3,630 research outputs found
Thermal Properties of Interacting Bose Fields and Imaginary-Time Stochastic Differential Equations
Matsubara Green's functions for interacting bosons are expressed as classical
statistical averages corresponding to a linear imaginary-time stochastic
differential equation. This makes direct numerical simulations applicable to
the study of equilibrium quantum properties of bosons in the non-perturbative
regime. To verify our results we discuss an oscillator with quartic
anharmonicity as a prototype model for an interacting Bose gas. An analytic
expression for the characteristic function in a thermal state is derived and a
Higgs-type phase transition discussed, which occurs when the oscillator
frequency becomes negative.Comment: Published versio
Unconditional two-mode squeezing of separated atomic ensembles
We propose schemes for the unconditional preparation of a two-mode squeezed
state of effective bosonic modes realized in a pair of atomic ensembles
interacting collectively with optical cavity and laser fields. The scheme uses
Raman transitions between stable atomic ground states and under ideal
conditions produces pure entangled states in the steady state. The scheme works
both for ensembles confined within a single cavity and for ensembles confined
in separate, cascaded cavities.Comment: 4 pages, 2 figure
Effect of the Coriolis Force on the Hydrodynamics of Colliding Wind Binaries
Using fully three-dimensional hydrodynamic simulations, we investigate the
effect of the Coriolis force on the hydrodynamic and observable properties of
colliding wind binary systems. To make the calculations tractable, we assume
adiabatic, constant velocity winds. The neglect of radiative driving,
gravitational deceleration, and cooling limit the application of our models to
real systems. However, these assumptions allow us to isolate the effect of the
Coriolis force, and by simplifying the calculations, allow us to use a higher
resolution (up to 640^3) and to conduct a larger survey of parameter space. We
study the dynamics of collidng winds with equal mass loss rates and velocities
emanating from equal mass stars on circular orbits, with a range of values for
the ratio of the wind to orbital velocity. We also study the dynamics of winds
from stars on elliptical orbits and with unequal strength winds. Orbital motion
of the stars sweeps the shocked wind gas into an Archimedean spiral, with
asymmetric shock strengths and therefore unequal postshock temperatures and
densities in the leading and trailing edges of the spiral. We observe the
Kelvin-Helmholtz instability at the contact surface between the shocked winds
in systems with orbital motion even when the winds are identical. The change in
shock strengths caused by orbital motion increases the volume of X-ray emitting
post-shock gas with T > 0.59 keV by 63% for a typical system as the ratio of
wind velocity to orbital velocity decreases to V_w/V_o = 2.5. This causes
increased free-free emission from systems with shorter orbital periods and an
altered time-dependence of the wind attenuation. We comment on the importance
of the effects of orbital motion on the observable properties of colliding wind
binaries.Comment: 12 pages, 17 figures, accepted for publication in Ap
Decoherence of number states in phase-sensitive reservoirs
The non-unitary evolution of initial number states in general Gaussian
environments is solved analytically. Decoherence in the channels is quantified
by determining explicitly the purity of the state at any time. The influence of
the squeezing of the bath on decoherence is discussed. The behavior of coherent
superpositions of number states is addressed as well.Comment: 5 pages, 2 figures, minor changes, references adde
Commuting Heisenberg operators as the quantum response problem: Time-normal averages in the truncated Wigner representation
The applicability of the so-called truncated Wigner approximation (-W) is
extended to multitime averages of Heisenberg field operators. This task splits
naturally in two. Firstly, what class of multitime averages the -W
approximates, and, secondly, how to proceed if the average in question does not
belong to this class. To answer the first question we develop an (in principle,
exact) path-integral approach in phase-space based on the symmetric (Weyl)
ordering of creation and annihilation operators. These techniques calculate a
new class of averages which we call time-symmetric. The -W equations emerge as
an approximation within this path-integral techniques. We then show that the
answer to the second question is associated with response properties of the
system. In fact, for two-time averages Kubo's renowned formula relating the
linear response function to two-time commutators suffices. The -W is trivially
generalised to the response properties of the system allowing one to calculate
approximate time-normally ordered two-time correlation functions with
surprising ease. The techniques we develop are demonstrated for the
Bose-Hubbard model.Comment: 20 pages, 6 figure
Semiclassical Spectrum of Small Bose-Hubbard Chains: A Normal Form Approach
We analyze the spectrum of the 3-site Bose-Hubbard model with periodic
boundary conditions using a semiclassical method. The Bohr-Sommerfeld
quantization is applied to an effective classical Hamiltonian which we derive
using resonance normal form theory. The derivation takes into account the 1:1
resonance between frequencies of a linearized classical system, and brings
nonlinear terms into a corresponding normal form. The obtained expressions
reproduce the exact low-energy spectrum of the system remarkably well even for
a small number of particles N corresponding to fillings of just two particles
per site. Such small fillings are often used in current experiments, and it is
inspiring to get insight into this quantum regime using essentially classical
calculations.Comment: Minor corrections to the coefficients of the effective Hamiltonian in
Eqs 14,15,18,19. Figs 1,2 are slightly modified, correspondingl
Quantum trajectory approach to circuit QED: Quantum jumps and the Zeno effect
We present a theoretical study of a superconducting charge qubit dispersively
coupled to a transmission line resonator. Starting from a master equation
description of this coupled system and using a polaron transformation, we
obtain an exact effective master equation for the qubit. We then use quantum
trajectory theory to investigate the measurement of the qubit by continuous
homodyne measurement of the resonator out-field. Using the same porlaron
transformation, a stochastic master equation for the conditional state of the
qubit is obtained. From this result, various definitions of the measurement
time are studied. Furthermore, we find that in the limit of strong homodyne
measurement, typical quantum trajectories for the qubit exhibit a crossover
from diffusive to jump-like behavior. Finally, in the presence of Rabi drive on
the qubit, the qubit dynamics is shown to exhibit quantum Zeno behavior.Comment: 20 pages, 12 figure
- …