1,171 research outputs found
Holstein model and Peierls instability in 1D boson-fermion lattice gases
We study an ultracold bose-fermi mixture in a one dimensional optical
lattice. When boson atoms are heavier then fermion atoms the system is
described by an adiabatic Holstein model, exhibiting a Peierls instability for
commensurate fermion filling factors. A Bosonic density wave with a wavenumber
of twice the Fermi wavenumber will appear in the quasi one-dimensional system.Comment: 5 pages, 4 figure
Incoherent matter-wave solitons
The dynamics of matter-wave solitons in Bose-Einstein condensates (BEC) is
considerably affected by the presence of a surrounding thermal cloud and by
condensate depletion during its evolution. We analyze these aspects of BEC
soliton dynamics, using time-dependent Hartree-Fock-Bogoliubov (TDHFB) theory.
The condensate is initially prepared within a harmonic trap at finite
temperature, and solitonic behavior is studied by subsequently propagating the
TDHFB equations without confinement. Numerical results demonstrate the collapse
of the BEC via collisional emission of atom pairs into the thermal cloud,
resulting in splitting of the initial density into two solitonic structures
with opposite momentum. Each one of these solitary matter waves is a mixture of
condensed and noncondensed particles, constituting an analog of optical
random-phase solitons.Comment: 4 pages, 2 figures, new TDHFB result
Squeezing in driven bimodal Bose-Einstein Condensates: Erratic driving versus noise
We study the interplay of squeezing and phase randomization near the
hyperbolic instability of a two-site Bose-Hubbard model in the Josephson
interaction regime. We obtain results for the quantum Zeno suppression of
squeezing, far beyond the previously found short time behavior. More
importantly, we contrast the expected outcome with the case where randomization
is induced by erratic driving with the same fluctuations as the quantum noise
source, finding significant differences. These are related to the distribution
of the squeezing factor, which has log-normal characteristics: hence its
average is significantly different from its median due to the occurrence of
rare events.Comment: 5 pages, 4 figure
Propositional Dynamic Logic with Converse and Repeat for Message-Passing Systems
The model checking problem for propositional dynamic logic (PDL) over message
sequence charts (MSCs) and communicating finite state machines (CFMs) asks,
given a channel bound , a PDL formula and a CFM ,
whether every existentially -bounded MSC accepted by
satisfies . Recently, it was shown that this problem is
PSPACE-complete.
In the present work, we consider CRPDL over MSCs which is PDL equipped with
the operators converse and repeat. The former enables one to walk back and
forth within an MSC using a single path expression whereas the latter allows to
express that a path expression can be repeated infinitely often. To solve the
model checking problem for this logic, we define message sequence chart
automata (MSCAs) which are multi-way alternating parity automata walking on
MSCs. By exploiting a new concept called concatenation states, we are able to
inductively construct, for every CRPDL formula , an MSCA precisely
accepting the set of models of . As a result, we obtain that the model
checking problem for CRPDL and CFMs is still in PSPACE
Biased tomography schemes: an objective approach
We report on an intrinsic relationship between the maximum-likelihood
quantum-state estimation and the representation of the signal. A quantum
analogy of the transfer function determines the space where the reconstruction
should be done without the need for any ad hoc truncations of the Hilbert
space. An illustration of this method is provided by a simple yet practically
important tomography of an optical signal registered by realistic binary
detectors.Comment: 4 pages, 3 figures, accepted in PR
Collisional shifts in optical-lattice atom clocks
We theoretically study the effects of elastic collisions on the determination
of frequency standards via Ramsey fringe spectroscopy in optical-lattice atom
clocks. Interparticle interactions of bosonic atoms in multiply-occupied
lattice sites can cause a linear frequency shift, as well as generate
asymmetric Ramsey fringe patterns and reduce fringe visibility due to
interparticle entanglement. We propose a method of reducing these collisional
effects in an optical lattice by introducing a phase difference of
between the Ramsey driving fields in adjacent sites. This configuration
suppresses site to site hopping due to interference of two tunneling pathways,
without degrading fringe visibility. Consequently, the probability of double
occupancy is reduced, leading to cancellation of collisional shifts.Comment: 15 pages, 11 figure
Coherently Controlled Nanoscale Molecular Deposition
Quantum interference effects are shown to provide a means of controlling and
enhancing the focusing a collimated neutral molecular beam onto a surface. The
nature of the aperiodic pattern formed can be altered by varying laser field
characteristics and the system geometry.Comment: 13 pages (inculding 4 figures), LaTeX (Phys. Rev. Lett., 2000, in
Press
Reconstruction of photon statistics using low performance photon counters
The output of a photodetector consists of a current pulse whose charge has
the statistical distribution of the actual photon numbers convolved with a
Bernoulli distribution. Photodetectors are characterized by a nonunit quantum
efficiency, i.e. not all the photons lead to a charge, and by a finite
resolution, i.e. a different number of detected photons leads to a
discriminable values of the charge only up to a maximum value. We present a
detailed comparison, based on Monte Carlo simulated experiments and real data,
among the performances of detectors with different upper limits of counting
capability. In our scheme the inversion of Bernoulli convolution is performed
by maximum-likelihood methods assisted by measurements taken at different
quantum efficiencies. We show that detectors that are only able to discriminate
between zero, one and more than one detected photons are generally enough to
provide a reliable reconstruction of the photon statistics for single-peaked
distributions, while detectors with higher resolution limits do not lead to
further improvements. In addition, we demonstrate that, for semiclassical
states, even on/off detectors are enough to provide a good reconstruction.
Finally, we show that a reliable reconstruction of multi-peaked distributions
requires either higher quantum efficiency or better capability in
discriminating high number of detected photons.Comment: 8 pages, 3 figure
Exact solution, scaling behaviour and quantum dynamics of a model of an atom-molecule Bose-Einstein condensate
We study the exact solution for a two-mode model describing coherent coupling
between atomic and molecular Bose-Einstein condensates (BEC), in the context of
the Bethe ansatz. By combining an asymptotic and numerical analysis, we
identify the scaling behaviour of the model and determine the zero temperature
expectation value for the coherence and average atomic occupation. The
threshold coupling for production of the molecular BEC is identified as the
point at which the energy gap is minimum. Our numerical results indicate a
parity effect for the energy gap between ground and first excited state
depending on whether the total atomic number is odd or even. The numerical
calculations for the quantum dynamics reveals a smooth transition from the
atomic to the molecular BEC.Comment: 5 pages, 4 figure
Verification of Hierarchical Artifact Systems
Data-driven workflows, of which IBM's Business Artifacts are a prime
exponent, have been successfully deployed in practice, adopted in industrial
standards, and have spawned a rich body of research in academia, focused
primarily on static analysis. The present work represents a significant advance
on the problem of artifact verification, by considering a much richer and more
realistic model than in previous work, incorporating core elements of IBM's
successful Guard-Stage-Milestone model. In particular, the model features task
hierarchy, concurrency, and richer artifact data. It also allows database key
and foreign key dependencies, as well as arithmetic constraints. The results
show decidability of verification and establish its complexity, making use of
novel techniques including a hierarchy of Vector Addition Systems and a variant
of quantifier elimination tailored to our context.Comment: Full version of the accepted PODS pape
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