6,811 research outputs found
Quantum turbulence and correlations in Bose-Einstein condensate collisions
We investigate numerically simulated collisions between experimentally
realistic Bose-Einstein condensate wavepackets, within a regime where highly
populated scattering haloes are formed. The theoretical basis for this work is
the truncated Wigner method, for which we present a detailed derivation, paying
particular attention to its validity regime for colliding condensates. This
paper is an extension of our previous Letter [A. A. Norrie, R. J. Ballagh, and
C. W. Gardiner, Phys. Rev. Lett. 94, 040401 (2005)] and we investigate both
single-trajectory solutions, which reveal the presence of quantum turbulence in
the scattering halo, and ensembles of trajectories, which we use to calculate
quantum-mechanical correlation functions of the field
Bogoliubov dynamics of condensate collisions using the positive-P representation
We formulate the time-dependent Bogoliubov dynamics of colliding
Bose-Einstein condensates in terms of a positive-P representation of the
Bogoliubov field. We obtain stochastic evolution equations for the field which
converge to the full Bogoliubov description as the number of realisations
grows. The numerical effort grows linearly with the size of the computational
lattice. We benchmark the efficiency and accuracy of our description against
Wigner distribution and exact positive-P methods. We consider its regime of
applicability, and show that it is the most efficient method in the common
situation - when the total particle number in the system is insufficient for a
truncated Wigner treatment.Comment: 9 pages. 5 figure
Three-body recombination of ultracold Bose gases using the truncated Wigner method
We apply the truncated Wigner method to the process of three-body
recombination in ultracold Bose gases. We find that within the validity regime
of the Wigner truncation for two-body scattering, three-body recombination can
be treated using a set of coupled stochastic differential equations that
include diffusion terms, and can be simulated using known numerical methods. As
an example we investigate the behaviour of a simple homogeneous Bose gas.Comment: Replaced paper same as original; correction to author list on
cond-mat mad
Non-Markovian Open Quantum Systems: Input-Output Fields, Memory, Monitoring
Principles of monitoring non-Markovian open quantum systems are analyzed. We
use the field representation of the environment (Gardiner and Collet, 1985) for
the separation of its memory and detector part, respectively. We claim the
system-plus-memory compound becomes Markovian, the detector part is tractable
by standard Markovian monitoring. Because of non-Markovianity, only the mixed
state of the system can be predicted, the pure state of the system can be
retrodicted. We present the corresponding non-Markovian stochastic
Schr\"odinger equation.Comment: 5 pages, 3 postscript figures; version with brief important
improvement
ScotPID - a model of collaboration
ScotPID is a national personal development initiative in Scotland, with thirteen higher education institutions taking part in the development of case studies which enhance personal development planning for students. As a model of collaboration, ScotPID involves all stakeholders: each core project group is composed of an academic, IT support manager, careers service adviser and undergraduate student, with support from QAA Scotland. The case study is developed by the contribution of all of the members of the team. The strength of the ScotPID collaboration is the varied background of the team members. However, collaboration between the ScotPID teams should also be encouraged, to strengthen the inter-institutional approach further
On the optimal feedback control of linear quantum systems in the presence of thermal noise
We study the possibility of taking bosonic systems subject to quadratic
Hamiltonians and a noisy thermal environment to non-classical stationary states
by feedback loops based on weak measurements and conditioned linear driving. We
derive general analytical upper bounds for the single mode squeezing and
multimode entanglement at steady state, depending only on the Hamiltonian
parameters and on the number of thermal excitations of the bath. Our findings
show that, rather surprisingly, larger number of thermal excitations in the
bath allow for larger steady-state squeezing and entanglement if the efficiency
of the optimal continuous measurements conditioning the feedback loop is high
enough. We also consider the performance of feedback strategies based on
homodyne detection and show that, at variance with the optimal measurements, it
degrades with increasing temperature.Comment: 10 pages, 2 figures. v2: minor changes to the letter; better
explanation of the necessary and sufficient conditions to achieve the bounds
(in the supplemental material); v3: title changed; comparison between optimal
general-dyne strategy and homodyne strategy is discussed; supplemental
material included in the manuscript and few references added. v4: published
versio
Phase dynamics of a multimode Bose condensate controlled by decay
The relative phase between two uncoupled BE condensates tends to attain a
specific value when the phase is measured. This can be done by observing their
decay products in interference. We discuss exactly solvable models for this
process in cases where competing observation channels drive the phases to
different sets of values. We treat the case of two modes which both emit into
the input ports of two beam splitters, and of a linear or circular chain of
modes. In these latter cases, the transitivity of relative phase becomes an
issue
Number-Phase Wigner Representation for Scalable Stochastic Simulations of Controlled Quantum Systems
Simulation of conditional master equations is important to describe systems
under continuous measurement and for the design of control strategies in
quantum systems. For large bosonic systems, such as BEC and atom lasers, full
quantum field simulations must rely on scalable stochastic methods whose
convergence time is restricted by the use of representations based on coherent
states. Here we show that typical measurements on atom-optical systems have a
common form that allows for an efficient simulation using the number-phase
Wigner (NPW) phase-space representation. We demonstrate that a stochastic
method based on the NPW can converge over an order of magnitude longer and more
precisely than its coherent equivalent. This opens the possibility of realistic
simulations of controlled multi-mode quantum systems.Comment: 5 pages, 1 figur
Dissipation in a rotating frame: master equation, effective temperature and Lamb-shift
Motivated by recent realizations of microwave-driven nonlinear resonators in
superconducting circuits, the impact of environmental degrees of freedom is
analyzed as seen from a rotating frame. A system plus reservoir model is
applied to consistently derive in the weak coupling limit the master equation
for the reduced density in the moving frame and near the first bifurcation
threshold. It turns out that additional interactions between momenta of system
and bath appear which have been omitted in previous studies. Explicit
expressions for the effective temperature and the Lamb-shift are given which
for ohmic baths are in agreement with experimental findings, while for
structured environments population inversion is predicted that may
qualitatively explain recent observations.Comment: 7 pages, 5 figure
All-optical versus electro-optical quantum-limited feedback
All-optical feedback can be effected by putting the output of a source cavity
through a Faraday isolator and into a second cavity which is coupled to the
source cavity by a nonlinear crystal. If the driven cavity is heavily damped,
then it can be adiabatically eliminated and a master equation or quantum
Langevin equation derived for the first cavity alone. This is done for an input
bath in an arbitrary state, and for an arbitrary nonlinear coupling. If the
intercavity coupling involves only the intensity (or one quadrature) of the
driven cavity, then the effect on the source cavity is identical to that which
can be obtained from electro-optical feedback using direct (or homodyne)
detection. If the coupling involves both quadratures, this equivalence no
longer holds, and a coupling linear in the source amplitude can produce a
nonclassical state in the source cavity. The analogous electro-optic scheme
using heterodyne detection introduces extra noise which prevents the production
of nonclassical light. Unlike the electro-optic case, the all-optical feedback
loop has an output beam (reflected from the second cavity). We show that this
may be squeezed, even if the source cavity remains in a classical state.Comment: 21 pages. This is an old (1994) paper, but one which I thought was
worth posting because in addition to what is described in abstract it has:
(1) the first formulation (to my knowledge) of quantum trajectories for an
arbitrary (i.e. squeezed, thermal etc.) broadband bath; (2) the prediction of
a periodic modification to the detuning and damping of an oscillator for the
simplest sort of all-optical feedback (i.e. a mirror) as seen in the recent
experiment "Forces between a Single Atom and Its Distant Mirror Image", P.
Bushev et al, Phys. Rev. Lett. 92, 223602 (2004
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