1,618 research outputs found
p-Wave Optical Feshbach Resonances in Yb-171
We study the use of an optical Feshbach resonance to modify the p-wave
interaction between ultracold polarized Yb-171 spin-1/2 fermions. A laser
exciting two colliding atoms to the 1S_0 + 3P_1 channel can be detuned near a
purely-long-range excited molecular bound state. Such an exotic molecule has an
inner turning point far from the chemical binding region and thus
three-body-recombination in the Feshbach resonance will be highly suppressed in
contrast to that typically seen in a ground state p-wave magnetic Feshbach
resonance. We calculate the excited molecular bound-state spectrum using a
multichannel integration of the Schr\"{o}dinger equation, including an external
perturbation by a magnetic field. From the multichannel wave functions, we
calculate the Feshbach resonance properties, including the modification of the
elastic p-wave scattering volume and inelastic spontaneous scattering rate. The
use of magnetic fields and selection rules for polarized light yields a highly
controllable system. We apply this control to propose a toy model for
three-color superfluidity in an optical lattice for spin-polarized Yb-171,
where the three colors correspond to the three spatial orbitals of the first
excited p-band. We calculate the conditions under which tunneling and on-site
interactions are comparable, at which point quantum critical behavior is
possible.Comment: 8 pages, 4 figure
Sideband cooling while preserving coherences in the nuclear spin state in group-II-like atoms
We propose a method for laser cooling group-II-like atoms without changing
the quantum state of their nuclear spins, thus preserving coherences that are
usually destroyed by optical pumping. As group-II-like atoms have a
closed-shell ground state, nuclear spin and electronic degrees of freedom are
decoupled, allowing for independent manipulation. The hyperfine interaction
that couples these degrees of freedom in excited states can be suppressed
through the application of external magnetic fields. Our protocol employs
resolved-sideband cooling on the forbidden clock transition, , with quenching via coupling to the rapidly decaying state,
deep in the Paschen-Back regime. This makes it possible to laser cool neutral
atomic qubits without destroying the quantum information stored in their
nuclear spins, as shown in two examples, Yb and Sr.Comment: 4 pages, 3 figures v4: minor changes in text, changes in the
references, published versio
Noise and Correlations in a Spatial Population Model with Cyclic Competition
Noise and spatial degrees of freedom characterize most ecosystems. Some
aspects of their influence on the coevolution of populations with cyclic
interspecies competition have been demonstrated in recent experiments [e.g. B.
Kerr et al., Nature {\bf 418}, 171 (2002)]. To reach a better theoretical
understanding of these phenomena, we consider a paradigmatic spatial model
where three species exhibit cyclic dominance. Using an individual-based
description, as well as stochastic partial differential and deterministic
reaction-diffusion equations, we account for stochastic fluctuations and
spatial diffusion at different levels, and show how fascinating patterns of
entangled spirals emerge. We rationalize our analysis by computing the
spatio-temporal correlation functions and provide analytical expressions for
the front velocity and the wavelength of the propagating spiral waves.Comment: 4 pages of main text, 3 color figures + 2 pages of supplementary
material (EPAPS Document). Final version for Physical Review Letter
Effects and Propositions
The quantum logical and quantum information-theoretic traditions have exerted
an especially powerful influence on Bub's thinking about the conceptual
foundations of quantum mechanics. This paper discusses both the quantum logical
and information-theoretic traditions from the point of view of their
representational frameworks. I argue that it is at this level, at the level of
its framework, that the quantum logical tradition has retained its centrality
to Bub's thought. It is further argued that there is implicit in the quantum
information-theoretic tradition a set of ideas that mark a genuinely new
alternative to the framework of quantum logic. These ideas are of considerable
interest for the philosophy of quantum mechanics, a claim which I defend with
an extended discussion of their application to our understanding of the
philosophical significance of the no hidden variable theorem of Kochen and
Specker.Comment: Presented to the 2007 conference, New Directions in the Foundations
of Physic
A model for bidirectional traffic of cytoskeletal motors
We introduce a stochastic lattice gas model including two particle species
and two parallel lanes. One lane with exclusion interaction and directed motion
and the other lane without exclusion and unbiased diffusion, mimicking a
micotubule filament and the surrounding solution. For a high binding affinity
to the filament, jam-like situations dominate the system's behaviour. The
fundamental process of position exchange of two particles is approximated. In
the case of a many-particle system, we were able to identify a regime in which
the system is rather homogenous presenting only small accumulations of
particles and a regime in which an important fraction of all particles
accumulates in the same cluster. Numerical data proposes that this cluster
formation will occur at all densities for large system sizes. Coupling of
several filaments leads to an enhanced cluster formation compared to the
uncoupled system, suggesting that efficient bidirectional transport on
one-dimensional filaments relies on long-ranged interactions and track
formation.Comment: 20 pages, 9 figure
On the entropy production of time series with unidirectional linearity
There are non-Gaussian time series that admit a causal linear autoregressive
moving average (ARMA) model when regressing the future on the past, but not
when regressing the past on the future. The reason is that, in the latter case,
the regression residuals are only uncorrelated but not statistically
independent of the future. In previous work, we have experimentally verified
that many empirical time series indeed show such a time inversion asymmetry.
For various physical systems, it is known that time-inversion asymmetries are
linked to the thermodynamic entropy production in non-equilibrium states. Here
we show that such a link also exists for the above unidirectional linearity.
We study the dynamical evolution of a physical toy system with linear
coupling to an infinite environment and show that the linearity of the dynamics
is inherited to the forward-time conditional probabilities, but not to the
backward-time conditionals. The reason for this asymmetry between past and
future is that the environment permanently provides particles that are in a
product state before they interact with the system, but show statistical
dependencies afterwards. From a coarse-grained perspective, the interaction
thus generates entropy. We quantitatively relate the strength of the
non-linearity of the backward conditionals to the minimal amount of entropy
generation.Comment: 16 page
Dynamics at barriers in bidirectional two-lane exclusion processes
A two-lane exclusion process is studied where particles move in the two lanes
in opposite directions and are able to change lanes. The focus is on the steady
state behavior in situations where a positive current is constrained to an
extended subsystem (either by appropriate boundary conditions or by the
embedding environment) where, in the absence of the constraint, the current
would be negative. We have found two qualitatively different types of steady
states and formulated the conditions of them in terms of the transition rates.
In the first type of steady state, a localized cluster of particles forms with
an anti-shock located in the subsystem and the current vanishes exponentially
with the extension of the subsystem. This behavior is analogous to that of the
one-lane partially asymmetric simple exclusion process, and can be realized
e.g. when the local drive is induced by making the jump rates in two lanes
unequal. In the second type of steady state, which is realized e.g. if the
local drive is induced purely by the bias in the lane change rates, and which
has thus no counterpart in the one-lane model, a delocalized cluster of
particles forms which performs a diffusive motion as a whole and, as a
consequence, the current vanishes inversely proportionally to the extension of
the subsystem. The model is also studied in the presence of quenched
disordered, where, in case of delocalization, phenomenological considerations
predict anomalously slow, logarithmic decay of the current with the system size
in contrast with the usual power-law.Comment: 24 pages, 13 figure
Parallel Coupling of Symmetric and Asymmetric Exclusion Processes
A system consisting of two parallel coupled channels where particles in one
of them follow the rules of totally asymmetric exclusion processes (TASEP) and
in another one move as in symmetric simple exclusion processes (SSEP) is
investigated theoretically. Particles interact with each other via hard-core
exclusion potential, and in the asymmetric channel they can only hop in one
direction, while on the symmetric lattice particles jump in both directions
with equal probabilities. Inter-channel transitions are also allowed at every
site of both lattices. Stationary state properties of the system are solved
exactly in the limit of strong couplings between the channels. It is shown that
strong symmetric couplings between totally asymmetric and symmetric channels
lead to an effective partially asymmetric simple exclusion process (PASEP) and
properties of both channels become almost identical. However, strong asymmetric
couplings between symmetric and asymmetric channels yield an effective TASEP
with nonzero particle flux in the asymmetric channel and zero flux on the
symmetric lattice. For intermediate strength of couplings between the lattices
a vertical cluster mean-field method is developed. This approximate approach
treats exactly particle dynamics during the vertical transitions between the
channels and it neglects the correlations along the channels. Our calculations
show that in all cases there are three stationary phases defined by particle
dynamics at entrances, at exits or in the bulk of the system, while phase
boundaries depend on the strength and symmetry of couplings between the
channels. Extensive Monte Carlo computer simulations strongly support our
theoretical predictions.Comment: 16 page
Influence of local carrying capacity restrictions on stochastic predator-prey models
We study a stochastic lattice predator-prey system by means of Monte Carlo
simulations that do not impose any restrictions on the number of particles per
site, and discuss the similarities and differences of our results with those
obtained for site-restricted model variants. In accord with the classic
Lotka-Volterra mean-field description, both species always coexist in two
dimensions. Yet competing activity fronts generate complex, correlated
spatio-temporal structures. As a consequence, finite systems display transient
erratic population oscillations with characteristic frequencies that are
renormalized by fluctuations. For large reaction rates, when the processes are
rendered more local, these oscillations are suppressed. In contrast with
site-restricted predator-prey model, we observe species coexistence also in one
dimension. In addition, we report results on the steady-state prey age
distribution.Comment: Latex, IOP style, 17 pages, 9 figures included, related movies
available at http://www.phys.vt.edu/~tauber/PredatorPrey/movies
Spontaneous symmetry breaking in a two-lane model for bidirectional overtaking traffic
First we consider a unidirectional flux \omega_bar of vehicles each of which
is characterized by its `natural' velocity v drawn from a distribution P(v).
The traffic flow is modeled as a collection of straight `world lines' in the
time-space plane, with overtaking events represented by a fixed queuing time
tau imposed on the overtaking vehicle. This geometrical model exhibits platoon
formation and allows, among many other things, for the calculation of the
effective average velocity w=\phi(v) of a vehicle of natural velocity v.
Secondly, we extend the model to two opposite lanes, A and B. We argue that the
queuing time \tau in one lane is determined by the traffic density in the
opposite lane. On the basis of reasonable additional assumptions we establish a
set of equations that couple the two lanes and can be solved numerically. It
appears that above a critical value \omega_bar_c of the control parameter
\omega_bar the symmetry between the lanes is spontaneously broken: there is a
slow lane where long platoons form behind the slowest vehicles, and a fast lane
where overtaking is easy due to the wide spacing between the platoons in the
opposite direction. A variant of the model is studied in which the spatial
vehicle density \rho_bar rather than the flux \omega_bar is the control
parameter. Unequal fluxes \omega_bar_A and \omega_bar_B in the two lanes are
also considered. The symmetry breaking phenomenon exhibited by this model, even
though no doubt hard to observe in pure form in real-life traffic, nevertheless
indicates a tendency of such traffic.Comment: 50 pages, 16 figures; extra references adde
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