458 research outputs found
Presence of temporal dynamical instabilities in topological insulator lasers
Topological insulator lasers are a newly introduced kind of lasers in which
light snakes around a cavity without scattering. Like for an electron current
in a topological insulator material, a topologically protected lasing mode
travels along the cavity edge, steering neatly around corners and imperfections
without scattering or leaking out. In a recent experiment, topological
insulator lasers have been demonstrated using a square lattice of coupled
semiconductor microring resonators with a synthetic magnetic field. However,
laser arrays with slow population dynamics are likely to show dynamical
instabilities in a wide range of parameter space corresponding to realistic
experimental conditions, thus preventing stable laser operation. While
topological insulator lasers provide an interesting mean for combating disorder
and help collective oscillation of lasers at the edge of the lattice, it is not
clear whether chiral edge states are immune to dynamical instabilities. In this
work we consider a realistic model of semiconductor class-B topological
insulator laser and show that chiral edge states are not immune to dynamical
instabilities.Comment: 7 pages, 6 figure
Quantum Zeno Effect Explains Magnetic-Sensitive Radical-Ion-Pair Reactions
Chemical reactions involving radical-ion pairs are ubiquitous in biology,
since not only are they at the basis of the photosynthetic reaction chain, but
are also assumed to underlie the biochemical magnetic compass used by avian
species for navigation. Recent experiments with magnetic-sensitive radical-ion
pair reactions provided strong evidence for the radical-ion-pair
magnetoreception mechanism, verifying the expected magnetic sensitivities and
chemical product yield changes. It is here shown that the theoretical
description of radical-ion-pair reactions used since the 70's cannot explain
the observed data, because it is based on phenomenological equations masking
quantum coherence effects. The fundamental density matrix equation derived here
from basic quantum measurement theory considerations naturally incorporates the
quantum Zeno effect and readily explains recent experimental observations on
low- and high-magnetic-field radical-ion-pair reactions.Comment: 10 pages, 5 figure
Anomalous diffusion in a symbolic model
We address this work to investigate some statistical properties of symbolic
sequences generated by a numerical procedure in which the symbols are repeated
following a power law probability density. In this analysis, we consider that
the sum of n symbols represents the position of a particle in erratic movement.
This approach revealed a rich diffusive scenario characterized by non-Gaussian
distributions and, depending on the power law exponent and also on the
procedure used to build the walker, we may have superdiffusion, subdiffusion or
usual diffusion. Additionally, we use the continuous-time random walk framework
to compare with the numerical data, finding a good agreement. Because of its
simplicity and flexibility, this model can be a candidate to describe real
systems governed by power laws probabilities densities.Comment: Accepted for publication in Physica Script
Observation of two-dimensional lattice interface solitons
We report on the experimental observation of two-dimensional solitons at the
interface between square and hexagonal waveguide arrays. In addition to the
different symmetry of the lattices, the influence of a varying refractive index
modulation depth is investigated. Such variation strongly affects the
properties of surface solitons residing at different sides of the interface.Comment: 14 pages, 5 figures, to appear in Optics Letter
Scalars from Top-condensation Models at Hadron Colliders
We study the production and decay of neutral scalars and pseudo-scalars at
hadron colliders, in theories where the top-quark mass is the result of a
condensate. We show that the dominant decay channel for masses below
the threshold is the flavor changing mode . This is a consequence
of the non-universal nature of the underlying interactions in all
top-condensation models and provides a model-independent signature of these
scenarios. We show that an upgraded Tevatron is sensitive to a sizeable region
of the interesting parameter space and that the LHC will highly constrain these
models through this flavor violating channel.Comment: 4 pages, 4 figures. Minor changes in figures for readibility. final
version to appear in PR
Interaction-based quantum metrology showing scaling beyond the Heisenberg limit
Quantum metrology studies the use of entanglement and other quantum resources
to improve precision measurement. An interferometer using N independent
particles to measure a parameter X can achieve at best the "standard quantum
limit" (SQL) of sensitivity {\delta}X \propto N^{-1/2}. The same interferometer
using N entangled particles can achieve in principle the "Heisenberg limit"
{\delta}X \propto N^{-1}, using exotic states. Recent theoretical work argues
that interactions among particles may be a valuable resource for quantum
metrology, allowing scaling beyond the Heisenberg limit. Specifically, a
k-particle interaction will produce sensitivity {\delta}X \propto N^{-k} with
appropriate entangled states and {\delta}X \propto N^{-(k-1/2)} even without
entanglement. Here we demonstrate this "super-Heisenberg" scaling in a
nonlinear, non-destructive measurement of the magnetisation of an atomic
ensemble. We use fast optical nonlinearities to generate a pairwise
photon-photon interaction (k = 2) while preserving quantum-noise-limited
performance, to produce {\delta}X \propto N^{-3/2}. We observe super-Heisenberg
scaling over two orders of magnitude in N, limited at large N by higher-order
nonlinear effects, in good agreement with theory. For a measurement of limited
duration, super-Heisenberg scaling allows the nonlinear measurement to overtake
in sensitivity a comparable linear measurement with the same number of photons.
In other scenarios, however, higher-order nonlinearities prevent this crossover
from occurring, reflecting the subtle relationship of scaling to sensitivity in
nonlinear systems. This work shows that inter-particle interactions can improve
sensitivity in a quantum-limited measurement, and introduces a fundamentally
new resource for quantum metrology
Pseudo-Goldstone Boson Effects in Top-Antitop Productions at High Energy Hadron Colliders and Testing Technicolor Models
We study the top quark pair production process p+p(anti-p)-->top+antitop in
various kinds of technicolor (TC) models at the Fermilab Tevatron Run II and
the CERN LHC. The s-channel neutral pseudo-Goldstone bosons (PGB's) contribute
dominately to the production amplitudes from its coupling to the gluons through
the triangle loops of techniquarks and the top quark. Cross sections in
different TC models with s-channel PGB contributions are calculated. It is
shown that the PGB effects can be experimentally tested and different TC models
under consideration can be distinguished at the LHC. Therefore, the
p+p-->top+antitop process at the LHC provides feasible tests of the TC models.Comment: 10 pages in RevTex and 4 PS-files for the figures. Paramemter range
is changed, and some references are added. Version for publication in Phys.
Rev.
A large sample study of spin relaxation and magnetometric sensitivity of paraffin-coated Cs vapor cells
We have manufactured more than 250 nominally identical paraffin-coated Cs
vapor cells (30 mm diameter bulbs) for multi-channel atomic magnetometer
applications. We describe our dedicated cell characterization apparatus. For
each cell we have determined the intrinsic longitudinal, \sGamma{01}, and
transverse, \sGamma{02}, relaxation rates. Our best cell shows
\sGamma{01}/2\pi\approx 0.5 Hz, and \sGamma{02}/2\pi\approx 2 Hz. We find a
strong correlation of both relaxation rates which we explain in terms of
reservoir and spin exchange relaxation. For each cell we have determined the
optimal combination of rf and laser powers which yield the highest sensitivity
to magnetic field changes. Out of all produced cells, 90% are found to have
magnetometric sensitivities in the range of 9 to 30 fTHz. Noise analysis shows
that the magnetometers operated with such cells have a sensitivity close to the
fundamental photon shot noise limit
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