332 research outputs found
Homodyne Measurements on a Bose-Einstein Condensate
We investigate a non-destructive measurement technique to monitor
Josephson-like oscillations between two spatially separated neutral atom
Bose-Einstein condensates. One condensate is placed in an optical cavity, which
is strongly driven by a coherent optical field. The cavity output field is
monitored using a homodyne detection scheme. The cavity field is well detuned
from an atomic resonance, and experiences a dispersive phase shift proportional
to the number of atoms in the cavity. The detected current is modulated by the
coherent tunneling oscillations of the condensate. Even when there is an equal
number of atoms in each well initially, a phase is established by the
measurement process and Josephson-like oscillations develop due to measurement
back-action noise alone.Comment: 8 pages, 12 figures to appear in PR
Quantum noise in optical fibers II: Raman jitter in soliton communications
The dynamics of a soliton propagating in a single-mode optical fiber with
gain, loss, and Raman coupling to thermal phonons is analyzed. Using both
soliton perturbation theory and exact numerical techniques, we predict that
intrinsic thermal quantum noise from the phonon reservoirs is a larger source
of jitter and other perturbations than the gain-related Gordon-Haus noise, for
short pulses, assuming typical fiber parameters. The size of the Raman timing
jitter is evaluated for both bright and dark (topological) solitons, and is
larger for bright solitons. Because Raman thermal quantum noise is a nonlinear,
multiplicative noise source, these effects are stronger for the more intense
pulses needed to propagate as solitons in the short-pulse regime. Thus Raman
noise may place additional limitations on fiber-optical communications and
networking using ultrafast (subpicosecond) pulses.Comment: 3 figure
Tripartite and bipartite entanglement in continuous-variable tripartite systems
We examine one asymmetric adnd two fully symmetric Gaussian
continuous-variable systems in terms of their tripartite and bipartite
entanglement properties. We treat pure states and are able to find analytic
solutions using the undepleted pump approximation for the Hamiltonian models,
and standard beamsplitter relations for a model that mixes the outputs of
optical parametric oscillators. Our two symmetric systems exhibit perfect
tripartite correlations, but only in the unphysical limit of infinite
squeezing. For more realistic squeezing parameters, all three systems exhibit
both tripartite and bipartite entanglement. We conclude that none of the
outputs are completely analogous to either GHZ or W states, but there are
parameter regions where they produce T states introduced by Adesso \etal The
qualitative differences in the output states for different interaction
parameters indicate that continuous-variable tripartite quantum information
systems offer a versatility not found in bipartite systems.Comment: 18 pages, 6 figures. arXiv admin note: text overlap with
arXiv:1510.0182
Quantum noise in optical fibers I: stochastic equations
We analyze the quantum dynamics of radiation propagating in a single mode
optical fiber with dispersion, nonlinearity, and Raman coupling to thermal
phonons. We start from a fundamental Hamiltonian that includes the principal
known nonlinear effects and quantum noise sources, including linear gain and
loss. Both Markovian and frequency-dependent, non-Markovian reservoirs are
treated. This allows quantum Langevin equations to be calculated, which have a
classical form except for additional quantum noise terms. In practical
calculations, it is more useful to transform to Wigner or +
quasi-probability operator representations. These result in stochastic
equations that can be analyzed using perturbation theory or exact numerical
techniques. The results have applications to fiber optics communications,
networking, and sensor technology.Comment: 1 figur
Gaussian operator bases for correlated fermions
We formulate a general multi-mode Gaussian operator basis for fermions, to
enable a positive phase-space representation of correlated Fermi states. The
Gaussian basis extends existing bosonic phase-space methods to Fermi systems
and thus enables first-principles dynamical or equilibrium calculations in
quantum many-body Fermi systems. We prove the completeness and positivity of
the basis, and derive differential forms for products with one- and two-body
operators. Because the basis satisfies fermionic superselection rules, the
resulting phase space involves only c-numbers, without requiring anti-commuting
Grassmann variables
Improved quantum correlations in second harmonic generation with a squeezed pump
We investigate the effects of a squeezed pump on the quantum properties and
conversion efficiency of the light produced in single-pass second harmonic
generation. Using stochastic integration of the two-mode equations of motion in
the positive-P representation, we find that larger violations of
continuous-variable harmonic entanglement criteria are available for lesser
effective interaction strengths than with a coherent pump. This enhancement of
the quantum properties also applies to violations of the Reid-Drummond
inequalities used to demonstrate a harmonic version of the
Einstein-Podolsky-Rosen paradox. We find that the conversion efficiency is
largely unchanged except for very low pump intensities and high levels of
squeezing.Comment: 19 pages, 7 figure
Quantum dynamics of long-range interacting systems using the positive-P and gauge-P representations
We provide the necessary framework for carrying out stochastic positive-P and
gauge-P simulations of bosonic systems with long range interactions. In these
approaches, the quantum evolution is sampled by trajectories in phase space,
allowing calculation of correlations without truncation of the Hilbert space or
other approximations to the quantum state. The main drawback is that the
simulation time is limited by noise arising from interactions.
We show that the long-range character of these interactions does not further
increase the limitations of these methods, in contrast to the situation for
alternatives such as the density matrix renormalisation group. Furthermore,
stochastic gauge techniques can also successfully extend simulation times in
the long-range-interaction case, by making using of parameters that affect the
noise properties of trajectories, without affecting physical observables.
We derive essential results that significantly aid the use of these methods:
estimates of the available simulation time, optimized stochastic gauges, a
general form of the characteristic stochastic variance and adaptations for very
large systems. Testing the performance of particular drift and diffusion gauges
for nonlocal interactions, we find that, for small to medium systems, drift
gauges are beneficial, whereas for sufficiently large systems, it is optimal to
use only a diffusion gauge.
The methods are illustrated with direct numerical simulations of interaction
quenches in extended Bose-Hubbard lattice systems and the excitation of Rydberg
states in a Bose-Einstein condensate, also without the need for the typical
frozen gas approximation. We demonstrate that gauges can indeed lengthen the
useful simulation time.Comment: 19 pages, 11 appendix, 3 figure
First-principles quantum dynamics for fermions: Application to molecular dissociation
We demonstrate that the quantum dynamics of a many-body Fermi-Bose system can
be simulated using a Gaussian phase-space representation method. In particular,
we consider the application of the mixed fermion-boson model to ultracold
quantum gases and simulate the dynamics of dissociation of a Bose-Einstein
condensate of bosonic dimers into pairs of fermionic atoms. We quantify
deviations of atom-atom pair correlations from Wick's factorization scheme, and
show that atom-molecule and molecule-molecule correlations grow with time, in
clear departures from pairing mean-field theories. As a first-principles
approach, the method provides benchmarking of approximate approaches and can be
used to validate dynamical probes for characterizing strongly correlated phases
of fermionic systems.Comment: Final published versio
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