arXiv subjects

Kentaro Urasaki

Publications and source records attributed to Kentaro Urasaki.

9 recordsLinked to original sources

arXiv2016

Pointer States and Decoherence in Quantum Unitarity: Energy Conservation for Weak Interaction Model

The purpose of the present paper is to derive the pointer states of a macro-object using a simple perturbation method. We study the model Hamiltonian involving the weak interaction between the center of mass and its environment. The main conclusion is that the pointer states emerge as the small part of the total state vector when the interaction Hamiltonian is not dominant: the degrees of freedom being at the center of interaction can be localized. In the weak interaction case, if the disappearance of the energy contribution from the off-diagonal terms is caused by the decoherence, it threatens the conservation law. In this study, however, the energy conservation reasonably holds, since the non-classical (i.e., normal quantum) states give main contribution to it. The emergence of the pointer states also implies the special initial state in which the localized state makes the deep well of the potential. In the result, the unitary time evolution of a pointer state is very different from that of the majority of the original state vector of the total system.

quant-ph↗

arXiv2016

Classicality in Quantum Mechanics: model for pointer states and decoherence

We have studied the emergence of classical states in the perturbative interaction model. The states which interact with many other degrees of freedom, such as the center of mass of a macro-object, play important role. Although the random phase mechanism is effective as same as Zurek's strong correlation model, there are enormous states, each of which independently developes due to the orthogonality of the environmental states. In these privileged states, the subsystem picture with the separability is stable.

quant-ph↗

arXiv2014

A General Derivation of Pointer States: Decoherence and Classicality

The purpose of the present study is to derive the pointer states of a macroscopic system interacting with its environment, under the general assumptions, i.e., without assuming any form of the interaction Hamiltonian. The lowest order perturbation leads that the interaction energy shifts the phase factors of the state vectors. For a macroscopic system, these factors are the macroscopic quantities even for the very weak interaction. When we group the state vector of the total system by the view point of environmental side, the destructive interference occurs and the stationary phase approximation can be adopted. Only the pointer states then survive and the decoherence also occurs. The present approach is within the standard quantum mechanics as same as the standard decoherence theory, but the meaning of the classical state is much clear.

quant-ph↗

arXiv2011

Mixture of Quantum States: Thermal and Interaction Inducing Decoherence

In this study, we show that the interaction energy plays an important role on the quantum decoherence: If we pay attention to the oscillation phase factor, $e^{-iE_{int}t/\hbar},$ we see that the time average of the macro-system's density matrix becomes nearly diagonal, where the states giving extrema of interaction energy are privileged to describe the quantum decoherence. This approach is compatible with the von Neumann's old work, which has been recently studied with renewed interest: The thermal mixture of states can be reached by the time average of a density of matrix due to the oscillation phase factor, $e^{-i(E_i-E_j)t/\hbar}.$ One of the direct results is the localization of macroscopic objects.

quant-ph↗

arXiv2009

Quantum Decoherence and Pointer Basis: Dynamics in State Vectors

It is well-known that the pointer basis of a quantum system satisfies the condition to diagonalize the interaction Hamiltonian between the subsystems. We show that this condition can be translated into the form $δΛ=0,$ where $Λ$, so-called the action, is the time integrated interaction energy: it is found out naturally in the phase of state vectors due to diagonal interaction. The careful treatment of a two states system demonstrates that the states of the total system branch into the states with different values of the action. Mathematically the pointer states are selected out by the saddle point condition on the phase $Λ$. This study helps us to understand the precise mechanism and the general dynamics of decoherence.

quant-ph↗

arXiv2007

Macroscopic Reality in Quantum Mechanics; Origin and Dissipation

We study the connection between dissipation and reality in macroscopic quantum systems. We present the following scenario; if we consider the dynamics of a `partial' wave function, the dissipation is represented as a nonlocal term and it causes destructive interference to suppress the quantum fluctuation. Using the variational method, we confirm that this dissipation term is a reasonable extension of the standard (Schrödinger) description for isolated systems, from which we also derive the classical action. Consequently, in macroscopic systems, the states whose time-integrated dissipation takes an extreme value come true. This description, which is consistent with our sense of reality, coexists with the usual linear-time-dependent description.

quant-ph↗

arXiv2000

Correlation Effects in Multi-Band Hubbard Model and Anomalous Properties of FeSi

The two-band Hubbard model with the density of states obtained from the band calculation is applied for FeSi, which is suggested to be a Kondo insulator or a correlated band insulator. Using this model, the correlation effects on FeSi are investigated in terms of the self-consistent second-order perturbation theory combined with the local approximation. The calculated optical conductivity spectrum reproduces the experiments by Damascelli et al. semiquantitatively and the specific heat explains the anomalous contribution at about 250 K observed in FeSi. Inclusion of the spin fluctuation and the extension to the case of strong correlation are also discussed.

cond-mat.str-el↗

arXiv1999

Correlation Effects on Optical Conductivity of FeSi

Effects of electron correlation in FeSi are studied in terms of the two-band Hubbard model with the density of states obtained from the band calculation. Using the self-consistent second-order perturbation theory combined with the local approximation, the correlation effects are investigated on the density of states and the optical conductivity spectrum, which are found to reproduce the experiments done by Damascelli et al. semiquantitatively. It is also found that the peak at the gap edge shifts to lower energy region by correlation effects, as is seen in the experiments.

cond-mat.str-el↗

arXiv1998

Thermal and Dynamical Properties of the Two-band Hubbard Model Compared with FeSi

We study the two-band Hubbard model introduced by Fu and Doniach as a model for FeSi which is suggested to be a Kondo insulator. Using the self-consistent second-order perturbation theory combined with the local approximation which becomes exact in the limit of infinite dimensions, we calculate the specific heat, the spin susceptibility and the dynamical conductivity and point out that the reduction of the energy gap due to correlation is not significant in contrast to the previous calculation. It is also demonstrated that the gap at low temperatures in the optical conductivity is filled up at a rather low temperature than the gap size, which is consistent with the experiment.

cond-mat.str-el↗