is initially prepared in an equilibrium state. During the time evolution, we apply an external force to the system so that the actual density matrix is quantitatively different from the canonical state specified by the time-dependent system Hamiltonian. To express how the external forcing causes the deviation from equilibrium, we give a class of thermodynamic expressions for the lower bounds of the distance between the actual nonequilibrium state and the corresponding canonical state. The lower bounds are thermodynamically expressed only in terms of the strength of the forcing and its consequent entropy production rate.

We show that it is possible to calculate equilibrium expectation values of many-body correlated quantities such as the characteristic functions and probability distributions with the use of only a single typical pure state. It also means that we can apply the pure state approach to Heisenberg operators and their spectral fluctuation, and hence to nonequilibrium processes starting from equilibrium. In particular, we can accurately analyze the full statistics of entropy production in nonequilibrium mesoscopic systems. In this way, we can access the full information on higher-order fluctuations in the large deviation regime far from equilibrium.

The transition probability of an isolated system for a time-dependent unitary evolution is invariant under the reversal of protocols. In this paper, we generalize the expression of microscopic reversibility to externally perturbed large quantum open systems, which provides a model-independent equality between time forward and reversed joint transition probabilities. A time-dependent external perturbation acts on the subsystem during a transient duration, and subsequently the perturbation is switched off so that the total system would thermalize. We concern ourselves with the net transition probability for the subsystem from the initial to final states after a time evolution during which the energy is irreversibly exchanged between the subsystem and reservoir. The time-reversed probability is given by the reversal of the forcing protocol and the initial ensemble. Microscopic reversibility equates the time forward and reversed probabilities, and therefore appears as a thermodynamic symmetry for open quantum systems. © 2012 IOP Publishing Ltd.

A quantum mechanical explanation of the relaxation to equilibrium is shown for macroscopic systems for nonintegrable cases and numerically verified. The macroscopic system is initially in an equilibrium state, subsequently externally perturbed during a finite time, and then isolated for a sufficiently long time. We show a quantitative explanation that the initial microcanonical state typically reaches a state whose expectation values are well approximated by the average over another microcanonical ensemble.

We show that systems driven by an external force and described δiS = KbD[f(t, γ(t)) 7∥ fs (t, γs (t)) fr eq (ζ(t)] ≥ 0 by Nose-Hoover dynamics allow for a consistent nonequilibrium thermodynamics description when the thermostatted variable is initially assumed in a state of canonical equilibrium. By treating the "real" variables as the system and the thermostatted variable as the reservoir, we establish the first and second law of thermodynamics. As for Hamiltonian systems, the entropy production can be expressed as a relative entropy measuring the system-reservoir correlations established during the dynamics. © 2010 American Chemical Society.

We consider a quantum gas of noninteracting particles confined in the expanding cavity and investigate the nature of the nonadiabatic force which is generated from the gas and acts on the cavity wall. First, with use of the time-dependent canonical transformation, which transforms the expanding cavity to the nonexpanding one, we can define the force operator. Second, applying the perturbative theory, which works when the cavity wall begins to move at time origin, we find that the nonadiabatic force is quadratic in the wall velocity and thereby does not break the time-reversal symmetry, in contrast with general belief. Finally, using an assembly of the transitionless quantum states, we obtain the nonadiabatic force exactly. The exact result justifies the validity of both the definition of the force operator and the issue of the perturbative theory. The mysterious mechanism of nonadiabatic transition with the use of transitionless quantum states is also explained. The study is done for both cases of the hard-and soft-wall confinement with the time-dependent confining length.

We show a microscopic derivation of a quantum master equation with counting terms which describes the electron statistics. A localized spin behaves as a probe whose precession angle monitors the net electron current by the magnetic moment interaction. The probe Hamiltonian is proportional to the current and is determined self-consistently for a model of a quantum dot. Then it turns out that the quantum master equation for the spin precession contains the counting terms. As an application, we show the fluctuation theorem for the electron current.

We analytically explore the fluctuation of the work-induced entropy production of an externally perturbed quantum harmonic oscillator interacting with several reservoirs. The quantum fluctuation of the work amounts to a non-Gaussian fluctuation of the entropy production, which interpolates the Gaussian and the Poissonian distributions at high- and low-temperature regimes. Also, it is shown that the corresponding fluctuation theorem symmetry is rigorously satisfied. © 2010 The American Physical Society.

We derive analytical formulas for the firing rate of integrate-and-fire neurons endowed with realistic synaptic dynamics. In particular, we include the possibility of multiple synaptic inputs as well as the effect of an absolute refractory period into the description. The latter affects the firing rate through its interaction with the synaptic dynamics. © 2009 The American Physical Society.