190 research outputs found
A model of quantum collapse induced by gravity
We discuss a model where a spontaneous quantum collapse is induced by the
gravitational interaction, treated classically. Its dynamics couples the
standard wave function of a system with the Bohmian positions of its particles,
which are considered as the only source of the gravitational attraction. The
collapse is obtained by adding a small imaginary component to the gravitational
coupling. It predicts extremely small perturbations of microscopic systems, but
very fast collapse of QSMDS (quantum superpositions of macroscopically distinct
quantum states) of a solid object, varying as the fifth power of its size. The
model does not require adding any dimensional constant to those of standard
physics.Comment: Version pubished in EPJ
Quantum properties of a single beam splitter
When a single beam-splitter receives two beams of bosons described by Fock
states (Bose-Einstein condensates at very low temperatures), interesting
generalizations of the two-photon Hong-Ou-Mandel effect take place for larger
number of particles. The distributions of particles at two detectors behind the
beam splitter can be understood as resulting from the combination of two
effects, the spontaneous phase appearing during quantum measurement, and the
quantum angle. The latter introduces quantum "population oscillations", which
can be seen as a generalized Hong-Ou-Mandel effect, although they do not always
correspond to even-odd oscillations.Comment: 14 pages, 11 figure
Angular momentum conservation in measurements on spin Bose-Einstein condensates
We discuss a thought experiment where two operators, Alice and Bob, perform
transverse spin measurements on a quantum system; this system is initially in a
double Fock spin state, which extends over a large distance in space so that
the two operators are far away from each other. Standard quantum mechanics
predicts that, when Alice makes a few measurements, a large transverse
component of the spin angular momentum may appear in Bob's laboratory. A
paradox then arises since local angular momentum conservation seems to be
violated. It has been suggested that this angular momentum may be provided by
the interaction with the measurement apparatuses. We show that this solution of
the paradox is not appropriate, so that another explanation must be sought. The
general question is the retroaction of a quantum system onto a measurement
apparatus. For instance, when the measured system is entangled with another
quantum system, can its reaction on a measurement apparatus be completely
changed? Is angular momentum conserved only on average over several
measurements, but not during one realization of the experiment?Comment: 11 pages, 3 figure
Beyond spontaneously broken symmetry in Bose-Einstein condensates
Spontaneous symmetry breaking (SSB) for Bose-Einstein condensates cannot
treat phase off-diagonal effects, and thus not explain Bell inequality
violations. We describe another situation that is beyond a SSB treatment: an
experiment where particles from two (possibly macroscopic) condensate sources
are used for conjugate measurements of the relative phase and populations.
Off-diagonal phase effects are characterized by a "quantum angle" and observed
via "population oscillations", signaling quantum interference of
macroscopically distinct states (QIMDS).Comment: 10 pages 4 figure
Nonlocal appearance of a macroscopic angular momentum
We discuss a type of measurement in which a macroscopically large angular
momentum (spin) is "created" nonlocally by the measurement of just a few atoms
from a double Fock state. This procedure apparently leads to a blatant
nonconservation of a macroscopic variable - the local angular momentum. We
argue that while this gedankenexperiment provides a striking illustration of
several counter-intuitive features of quantum mechanics, it does not imply a
non-local violation of the conservation of angular momentum.Comment: 10 pages, 1 figur
Ursell Operators in Statistical Physics III: thermodynamic properties of degenerate gases
We study in more detail the properties of the generalized Beth Uhlenbeck
formula obtained in a preceding article. This formula leads to a simple
integral expression of the grand potential of the system, where the interaction
potential appears only through the matrix elements of the second order Ursell
operator . Our results remain valid for significant degree of degeneracy
of the gas, but not when Bose Einstein (or BCS) condensation is reached, or
even too close from this transition point. We apply them to the study of the
thermodynamic properties of degenerate quantum gases: equation of state,
magnetic susceptibility, effects of exchange between bound states and free
particles, etc. We compare our predictions to those obtained within other
approaches, especially the ``pseudo potential'' approximation, where the real
potential is replaced by a potential with zero range (Dirac delta function).
This comparison is conveniently made in terms of a temperature dependent
quantity, the ``Ursell length'', which we define in the text. This length plays
a role which is analogous to the scattering length for pseudopotentials, but it
is temperature dependent and may include more physical effects than just binary
collision effects; for instance at very low temperatures it may change sign or
increase almost exponentially, an effect which is reminiscent of a precursor of
the BCS pairing transition. As an illustration, numerical results for quantum
hard spheres are given.Comment: 26 pages, 4 figures, LaTeX (amssymb), slight changes to first versio
Cumulative identical spin rotation effects in collisionless trapped atomic gases
We discuss the strong spin segregation in a dilute trapped Fermi gas recently
observed by Du et al. with "anomalous" large time scale and amplitude. In a
collisionless regime, the atoms oscillate rapidly in the trap and average the
inhomogeneous external field in an energy dependent way, which controls their
transverse spin precession frequency. During interactions between atoms with
different spin directions, the identical spin rotation effect (ISRE) transfers
atoms to the up or down spin state, depending on their motional energy. Since
low energy atoms are closer to the center of the trap than high energy atoms,
the final outcome is a strong correlation between spins and positions.Comment: 4 pages, 2 figures; v2: comparison to experimental data adde
Surrealistic Bohmian trajectories do not occur with macroscopic pointers
We discuss whether position measurements in quantum mechanics can be
contradictory with Bohmian trajectories, leading to what has been called
\textquotedblleft surrealistic trajectories\textquotedblright\ in the
literature. Previous work has considered that a single Bohmian position can be
ascribed to the pointer. Nevertheless, a correct treatment of a macroscopic
pointer requires that many particle positions should be included in the
dynamics of the system, and that statistical averages should be made over their
random initial values. Using numerical as well as analytical calculations, we
show that these surrealistic trajectories exist only if the pointer contains a
small number of particles; they completely disappear with macroscopic pointers.
With microscopic pointers, non-local effects of quantum entanglement can
indeed take place and introduce unexpected trajectories, as in Bell
experiments; moreover, the initial values of the Bohmian positions associated
with the measurement apparatus may influence the trajectory of the test
particle, and determine the result of measurement. Nevertheless, a detailed
observation of the trajectories of the particles of the pointer can still
reveal the nature of the trajectory of the test particle; nothing looks
surrealistic if all trajectories are properly interpreted.Comment: 22 pages, 12 figure
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