5,954 research outputs found
The three-site Bose-Hubbard model subject to atom losses: the boson-pair dissipation channel and failure of the mean-field approach
We employ the perturbation series expansion for derivation of the reduced
master equations for the three-site Bose-Hubbard model subject to strong atom
losses from the central site. The model describes a condensate trapped in a
triple-well potential subject to externally controlled removal of atoms. We
find that the -phase state of the coherent superposition between the side
wells decays via two dissipation channels, the single-boson channel (similar to
the externally applied dissipation) and the boson-pair channel. The quantum
derivation is compared to the classical adiabatic elimination within the
mean-field approximation. We find that the boson-pair dissipation channel is
not captured by the mean-field model, whereas the single-boson channel is
described by it. Moreover, there is a matching condition between the zero-point
energy bias of the side wells and the nonlinear interaction parameter which
separates the regions where either the single-boson or the boson-pair
dissipation channel dominate. Our results indicate that the -site
Bose-Hubbard models, for , subject to atom losses may require an analysis
which goes beyond the usual mean-field approximation for correct description of
their dissipative features. This is an important result in view of the recent
experimental works on the single site addressability of condensates trapped in
optical lattices.Comment: 9 pages; 3 figures in color; submitted to PR
Temperature gradient driven lasing and stimulated cooling
A laser can be understood as thermodynamic engine converting heat to a
coherent single mode field close to Carnot efficiency. From this perspective
spectral shaping of the excitation light generates a higher effective
temperature on the pump than on the gain transition. Here, using a toy model of
a quantum well structure with two suitably designed tunnel-coupled wells kept
at different temperature, we study a laser operated on an actual spatial
temperature gradient between pump and gain region. We predict gain and narrow
band laser emission for a sufficient temperature gradient and resonator
quality. Lasing appears concurrent with amplified heat flow and points to a new
form of stimulated solid state cooling. Such a mechanism could raise the
operating temperature limit of quantum cascade lasers by substituting phonon
emission driven injection, which generates intrinsic heat, by an extended model
with phonon absorption steps
A Molecular Matter-Wave Amplifier
We describe a matter-wave amplifier for vibrational ground state molecules,
which uses a Feshbach resonance to first form quasi-bound molecules starting
from an atomic Bose-Einstein condensate. The quasi-bound molecules are then
driven into their stable vibrational ground state via a two-photon Raman
transition inside an optical cavity. The transition from the quasi-bound state
to the electronically excited state is driven by a classical field.
Amplification of ground state molecules is then achieved by using a strongly
damped cavity mode for the transition from the electronically excited molecules
to the molecular ground state
Entangled and disentangled evolution for a single atom in a driven cavity
For an atom in an externally driven cavity, we show that special initial
states lead to near-disentangled atom-field evolution, and superpositions of
these can lead to near maximally-entangled states. Somewhat counterintutively,
we find that (moderate) spontaneous emission in this system actually leads to a
transient increase in entanglement beyond the steady-state value. We also show
that a particular field correlation function could be used, in an experimental
setting, to track the time evolution of this entanglement
Time evolution and squeezing of the field amplitude in cavity QED
We present the conditional time evolution of the electromagnetic field
produced by a cavity QED system in the strongly coupled regime. We obtain the
conditional evolution through a wave-particle correlation function that
measures the time evolution of the field after the detection of a photon. A
connection exists between this correlation function and the spectrum of
squeezing which permits the study of squeezed states in the time domain. We
calculate the spectrum of squeezing from the master equation for the reduced
density matrix using both the quantum regression theorem and quantum
trajectories. Our calculations not only show that spontaneous emission degrades
the squeezing signal, but they also point to the dynamical processes that cause
this degradation.Comment: 12 pages. Submitted to JOSA
Non-Markovian Relaxation of a Three-Level System: Quantum Trajectory Approach
The non-Markovian dynamics of a three-level quantum system coupled to a
bosonic environment is a difficult problem due to the lack of an exact dynamic
equation such as a master equation. We present for the first time an exact
quantum trajectory approach to a dissipative three-level model. We have
established a convolutionless stochastic Schr\"{o}dinger equation called
time-local quantum state diffusion (QSD) equation without any approximations,
in particular, without Markov approximation. Our exact time-local QSD equation
opens a new avenue for exploring quantum dynamics for a higher dimensional
quantum system coupled to a non-Markovian environment.Comment: 4 pages, 2 figure
Model of the optical emission of a driven semiconductor quantum dot: phonon-enhanced coherent scattering and off-resonant sideband narrowing
We study the crucial role played by the solid-state environment in
determining the photon emission characteristics of a driven quantum dot. For
resonant driving, we predict a phonon-enhancement of the coherently emitted
radiation field with increasing driving strength, in stark contrast to the
conventional expectation of a rapidly decreasing fraction of coherent emission
with stronger driving. This surprising behaviour results from thermalisation of
the dot with respect to the phonon bath, and leads to a nonstandard regime of
resonance fluorescence in which significant coherent scattering and the Mollow
triplet coexist. Off-resonance, we show that despite the phonon influence,
narrowing of dot spectral sideband widths can occur in certain regimes,
consistent with an experimental trend.Comment: Published version. 5 pages, 2 figures, plus 4 page supplement. Title
changed, figure 1 revised, various edits and additions to the tex
From quantum feedback to probabilistic error correction: Manipulation of quantum beats in cavity QED
It is shown how to implement quantum feedback and probabilistic error
correction in an open quantum system consisting of a single atom, with ground-
and excited-state Zeeman structure, in a driven two-mode optical cavity. The
ground state superposition is manipulated and controlled through conditional
measurements and external fields, which shield the coherence and correct
quantum errors. Modeling of an experimentally realistic situation demonstrates
the robustness of the proposal for realization in the laboratory
Developmental Programming: Granulosa cell mRNA expression of differentiation, growth and apoptosis genes in abnormally large progestagenic follicles from androgenized ewes
Stochastic analysis and simulation of spin star systems
We discuss two methods of an exact stochastic representation of the
non-Markovian quantum dynamics of open systems. The first method employs a pair
of stochastic product vectors in the total system's state space, while the
second method uses a pair of state vectors in the open system's state space and
a random operator acting on the state space of the environment. Both techniques
lead to an exact solution of the von Neumann equation for the density matrix of
the total system. Employing a spin star model describing a central spin coupled
to bath of surrounding spins, we perform Monte Carlo simulations for both
variants of the stochastic dynamics. In addition, we derive analytical
expression for the expectation values of the stochastic dynamics to obtain the
exact solution for the density matrix of the central spin.Comment: 8 pages, 2 figure
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