8,409 research outputs found
Correction to the geometric phase by structured environments: the onset of non-Markovian effects
We study the geometric phase of a two-level system under the presence of a
structured environment, particularly analysing its correction with the ohmicity
parameter and the onset of non-Markovianity. We firstly examine the system
coupled to a set of harmonic oscillators and studied the decoherence factor as
function of the environment's ohmicity parameter. Secondly, we propose the
two-level system coupled to a non-equilibrium environment, and show that these
environments display non-Markovian effects for all values of the ohmicity
parameter. The geometric phase of the two-level system is therefore computed
under the presence of both types of environment. The correction to the unitary
geometric phase is analysed in both, Markovian and non-Markovian regimes. Under
Markovian environments, the correction induced on the system's phase is mainly
ruled by the coupling constant between the system and the environment, while in
the non-Markovian regime, memory effects seem to trigger a significant
correction to the unitary geometric phase. The result is significant to the
quantum information processing based on the geometric phase in quantum open
systemsComment: 7 figures. Accepted for publication in Phys. Rev. A. arXiv admin
note: text overlap with arXiv:1303.493
Decoherence in composite quantum open systems: the effectiveness of unstable degrees of freedom
The effect induced by an environment on a composite quantum system is
studied. The model considers the composite system as comprised by a subsystem A
coupled to a subsystem B which is also coupled to an external environment. We
study all possible four combinations of subsystems A and B made up with a
harmonic oscillator and an upside down oscillator. We analyzed the decoherence
suffered by subsystem A due to an effective environment composed by subsystem B
and the external reservoir. In all the cases we found that subsystem A
decoheres even though it interacts with the environment only through its sole
coupling to B. However, the effectiveness of the diffusion depends on the
unstable nature of subsystem A and B. Therefore, the role of this degree of
freedom in the effective environment is analyzed in detailComment: 20 pages, 4 figures. Version to appear in Int. J. Mod. Phys.
Decoherence of a solid-state qubit by different noise correlation spectra
The interaction between solid-state qubits and their environmental degrees of
freedom produces non-unitary effects like decoherence and dissipation.
Uncontrolled decoherence is one of the main obstacles that must be overcome in
quantum information processing. We study the dynamically decay of coherences in
a solid-state qubit by means of the use of a master equation. We analyse the
effects induced by thermal Ohmic environments and low-frequency 1/f noise. We
focus on the effect of longitudinal and transversal noise on the
superconducting qubit's dynamics. Our results can be used to design
experimental future setups when manipulating superconducting qubits.Comment: 14 pages, 9 figures. Version to appear in Physics Letters A. arXiv
admin note: text overlap with arXiv:0809.4716 by other author
Macroscopic tunneling, decoherence and noise-induced activation
We study the effects of the environment at zero temperature on tunneling in
an open system described by a static double-well potential. We show that the
evolution of the system in an initial Schrodinger cat state, can be summarized
in terms of three main physical phenomena, namely decoherence, quantum
tunneling and noise-induced activation. Using large-scale numerical
simulations, we obtain a detailed picture of the main stages of the evolution
and of the relevant dynamical processesComment: Contribution to the Proceedings of DICE'0
Using boundary methods to compute the Casimir energy
We discuss new approaches to compute numerically the Casimir interaction
energy for waveguides of arbitrary section, based on the boundary methods
traditionally used to compute eigenvalues of the 2D Helmholtz equation. These
methods are combined with the Cauchy's theorem in order to perform the sum over
modes. As an illustration, we describe a point-matching technique to compute
the vacuum energy for waveguides containing media with different
permittivities. We present explicit numerical evaluations for perfect
conducting surfaces in the case of concentric corrugated cylinders and a
circular cylinder inside an elliptic one.Comment: To be published in the Proceedings of QFEXT09, Norman, OK
Unconventional antiferromagnetic correlations of the doped Haldane gap system YBaNiZnO
We make a new proposal to describe the very low temperature susceptibility of
the doped Haldane gap compound YBaNiZnO. We propose a new
mean field model relevant for this compound. The ground state of this mean
field model is unconventional because antiferromagnetism coexists with random
dimers. We present new susceptibility experiments at very low temperature. We
obtain a Curie-Weiss susceptibility as expected
for antiferromagnetic correlations but we do not obtain a direct signature of
antiferromagnetic long range order. We explain how to obtain the ``impurity''
susceptibility by subtracting the Haldane gap contribution to
the total susceptibility. In the temperature range [1 K, 300 K] the
experimental data are well fitted by . In the temperature range [100 mK, 1 K] the experimental data are
well fitted by , where increases with
. This fit suggests the existence of a finite N\'eel temperature which is
however too small to be probed directly in our experiments. We also obtain a
maximum in the temperature dependence of the ac-susceptibility which
suggests the existence of antiferromagnetic correlations at very low
temperature.Comment: 19 pages, 17 figures, revised version (minor modifications
Visibility Fringe Reduction Due to Noise-Induced Effects: Microscopic Approach to Interference Experiments
Decoherence is the main process behind the quantum to classical transition.
It is a purely quantum mechanical effect by which the system looses its ability
to exhibit coherent behavior. The recent experimental observation of
diffraction and interference patterns for large molecules raises some
interesting questions. In this context, we identify possible agents of
decoherence to take into account when modeling these experiments and study
theirs visible (or not) effects on the interference pattern. Thereby, we
present an analysis of matter wave interferometry in the presence of a dynamic
quantum environment and study how much the visibility fringe is reduced and in
which timescale the decoherence effects destroy the interference of massive
objects. Finally, we apply our results to the experimental data reported on
fullerenes and cold neutrons.Comment: 14 pages, 5 figures. Version to appear in Mod. Phys.
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