4,866 research outputs found
Quantum Spin Dimers from Chiral Dissipation in Cold-Atom Chains
We consider the non-equilibrium dynamics of a driven dissipative spin chain
with chiral coupling to a 1D bosonic bath, and its atomic implementation with a
two-species mixture of cold quantum gases. The reservoir is represented by a
spin-orbit coupled 1D quasi-condensate of atoms in a magnetized phase, while
the spins are identified with motional states of a separate species of atoms in
an optical lattice. The chirality of reservoir excitations allows the spins to
couple differently to left and right moving modes, which in our atomic setup
can be tuned from bidirectional to purely unidirectional. Remarkably, this
leads to a pure steady state in which pairs of neighboring spins form dimers
that decouple from the remainder of the chain. Our results also apply to
current experiments with two-level emitters coupled to photonic waveguides.Comment: Replaced by published version (6 pages + 8 pages supplemental
material
Searching for a trail of evidence in a maze
Consider a graph with a set of vertices and oriented edges connecting pairs
of vertices. Each vertex is associated with a random variable and these are
assumed to be independent. In this setting, suppose we wish to solve the
following hypothesis testing problem: under the null, the random variables have
common distribution N(0,1) while under the alternative, there is an unknown
path along which random variables have distribution , , and
distribution N(0,1) away from it. For which values of the mean shift can
one reliably detect and for which values is this impossible? Consider, for
example, the usual regular lattice with vertices of the form and oriented edges , where . We show that for paths of length starting at
the origin, the hypotheses become distinguishable (in a minimax sense) if
, while they are not if . We derive
equivalent results in a Bayesian setting where one assumes that all paths are
equally likely; there, the asymptotic threshold is . We
obtain corresponding results for trees (where the threshold is of order 1 and
independent of the size of the tree), for distributions other than the Gaussian
and for other graphs. The concept of the predictability profile, first
introduced by Benjamini, Pemantle and Peres, plays a crucial role in our
analysis.Comment: Published in at http://dx.doi.org/10.1214/07-AOS526 the Annals of
Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical
Statistics (http://www.imstat.org
Ground states in the Many Interacting Worlds approach
Recently the Many-Interacting-Worlds (MIW) approach to a quantum theory
without wave functions was proposed. This approach leads quite naturally to
numerical integrators of the Schr\"odinger equation. It has been suggested that
such integrators may feature advantages over fixed-grid methods for higher
numbers of degrees of freedom. However, as yet, little is known about concrete
MIW models for more than one spatial dimension and/or more than one particle.
In this work we develop the MIW approach further to treat arbitrary degrees of
freedom, and provide a systematic study of a corresponding numerical
implementation for computing one-particle ground and excited states in one
dimension, and ground states in two spatial dimensions. With this step towards
the treatment of higher degrees of freedom we hope to stimulate their further
study.Comment: 16 pages, 8 figure
High Curie temperatures in (Ga,Mn)N from Mn clustering
The effect of microscopic Mn cluster distribution on the Curie temperature
(Tc) is studied using density-functional calculations. We find that the
calculated Tc depends crucially on the microscopic cluster distribution, which
can explain the abnormally large variations in experimental Tc values from a
few K to well above room temperature. The partially dimerized Mn_2-Mn_1
distribution is found to give the highest Tc > 500 K, and in general, the
presence of the Mn_2 dimer has a tendency to enhance Tc. The lowest Tc values
close to zero are obtained for the Mn_4-Mn_1 and Mn_4-Mn_3 distributions.Comment: To appear in Applied Phyiscs Letter
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