308 research outputs found
Schr\"odinger cat state of a Bose-Einstein condensate in a double-well potential
We consider a weakly interacting coherently coupled Bose-Einstein condensate
in a double-well potential. We show by means of stochastic simulations that the
system could possibly be driven to an entangled macroscopic superposition state
or a Schr\"odinger cat state by means of a continuous quantum measurement
process.Comment: 6 pages; to be published in memorial volume for Dan Wall
Light propagation beyond the mean-field theory of standard optics
With ready access to massive computer clusters we may now study light
propagation in a dense cold atomic gas by means of basically exact numerical
simulations. We report on a direct comparison between traditional optics, that
is, electrodynamics of a polarizable medium, and numerical simulations in an
elementary problem of light propagating through a slab of matter. The standard
optics fails already at quite low atom densities, and the failure becomes
dramatic when the average interatomic separation is reduced to around ,
where is the wave number of resonant light. The difference between the two
solutions originates from correlations between the atoms induced by
light-mediated dipole-dipole interactions
Classical stochastic measurement trajectories: Bosonic atomic gases in an optical cavity and quantum measurement backaction
We formulate computationally efficient classical stochastic measurement
trajectories for a multimode quantum system under continuous observation.
Specifically, we consider the nonlinear dynamics of an atomic Bose-Einstein
condensate contained within an optical cavity subject to continuous monitoring
of the light leaking out of the cavity. The classical trajectories encode
within a classical phase-space representation a continuous quantum measurement
process conditioned on a given detection record. We derive a Fokker-Planck
equation for the quasi-probability distribution of the combined
condensate-cavity system. We unravel the dynamics into stochastic classical
trajectories that are conditioned on the quantum measurement process of the
continuously monitored system, and that these trajectories faithfully represent
measurement records of individual experimental runs. Since the dynamics of a
continuously measured observable in a many-atom system can be closely
approximated by classical dynamics, the method provides a numerically efficient
and accurate approach to calculate the measurement record of a large multimode
quantum system. Numerical simulations of the continuously monitored dynamics of
a large atom cloud reveal considerably fluctuating phase profiles between
different measurement trajectories, while ensemble averages exhibit local
spatially varying phase decoherence. Individual measurement trajectories lead
to spatial pattern formation and optomechanical motion that solely result from
the measurement backaction. The backaction of the continuous quantum
measurement process, conditioned on the detection record of the photons,
spontaneously breaks the symmetry of the spatial profile of the condensate and
can be tailored to selectively excite collective modes.Comment: 22 pages, 11 figure
Energetically stable singular vortex cores in an atomic spin-1 Bose-Einstein condensate
We analyze the structure and stability of singular singly quantized vortices in a rotating spin-1 Bose-Einstein condensate. We show that the singular vortex can be energetically stable in both the ferromagnetic and polar phases despite the existence of a lower-energy nonsingular coreless vortex in the ferromagnetic phase. The spin-1 system exhibits energetic hierarchy of length scales resulting from different interaction strengths and we find that the vortex cores deform to a larger size determined by the characteristic length scale of the spin-dependent interaction. We show that in the ferromagnetic phase the resulting stable core structure, despite apparent complexity, can be identified as a single polar core with everywhere nonvanishing axially symmetric density profile. In the polar phase, the energetically favored core deformation leads to a splitting of a singly quantized vortex into a pair of half-quantum vortices that preserves the topology of the vortex outside the extended core region, but breaks the axial symmetry of the core. The resulting half-quantum vortices exhibit nonvanishing ferromagnetic cores.<br/
Optical response of superfluid state in dilute atomic Fermi-Dirac gases
We theoretically study the propagation of light in a Fermi-Dirac gas in the
presence of a superfluid state. BCS pairing between atoms in different
hyperfine levels may significantly increase the optical linewidth and line
shift of a quantum degenerate Fermi-Dirac gas and introduce a local-field
correction that, under certain conditions, dramatically dominates over the
Lorentz-Lorenz shift. These optical properties could possibly unambiguously
sign the presence of the superfluid state and determine the value of the BCS
order parameter.Comment: 5 pages, 2 figure
Spontaneous photon emission stimulated by two Bose condensates
We show that the phase difference of two overlapping ground state
Bose-Einstein condensates can effect the optical spontaneous emission rate of
excited atoms. Depending on the phase difference the atom stimulated
spontaneous emission rate can vary between zero and the rate corresponding to
all the ground state atoms in a single condensate. Besides giving control over
spontaneous emission this provides an optical method for detecting the
condensate phase difference. It differs from previous methods in that no light
fields are applied. Instead the light is spontaneously emitted when excited
atoms make a transition into either condensate.Comment: 14 pages, 2 postscript figures, Revtex. Corrections and significant
additions in revisio
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