526 research outputs found
Momentum spectroscopy of 1D phase fluctuations in Bose-Einstein condensates
We measure the axial momentum distribution of Bose-Einstein condensates with
an aspect ratio of 152 using Bragg spectroscopy. We observe the Lorentzian
momentum distribution characteristic of one-dimensional phase fluctuations. The
temperature dependence of the width of this distribution provides a
quantitative test of quasi-condensate theory. In addition, we observe a
condensate length consistent with the absence of density fluctuations, even
when phase fluctuations are large.Comment: 4 pages, 3 figures; submitted to Phys. Rev. Let
Lorentz-covariant quantum mechanics and preferred frame
In this paper the relativistic quantum mechanics is considered in the
framework of the nonstandard synchronization scheme for clocks. Such a
synchronization preserves Poincar{\'e} covariance but (at least formally)
distinguishes an inertial frame. This enables to avoid the problem of a
noncausal transmision of information related to breaking of the Bell's
inequalities in QM. Our analysis has been focused mainly on the problem of
existence of a proper position operator for massive particles. We have proved
that in our framework such an operator exists for particles with arbitrary
spin. It fulfills all the requirements: it is Hermitean and covariant, it has
commuting components and moreover its eigenvectors (localised states) are also
covariant. We have found the explicit form of the position operator and have
demonstrated that in the preferred frame our operator coincides with the
Newton--Wigner one. We have also defined a covariant spin operator and have
constructed an invariant spin square operator. Moreover, full algebra of
observables consisting of position operators, fourmomentum operators and spin
operators is manifestly Poincar\'e covariant in this framework. Our results
support expectations of other authors (Bell, Eberhard) that a consistent
formulation of quantum mechanics demands existence of a preferred frame.Comment: 21 pages, LaTeX file, no figure
Testing Bell's inequality using Aharonov-Casher effect
We propose the Aharonov-Casher (AC) effect for four entangled spin-half
particles carrying magnetic moments in the presence of impenetrable line
charge. The four particle state undergoes AC phase shift in two causually
disconnected region which can show up in the correlations between different
spin states of distant particles. This correlation can violate Bell's
inequality, thus displaying the non-locality for four particle entangled states
in an objective way. Also, we have suggested how to control the AC phase shift
locally at two distant locations to test Bell's inequality. We belive that
although the single particle AC effect may not be non-local but the entangled
state AC effect is a non-local one.Comment: Latex, 6 pages, no figures, submitted to Phys. Rev.
One-dimensional behavior of elongated Bose-Einstein condensates
We study the properties of elongated Bose-Einstein condensates. First, we
show that the dimensions of the condensate after expansion differs from the 3D
Thomas-Fermi regime. We also study the coherence length of such elongated
condensates.Comment: proceeding of Quantum Gases in Low Dimension, Les Houches 2003, 8
pages, 5 figure
Reality in quantum mechanics, Extended Everett Concept, and consciousness
Conceptual problems in quantum mechanics result from the specific quantum
concept of reality and require, for their solution, including the observer's
consciousness into quantum theory of measurements. Most naturally this is
achieved in the framework of Everett's "many-worlds interpretation" of quantum
mechanics. According to this interpretation, various classical alternatives are
perceived by consciousness separately from each other. In the Extended Everett
Concept (EEC) proposed by the present author, the separation of the
alternatives is identified with the phenomenon of consciousness. This explains
classical character of the alternatives and unusual manifestations of
consciousness arising "at the edge of consciousness" (i.e. in sleep or trance)
when its access to "other alternative classical realities" (other Everett's
worlds) becomes feasible. Because of reversibility of quantum evolution in EEC,
all time moments in the quantum world are equivalent while the impression of
flow of time appears only in consciousness. If it is assumed that consciousness
may influence onto probabilities of alternatives (which is consistent in case
of infinitely many Everett's worlds), EEC explains free will, "probabilistic
miracles" (observing low-probability events) and decreasing entropy in the
sphere of life.Comment: 17 pages, 2 figures in EP
The critical temperature of a trapped, weakly interacting Bose gas
We report on measurements of the critical temperature of a harmonically
trapped, weakly interacting Bose gas as a function of atom number. Our results
exclude ideal-gas behavior by more than two standard deviations, and agree
quantitatively with mean-field theory. At our level of sensitivity, we find no
additional shift due to critical fluctuations. In the course of this
measurement, the onset of hydrodynamic expansion in the thermal component has
been observed. Our thermometry method takes this feature into account.Comment: version 2, 20 octobre 200
Field fluctuations near a conducting plate and Casimir-Polder forces in the presence of boundary conditions
We consider vacuum fluctuations of the quantum electromagnetic field in the
presence of an infinite and perfectly conducting plate. We evaluate how the
change of vacuum fluctuations due to the plate modifies the Casimir-Polder
potential between two atoms placed near the plate. We use two different methods
to evaluate the Casimir-Polder potential in the presence of the plate. They
also give new insights on the role of boundary conditions in the Casimir-Polder
interatomic potential, as well as indications for possible generalizations to
more complicated boundary conditions.Comment: 10 page
Observation of atom pairs in spontaneous four wave mixing of two colliding Bose-Einstein Condensates
We study atom scattering from two colliding Bose-Einstein condensates using a
position sensitive, time resolved, single atom detector. In analogy to quantum
optics, the process can also be thought of as spontaneous, degenerate four wave
mixing of de Broglie waves. We find a clear correlation between atoms with
opposite momenta, demonstrating pair production in the scattering process. We
also observe a Hanbury Brown and Twiss correlation for collinear momenta, which
permits an independent measurement of the size of the pair production source
and thus the size of the spatial mode. The back to back pairs occupy very
nearly two oppositely directed spatial modes, a promising feature for future
quantum optics experiments.Comment: A few typos have been correcte
Dynamical Cooling of Trapped Gases I: One Atom Problem
We study the laser cooling of one atom in an harmonic trap beyond the
Lamb-Dicke regime. By using sequences of laser pulses of different detunings we
show that the atom can be confined into just one state of the trap, either the
ground state or an excited state of the harmonic potential. The last can be
achieved because under certain conditions an excited state becomes a dark
state. We study the problem in one and two dimensions. For the latter case a
new cooling mechanism is possible, based on the destructive interference
between the effects of laser fields in different directions, which allows the
creation of variety of dark states. For both, one and two dimensional cases,
Monte Carlo simulations of the cooling dynamics are presented.Comment: LaTeX file with 8 pages, 7 eps figures. Submitted to Phys. Rev.
Bell inequality and the locality loophole: Active versus passive switches
All experimental tests of the violation of Bell's inequality suffer from some
loopholes. We show that the locality loophole is not independent of the
detection loophole: in experiments using low efficient detectors, the locality
loophole can be closed equivalently using active or passive switches.Comment: 6 pages, 1 figur
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