6,670 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
Evaluation of heating effects on atoms trapped in an optical trap
We solve a stochastic master equation based on the theory of Savard et al. [T. A. Savard. K. M. O'Hara, and J. E. Thomas, Phys, Rev. A 56, R1095 (1997)] for heating arising from fluctuations in the trapping laser intensity. We compare with recent experiments of Ye et al. [J. Ye, D. W. Vernooy, and H. J. Kimble, Phys. Rev. Lett. 83, 4987 (1999)], and find good agreement with the experimental measurements of the distribution of trap occupancy times. The major cause of trap loss arises from the broadening of the energy distribution of the trapped atom, rather than the mean heating rate, which is a very much smaller effect
Simulation of complete many-body quantum dynamics using controlled quantum-semiclassical hybrids
A controlled hybridization between full quantum dynamics and semiclassical
approaches (mean-field and truncated Wigner) is implemented for interacting
many-boson systems. It is then demonstrated how simulating the resulting hybrid
evolution equations allows one to obtain the full quantum dynamics for much
longer times than is possible using an exact treatment directly. A collision of
sodium BECs with 1.x10^5 atoms is simulated, in a regime that is difficult to
describe semiclassically. The uncertainty of physical quantities depends on the
statistics of the full quantum prediction. Cutoffs are minimised to a
discretization of the Hamiltonian. The technique presented is quite general and
extension to other systems is considered.Comment: Published version. Broader background and discussion, slightly
shortened, less figures in epaps. Research part unchanged. Article + epaps
(4+4 pages), 8 figure
Diagnosis of antiphospholipid syndrome in routine clinical practice.
The updated international consensus criteria for definite antiphospholipid syndrome (APS) are useful for scientific clinical studies. However, there remains a need for diagnostic criteria for routine clinical use. We audited the results of routine antiphospholipid antibodies (aPLs) in a cohort of 193 consecutive patients with aPL positivity-based testing for lupus anticoagulant (LA), IgG and IgM anticardiolipin (aCL) and anti-ß(2)glycoprotein-1 antibodies (aß(2)GPI). Medium/high-titre aCL/aβ(2)GPI was defined as >99th percentile. Low-titre aCL/aβ(2)GPI positivity (>95(th )< 99(th) percentile) was considered positive for obstetric but not for thrombotic APS. One hundred of the 145 patients fulfilled both clinical and laboratory criteria for definite APS. Twenty-six women with purely obstetric APS had persistent low-titre aCL and/or aβ(2)GPI. With the inclusion of these patients, 126 of the 145 patients were considered to have APS. Sixty-seven out of 126 patients were LA-negative, of whom 12 had aCL only, 37 had aβ(2)GPI only and 18 positive were for both. The omission of aCL or aβ(2)GPI testing from investigation of APS would have led to a failure to diagnose APS in 9.5% and 29.4% of patients, respectively. Our data suggest that LA, aCL and aβ(2)GPI testing are all required for the accurate diagnosis of APS and that low-titre antibodies should be included in the diagnosis of obstetric APS
Emergent classicality in continuous quantum measurements
We develop a classical theoretical description for nonlinear many-body
dynamics that incorporates the back-action of a continuous measurement process.
The classical approach is compared with the exact quantum solution in an
example with an atomic Bose-Einstein condensate in a double-well potential
where the atom numbers in both potential wells are monitored by light
scattering. In the classical description the back-action of the measurements
appears as diffusion of the relative phase of the condensates on each side of
the trap. When the measurements are frequent enough to resolve the system
dynamics, the system behaves classically. This happens even deep in the quantum
regime, and demonstrates how classical physics emerges from quantum mechanics
as a result of measurement back-action
Winding up by a quench: vortices in the wake of rapid Bose-Einstein condensation
A second order phase transition induced by a rapid quench can lock out
topological defects with densities far exceeding their equilibrium expectation
values. We use quantum kinetic theory to show that this mechanism, originally
postulated in the cosmological context, and analysed so far only on the mean
field classical level, should allow spontaneous generation of vortex lines in
trapped Bose-Einstein condensates of simple topology, or of winding number in
toroidal condensates.Comment: 4 pages, 2 figures; misprint correcte
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
Quantum Kinetic Theory VI: The Growth of a Bose-Einstein Condensate
A detailed analysis of the growth of a BEC is given, based on quantum kinetic
theory, in which we take account of the evolution of the occupations of lower
trap levels, and of the full Bose-Einstein formula for the occupations of
higher trap levels, as well as the Bose stimulated direct transfer of atoms to
the condensate level introduced by Gardiner et al. We find good agreement with
experiment at higher temperatures, but at lower temperatures the experimentally
observed growth rate is somewhat more rapid. We also confirm the picture of the
``kinetic'' region of evolution, introduced by Kagan et al., for the time up to
the initiation of the condensate. The behavior after initiation essentially
follows our original growth equation, but with a substantially increased rate
coefficient.
Our modelling of growth implicitly gives a model of the spatial shape of the
condensate vapor system as the condensate grows, and thus provides an
alternative to the present phenomenological fitting procedure, based on the sum
of a zero-chemical potential vapor and a Thomas-Fermi shaped condensate. Our
method may give substantially different results for condensate numbers and
temperatures obtained from phenomentological fits, and indicates the need for
more systematic investigation of the growth dynamics of the condensate from a
supersaturated vapor.Comment: TeX source; 29 Pages including 26 PostScript figure
Quantum feedback cooling of a single trapped ion in front of a mirror
We develop a theory of quantum feedback cooling of a single ion trapped in
front of a mirror. By monitoring the motional sidebands of the light emitted
into the mirror mode we infer the position of the ion, and act back with an
appropriate force to cool the ion. We derive a feedback master equation along
the lines of the quantum feedback theory developed by Wiseman and Milburn,
which provides us with cooling times and final temperatures as a function of
feedback gain and various system parameters.Comment: 15 pages, 11 Figure
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