643 research outputs found
Relativistic formulation of quantum state diffusion?
The recently reported relativistic formulation of the well-known
non-relativistic quantum state diffusion is seriously mistaken. It predicts,
for instance, inconsistent measurement outcomes for the same system when seen
by two different inertial observers.Comment: 5 pages LaTeX, submitted to J. Phys.
Decoherence and the Nature of System-Environment Correlations
We investigate system-environment correlations based on the exact dynamics of
a qubit and its environment in the framework of pure decoherence (phase
damping). We focus on the relation of decoherence and the build-up of
system-reservoir entanglement for an arbitrary (possibly mixed) initial qubit
state. In the commonly employed regime where the qubit dynamics can be
described by a Markov master equation of Lindblad type, we find that for almost
all qubit initial states inside the Bloch sphere, decoherence is complete while
the total state is still separable - no entanglement is involved. In general,
both "separable" and "entangling" decoherence occurs, depending on temperature
and initial qubit state. Moreover, we find situations where classical and
quantum correlations periodically alternate as a function of time in the regime
of low temperatures
Breakdown of a conservation law in incommensurate systems
We show that invariance properties of the Lagrangian of an incommensurate
system, as described by the Frenkel Kontorova model, imply the existence of a
generalized angular momentum which is an integral of motion if the system
remains floating. The behavior of this quantity can therefore monitor the
character of the system as floating (when it is conserved) or locked (when it
is not). We find that, during the dynamics, the non-linear couplings of our
model cause parametric phonon excitations which lead to the appearance of
Umklapp terms and to a sudden deviation of the generalized momentum from a
constant value, signalling a dynamical transition from a floating to a pinned
state. We point out that this transition is related but does not coincide with
the onset of sliding friction which can take place when the system is still
floating.Comment: 7 pages, 6 figures, typed with RevTex, submitted to Phys. Rev. E
Replaced 27-03-2001: changes to text, minor revision of figure
Electron spin tomography through counting statistics: a quantum trajectory approach
We investigate the dynamics of electron spin qubits in quantum dots.
Measurement of the qubit state is realized by a charge current through the dot.
The dynamics is described in the framework of the quantum trajectory approach,
widely used in quantum optics, and we show that it can be applied successfully
to problems in condensed matter physics. The relevant master equation dynamics
is unravelled to simulate stochastic tunneling events of the current through
the dot.Quantum trajectories are then used to extract the counting statistics
of the current. We show how, in combination with an electron spin resonance
(ESR) field, counting statistics can be employed for quantum state tomography
of the qubit state. Further, it is shown how decoherence and relaxation time
scales can be estimated with the help of counting statistics, in the time
domain. Finally, we discuss a setup for single shot measurement of the qubit
state without the need for spin-polarized leads.Comment: 23 pages, 10 figures, RevTeX4, submitted to PR
System-environment correlations and Non-Markovian dynamics
We determine the total state dynamics of a dephasing open quantum system
using the standard environment of harmonic oscillators. Of particular interest
are random unitary approaches to the same reduced dynamics and
system-environment correlations in the full model. Concentrating on a model
with an at times negative dephasing rate, the issue of "non-Markovianity" will
also be addressed. Crucially, given the quantum environment, the appearance of
non-Markovian dynamics turns out to be accompanied by a loss of
system-environment correlations. Depending on the initial purity of the qubit
state, these system-environment correlations may be purely classical over the
whole relevant time scale, or there may be intervals of genuine
system-environment entanglement. In the latter case, we see no obvious relation
between the build-up or decay of these quantum correlations and
"Non-Markovianity"
Non-Markovian quantum state diffusion for absorption spectra of molecular aggregates
In many molecular systems one encounters the situation where electronic
excitations couple to a quasi-continuum of phonon modes. That continuum may be
highly structured e.g. due to some weakly damped high frequency modes. To
handle such a situation, an approach combining the non-Markovian quantum state
diffusion (NMQSD) description of open quantum systems with an efficient but
abstract approximation was recently applied to calculate energy transfer and
absorption spectra of molecular aggregates [Roden, Eisfeld, Wolff, Strunz, PRL
103 (2009) 058301]. To explore the validity of the used approximation for such
complicated systems, in the present work we compare the calculated
(approximative) absorption spectra with exact results. These are obtained from
the method of pseudomodes, which we show to be capable of determining the exact
spectra for small aggregates and a few pseudomodes. It turns out that in the
cases considered, the results of the two approaches mostly agree quite well.
The advantages and disadvantages of the two approaches are discussed
Exact c-number Representation of Non-Markovian Quantum Dissipation
The reduced dynamics of a quantum system interacting with a linear heat bath
finds an exact representation in terms of a stochastic Schr{\"o}dinger
equation. All memory effects of the reservoir are transformed into noise
correlations and mean-field friction. The classical limit of the resulting
stochastic dynamics is shown to be a generalized Langevin equation, and
conventional quantum state diffusion is recovered in the Born--Markov
approximation. The non-Markovian exact dynamics, valid at arbitrary temperature
and damping strength, is exemplified by an application to the dissipative
two-state system.Comment: 4 pages, 2 figures. To be published in Phys. Rev. Let
Quantum trajectories for Brownian motion
We present the stochastic Schroedinger equation for the dynamics of a quantum
particle coupled to a high temperature environment and apply it the dynamics of
a driven, damped, nonlinear quantum oscillator. Apart from an initial slip on
the environmental memory time scale, in the mean, our result recovers the
solution of the known non-Lindblad quantum Brownian motion master equation. A
remarkable feature of our approach is its localization property: individual
quantum trajectories remain localized wave packets for all times, even for the
classically chaotic system considered here, the localization being stronger the
smaller .Comment: 4 pages, 3 eps figure
Non-Markovian Quantum Trajectories of Many-Body Quantum Open Systems
A long-standing open problem in non-Markovian quantum state diffusion (QSD)
approach to open quantum systems is to establish the non-Markovian QSD
equations for multiple qubit systems. In this paper, we settle this important
question by explicitly constructing a set of exact time-local QSD equations for
-qubit systems. Our exact time-local (convolutionless) QSD equations have
paved the way towards simulating quantum dynamics of many-body open systems
interacting with a common bosonic environment. The applicability of this
multiple-qubit stochastic equation is exemplified by numerically solving
several quantum open many-body systems concerning quantum coherence dynamics
and dynamical control.Comment: 8 pages, 2 figures. manuscript revised and reference update
Electronic and phononic properties of the chalcopyrite CuGaS2
The availability of ab initio electronic calculations and the concomitant
techniques for deriving the corresponding lattice dynamics have been profusely
used for calculating thermodynamic and vibrational properties of
semiconductors, as well as their dependence on isotopic masses. The latter have
been compared with experimental data for elemental and binary semiconductors
with different isotopic compositions. Here we present theoretical and
experimental data for several vibronic and thermodynamic properties of CuGa2, a
canonical ternary semiconductor of the chalcopyrite family. Among these
properties are the lattice parameters, the phonon dispersion relations and
densities of states (projected on the Cu, Ga, and S constituents), the specific
heat and the volume thermal expansion coefficient. The calculations were
performed with the ABINIT and VASP codes within the LDA approximation for
exchange and correlation and the results are compared with data obtained on
samples with the natural isotope composition for Cu, Ga and S, as well as for
isotope enriched samples.Comment: 9 pages, 8 Figures, submitted to Phys. Rev
- …