4,682 research outputs found
Non-Markovian reservoir-dependent squeezing
The squeezing dynamics of a damped harmonic oscillator are studied for
different types of environment without making the Markovian approximation. The
squeezing dynamics of a coherent state depend on the reservoir spectrum in a
unique way that can, in the weak coupling approximation, be analyzed
analytically. Comparison of squeezing dynamics for Ohmic, sub-Ohmic and
super-Ohmic environments is done showing a clear connection between the
squeezing--non-squeezing oscillations and reservoir structure. Understanding
the effects occurring due to structured reservoirs is important both from a
purely theoretical point of view and in connection with evolving experimental
techniques and future quantum computing applications.Comment: 8 pages, 2 figures, submitted to Proceedings of CEWQO200
Sudden transition between classical and quantum decoherence
We study the dynamics of quantum and classical correlations in the presence
of nondissipative decoherence. We discover a class of initial states for which
the quantum correlations, quantified by the quantum discord, are not destroyed
by decoherence for times t < \bar{t}. In this initial time interval classical
correlations decay. For t > \bar{t}, on the other hand, classical correlations
do not change in time and only quantum correlations are lost due to the
interaction with the environment. Therefore, at the transition time \bar{t} the
open system dynamics exhibits a sudden transition from classical to quantum
decoherence regime.Comment: version accepted for publication by Physical Review Letter
Phenomenological memory-kernel master equations and time-dependent Markovian processes
Do phenomenological master equations with memory kernel always describe a
non-Markovian quantum dynamics characterized by reverse flow of information? Is
the integration over the past states of the system an unmistakable signature of
non-Markovianity? We show by a counterexample that this is not always the case.
We consider two commonly used phenomenological integro-differential master
equations describing the dynamics of a spin 1/2 in a thermal bath. By using a
recently introduced measure to quantify non-Markovianity [H.-P. Breuer, E.-M.
Laine, and J. Piilo, Phys. Rev. Lett. 103, 210401 (2009)] we demonstrate that
as far as the equations retain their physical sense, the key feature of
non-Markovian behavior does not appear in the considered memory kernel master
equations. Namely, there is no reverse flow of information from the environment
to the open system. Therefore, the assumption that the integration over a
memory kernel always leads to a non-Markovian dynamics turns out to be
vulnerable to phenomenological approximations. Instead, the considered
phenomenological equations are able to describe time-dependent and
uni-directional information flow from the system to the reservoir associated to
time-dependent Markovian processes.Comment: 5 pages, no figure
Local in time master equations with memory effects: Applicability and interpretation
Non-Markovian local in time master equations give a relatively simple way to
describe the dynamics of open quantum systems with memory effects. Despite
their simple form, there are still many misunderstandings related to the
physical applicability and interpretation of these equations. Here we clarify
these issues both in the case of quantum and classical master equations. We
further introduce the concept of a classical non-Markov chain signified through
negative jump rates in the chain configuration.Comment: Special issue on loss of coherence and memory effects in quantum
dynamics, J. Phys. B., to appea
Decoherence in a quantum harmonic oscillator monitored by a Bose-Einstein condensate
We investigate the dynamics of a quantum oscillator, whose evolution is
monitored by a Bose-Einstein condensate (BEC) trapped in a symmetric double
well potential. It is demonstrated that the oscillator may experience various
degrees of decoherence depending on the variable being measured and the state
in which the BEC is prepared. These range from a `coherent' regime in which
only the variances of the oscillator position and momentum are affected by
measurement, to a slow (power law) or rapid (Gaussian) decoherence of the mean
values themselves.Comment: 4 pages, 3 figures, lette
Non-Markovianity, Loschmidt echo and criticality: a unified picture
A simple relationship between recently proposed measures of non-Markovianity
and the Loschmidt echo is established, holding for a two-level system (qubit)
undergoing pure dephasing due to a coupling with a many-body environment. We
show that the Loschmidt echo is intimately related to the information flowing
out from and occasionally back into the system. This, in turn, determines the
non-Markovianity of the reduced dynamics. In particular, we consider a central
qubit coupled to a quantum Ising ring in the transverse field. In this context,
the information flux between system and environment is strongly affected by the
environmental criticality; the qubit dynamics is shown to be Markovian exactly
and only at the critical point. Therefore non-Markovianity is an indicator of
criticality in the model considered here
Environment-dependent dissipation in quantum Brownian motion
The dissipative dynamics of a quantum Brownian particle is studied for
different types of environment. We derive analytic results for the time
evolution of the mean energy of the system for Ohmic, sub-Ohmic and super-Ohmic
environments, without performing the Markovian approximation. Our results allow
to establish a direct link between the form of the environmental spectrum and
the thermalization dynamics. This in turn leads to a natural explanation of the
microscopic physical processes ruling the system time evolution both in the
short-time non-Markovian region and in the long-time Markovian one. Our
comparative study of thermalization for different environments sheds light on
the physical contexts in which non-Markovian dissipation effects are dominant.Comment: 10 pages, 6 figures, v2: added new references and paragraph
New method to simulate quantum interference using deterministic processes and application to event-based simulation of quantum computation
We demonstrate that networks of locally connected processing units with a
primitive learning capability exhibit behavior that is usually only attributed
to quantum systems. We describe networks that simulate single-photon
beam-splitter and Mach-Zehnder interferometer experiments on a causal,
event-by-event basis and demonstrate that the simulation results are in
excellent agreement with quantum theory. We also show that this approach can be
generalized to simulate universal quantum computers.Comment: J. Phys. Soc. Jpn. (in press) http://www.compphys.net/dl
Dynamical typicality of quantum expectation values
We show that the vast majority of all pure states featuring a common
expectation value of some generic observable at a given time will yield very
similar expectation values of the same observable at any later time. This is
meant to apply to Schroedinger type dynamics in high dimensional Hilbert
spaces. As a consequence individual dynamics of expectation values are then
typically well described by the ensemble average. Our approach is based on the
Hilbert space average method. We support the analytical investigations with
numerics obtained by exact diagonalization of the full time-dependent
Schroedinger equation for some pertinent, abstract Hamiltonian model.
Furthermore, we discuss the implications on the applicability of projection
operator methods with respect to initial states, as well as on irreversibility
in general.Comment: 4 pages, 1 figure, accepted for publication in Phys. Rev. Let
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