20,551 research outputs found
Polarization-controlled single photons
Vacuum-stimulated Raman transitions are driven between two magnetic substates
of a rubidium-87 atom strongly coupled to an optical cavity. A magnetic field
lifts the degeneracy of these states, and the atom is alternately exposed to
laser pulses of two different frequencies. This produces a stream of single
photons with alternating circular polarization in a predetermined
spatio-temporal mode. MHz repetition rates are possible as no recycling of the
atom between photon generations is required. Photon indistinguishability is
tested by time-resolved two-photon interference.Comment: 4 pages, 3 figure
A nonlinear model dynamics for closed-system, constrained, maximal-entropy-generation relaxation by energy redistribution
We discuss a nonlinear model for the relaxation by energy redistribution
within an isolated, closed system composed of non-interacting identical
particles with energy levels e_i with i=1,2,...,N. The time-dependent
occupation probabilities p_i(t) are assumed to obey the nonlinear rate
equations tau dp_i/dt=-p_i ln p_i+ alpha(t)p_i-beta(t)e_ip_i where alpha(t) and
beta(t) are functionals of the p_i(t)'s that maintain invariant the mean energy
E=sum_i e_ip_i(t) and the normalization condition 1=sum_i p_i(t). The entropy
S(t)=-k sum_i p_i(t) ln p_i(t) is a non-decreasing function of time until the
initially nonzero occupation probabilities reach a Boltzmann-like canonical
distribution over the occupied energy eigenstates. Initially zero occupation
probabilities, instead, remain zero at all times. The solutions p_i(t) of the
rate equations are unique and well-defined for arbitrary initial conditions
p_i(0) and for all times. Existence and uniqueness both forward and backward in
time allows the reconstruction of the primordial lowest entropy state. The time
evolution is at all times along the local direction of steepest entropy ascent
or, equivalently, of maximal entropy generation. These rate equations have the
same mathematical structure and basic features of the nonlinear dynamical
equation proposed in a series of papers ended with G.P.Beretta, Found.Phys.,
17, 365 (1987) and recently rediscovered in S. Gheorghiu-Svirschevski,
Phys.Rev.A, 63, 022105 and 054102 (2001). Numerical results illustrate the
features of the dynamics and the differences with the rate equations recently
considered for the same problem in M.Lemanska and Z.Jaeger, Physica D, 170, 72
(2002).Comment: 11 pages, 7 eps figures (psfrag use removed), uses subeqn, minor
revisions, accepted for Physical Review
A Unified Nanopublication Model for Effective and User-Friendly Access to the Elements of Scientific Publishing
Scientific publishing is the means by which we communicate and share
scientific knowledge, but this process currently often lacks transparency and
machine-interpretable representations. Scientific articles are published in
long coarse-grained text with complicated structures, and they are optimized
for human readers and not for automated means of organization and access. Peer
reviewing is the main method of quality assessment, but these peer reviews are
nowadays rarely published and their own complicated structure and linking to
the respective articles is not accessible. In order to address these problems
and to better align scientific publishing with the principles of the Web and
Linked Data, we propose here an approach to use nanopublications as a unifying
model to represent in a semantic way the elements of publications, their
assessments, as well as the involved processes, actors, and provenance in
general. To evaluate our approach, we present a dataset of 627 nanopublications
representing an interlinked network of the elements of articles (such as
individual paragraphs) and their reviews (such as individual review comments).
Focusing on the specific scenario of editors performing a meta-review, we
introduce seven competency questions and show how they can be executed as
SPARQL queries. We then present a prototype of a user interface for that
scenario that shows different views on the set of review comments provided for
a given manuscript, and we show in a user study that editors find the interface
useful to answer their competency questions. In summary, we demonstrate that a
unified and semantic publication model based on nanopublications can make
scientific communication more effective and user-friendly
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