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