3,343 research outputs found
Initial correlations in open system's dynamics: The Jaynes-Cummings model
Employing the trace distance as a measure for the distinguishability of
quantum states, we study the influence of initial correlations on the dynamics
of open systems. We concentrate on the Jaynes-Cummings model for which the
knowledge of the exact joint dynamics of system and reservoir allows the
treatment of initial states with arbitrary correlations. As a measure for the
correlations in the initial state we consider the trace distance between the
system-environment state and the product of its marginal states. In particular,
we examine the correlations contained in the thermal equilibrium state for the
total system, analyze their dependence on the temperature and on the coupling
strength, and demonstrate their connection to the entanglement properties of
the eigenstates of the Hamiltonian. A detailed study of the time dependence of
the distinguishability of the open system states evolving from the thermal
equilibrium state and its corresponding uncorrelated product state shows that
the open system dynamically uncovers typical features of the initial
correlations.Comment: 12 pages, 7 figure
Initial state preparation with dynamically generated system-environment correlations
The dependence of the dynamics of open quantum systems upon initial
correlations between the system and environment is an utterly important yet
poorly understood subject. For technical convenience most prior studies assume
factorizable initial states where the system and its environments are
uncorrelated, but these conditions are not very realistic and give rise to
peculiar behaviors. One distinct feature is the rapid build up or a sudden jolt
of physical quantities immediately after the system is brought in contact with
its environments. The ultimate cause of this is an initial imbalance between
system-environment correlations and coupling. In this note we demonstrate
explicitly how to avoid these unphysical behaviors by proper adjustments of
correlations and/or the coupling, for setups of both theoretical and
experimental interest. We provide simple analytical results in terms of
quantities that appear in linear (as opposed to affine) master equations
derived for factorized initial states.Comment: 6 pages, 2 figure
Magnetism of the LTT phase of Eu doped La_{2-x}Sr_xCuO_4
The ESR signal of Gd spin probes (0.5 at %) as well as the static normal
state susceptibility of Eu (J(Eu^{3+})=0) doped La_{2-x-y}Sr_xEu_yCuO_4 reveal
pronounced changes of the Cu magnetism at the structural transition from the
orthorhombic to the low temperature tetragonal phase for all
non-superconducting compositions. Both a jumplike decrease of \chi as well as
the ESR data show an increase of the in-plane magnetic correlation length in
the LTT phase. From the Gd^{3+} ESR linewidth we find that for specific Eu and
Sr concentrations in the LTT phase the correlation length increases up to more
than 100 lattice constants and the fluctuation frequency of the CuO_2 spin
system slows down to 10^{10}- 10^{11}sec^{-1}. However, there is no static
order above T ~ 8K in contrast to the LTT phase of Nd doped La_{2-x}Sr_xCuO_4
with pinned stripe correlations.Comment: 7 pages, RevTex, 3 eps figures. To appear in the Proceedings of the
International Conference "Stripes, Lattice Instabilities and High Tc
Superconductivity", (Rome, Dec. 1996
The equilibrium states of open quantum systems in the strong coupling regime
In this work we investigate the late-time stationary states of open quantum
systems coupled to a thermal reservoir in the strong coupling regime. In
general such systems do not necessarily relax to a Boltzmann distribution if
the coupling to the thermal reservoir is non-vanishing or equivalently if the
relaxation timescales are finite. Using a variety of non-equilibrium formalisms
valid for non-Markovian processes, we show that starting from a product state
of the closed system = system + environment, with the environment in its
thermal state, the open system which results from coarse graining the
environment will evolve towards an equilibrium state at late-times. This state
can be expressed as the reduced state of the closed system thermal state at the
temperature of the environment. For a linear (harmonic) system and environment,
which is exactly solvable, we are able to show in a rigorous way that all
multi-time correlations of the open system evolve towards those of the closed
system thermal state. Multi-time correlations are especially relevant in the
non-Markovian regime, since they cannot be generated by the dynamics of the
single-time correlations. For more general systems, which cannot be exactly
solved, we are able to provide a general proof that all single-time
correlations of the open system evolve to those of the closed system thermal
state, to first order in the relaxation rates. For the special case of a
zero-temperature reservoir, we are able to explicitly construct the reduced
closed system thermal state in terms of the environmental correlations.Comment: 20 pages, 2 figure
Dynamic entanglement in oscillating molecules and potential biological implications
We demonstrate that entanglement can persistently recur in an oscillating
two-spin molecule that is coupled to a hot and noisy environment, in which no
static entanglement can survive. The system represents a non-equilibrium
quantum system which, driven through the oscillatory motion, is prevented from
reaching its (separable) thermal equilibrium state. Environmental noise,
together with the driven motion, plays a constructive role by periodically
resetting the system, even though it will destroy entanglement as usual. As a
building block, the present simple mechanism supports the perspective that
entanglement can exist also in systems which are exposed to a hot environment
and to high levels of de-coherence, which we expect e.g. for biological
systems. Our results furthermore suggest that entanglement plays a role in the
heat exchange between molecular machines and environment. Experimental
simulation of our model with trapped ions is within reach of the current
state-of-the-art quantum technologies.Comment: Extended version, including supplementary information. 9 pages, 8
figure
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
Entanglement in the adiabatic limit of a two-atom Tavis-Cummings model
We study the adiabatic limit for the sequential passage of atoms through a
high-Q cavity, in the presence of frequency chirps. Despite the fact that the
adiabatic approximation might be expected to fail, we were able to show that
for proper choice of Stark-pulses this is not the case. Instead, a connection
to the resonant limit is established, where the robust creation of entanglement
is demonstrated. Recent developments in the fabrication of high-Q cavities
allow fidelities for a maximally entangled state up to 97%.Comment: 12 pages, 5 figures, Submitted to Physica Scripta as part of the
Proceedings of the 15th CEWQO 200
Stochastic wave function approach to the calculation of multitime correlation functions of open quantum systems
Within the framework of probability distributions on projective Hilbert space
a scheme for the calculation of multitime correlation functions is developed.
The starting point is the Markovian stochastic wave function description of an
open quantum system coupled to an environment consisting of an ensemble of
harmonic oscillators in arbitrary pure or mixed states. It is shown that matrix
elements of reduced Heisenberg picture operators and general time-ordered
correlation functions can be expressed by time-symmetric expectation values of
extended operators in a doubled Hilbert space. This representation allows the
construction of a stochastic process in the doubled Hilbert space which enables
the determination of arbitrary matrix elements and correlation functions. The
numerical efficiency of the resulting stochastic simulation algorithm is
investigated and compared with an alternative Monte Carlo wave function method
proposed first by Dalibard et al. [Phys. Rev. Lett. {\bf 68}, 580 (1992)]. By
means of a standard example the suggested algorithm is shown to be more
efficient numerically and to converge faster. Finally, some specific examples
from quantum optics are presented in order to illustrate the proposed method,
such as the coupling of a system to a vacuum, a squeezed vacuum within a finite
solid angle, and a thermal mixture of coherent states.Comment: RevTex, 19 pages, 3 figures, uses multico
Stochastic wave function method for non-Markovian quantum master equations
A generalization of the stochastic wave function method to quantum master
equations which are not in Lindblad form is developed. The proposed stochastic
unravelling is based on a description of the reduced system in a doubled
Hilbert space and it is shown, that this method is capable of simulating
quantum master equations with negative transition rates. Non-Markovian effects
in the reduced systems dynamics can be treated within this approach by
employing the time-convolutionless projection operator technique. This ansatz
yields a systematic perturbative expansion of the reduced systems dynamics in
the coupling strength. Several examples such as the damped Jaynes Cummings
model and the spontaneous decay of a two-level system into a photonic band gap
are discussed. The power as well as the limitations of the method are
demonstrated.Comment: RevTex, 14 pages, 9 figures, uses multico
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