431 research outputs found
Coherent Backscattering with Nonlinear Atomic Scatterers
We study coherent backscattering of a quasi-monochromatic laser by a dilute
gas of cold two-level atoms. We consider the perturbative regime of weak
intensities, where nonlinear effects arise from {\em inelastic} two-photon
scattering processes. Here, coherent backscattering can be formed by
interference between {\em three} different scattering amplitudes. Consequently,
if elastically scattered photons are filtered out from the photodetection
signal by means of suitable frequency-selective detection, we find the
nonlinear backscattering enhancement factor to exceed the linear barrier two.Comment: 4 pages, 3 figure
Coherent Backscattering of Light with Nonlinear Atomic Scatterers
We study coherent backscattering of a monochromatic laser by a dilute gas of
cold two-level atoms in the weakly nonlinear regime. The nonlinear response of
the atoms results in a modification of both the average field propagation
(nonlinear refractive index) and the scattering events. Using a perturbative
approach, the nonlinear effects arise from inelastic two-photon scattering
processes. We present a detailed diagrammatic derivation of the elastic and
inelastic components of the backscattering signal both for scalar and vectorial
photons. Especially, we show that the coherent backscattering phenomenon
originates in some cases from the interference between three different
scattering amplitudes. This is in marked contrast with the linear regime where
it is due to the interference between two different scattering amplitudes. In
particular we show that, if elastically scattered photons are filtered out from
the photo-detection signal, the nonlinear backscattering enhancement factor
exceeds the linear barrier two, consistently with a three-amplitude
interference effect.Comment: 18 pages, 13 figures, submitted to Phys. Rev.
Diagrammatic approach to coherent backscattering of laser light by cold atoms: Double scattering revisited
We present a diagrammatic derivation of the coherent backscattering spectrum
from two two-level atoms using the pump-probe approach, wherein the multiple
scattering signal is deduced from single-atom responses, and provide a physical
interpretation of the single-atom building blocks.Comment: 16 pages, 7 figure
Emission of photon echoes in a strongly scattering medium
We observe the two- and three-pulse photon echo emission from a scattering
powder, obtained by grinding a Pr:YSiO rare earth doped single
crystal. We show that the collective emission is coherently constructed over
several grains. A well defined atomic coherence can therefore be created
between randomly placed particles. Observation of photon echo on powders as
opposed to bulk materials opens the way to faster material development. More
generally, time-domain resonant four-wave mixing offers an attractive approach
to investigate coherent propagation in scattering media
Combinations of workplace stressors and work related injuries
Consideration of factors associated with increased accident likelihood has tended to concentrate on the influence of one specific factor (for example, noise) and other influences are either not considered or are controlled for. The aim here was to examine the influence of combinations of stressors on the prevalence of workplace accidents using self-report measures of exposure, health and work outcomes. Logistic regression analyses were carried out, with 'work-related/non-work-related accident' as the dependent variable. The main predictors were combinations of physical agents (noise, fumes, hazardous substances) and temporal stressors (night and shift work, long working hours). Additional predictors - the job demand-control-support model (Karasek 1979; Johnson and Hall 1988) and home-work interface (HWI) were also investigated. Other measured predictors (i.e., age, sex and social class based on occupation) were included in all analyses. There was some evidence for an increased likelihood of work-related accidents in those exposed to combinations of stressors - increased likelihood was largely due to independent effects of stressors, particularly noise. Certain stressors were also associated with a decreased risk of having a work-related accident (i.e. unpredictable work hours). Job-demand-control-support did not have a major impact on predicting work-/nonwork-related accident likelihood. Prevalence of accidents at work largely reflected social class based on occupation - 'skilled manual workers' or 'partly skilled workers' were most likely to have an accident
Topological transition in measurement-induced geometric phases
The state of a quantum system, adiabatically driven in a cycle, may acquire a measurable phase depending only on the closed trajectory in parameter space. Such geometric phases are ubiquitous and also underline the physics of robust topological phenomena such as the quantum Hall effect. Equivalently, a geometric phase may be induced through a cyclic sequence of quantum measurements. We show that the application of a sequence of weak measurements renders the closed trajectories, hence the geometric phase, stochastic. We study the concomitant probability distribution and show that, when varying the measurement strength, the mapping between the measurement sequence and the geometric phase undergoes a topological transition. Our finding may impact measurement-induced control and manipulation of quantum states-a promising approach to quantum information processing. It also has repercussions on understanding the foundations of quantum measurement
Optimal Lewenstein-Sanpera Decomposition for some Biparatite Systems
It is shown that for a given bipartite density matrix and by choosing a
suitable separable set (instead of product set) on the separable-entangled
boundary, optimal Lewenstein-Sanpera (L-S) decomposition can be obtained via
optimization for a generic entangled density matrix. Based on this, We obtain
optimal L-S decomposition for some bipartite systems such as and
Bell decomposable states, generic two qubit state in Wootters
basis, iso-concurrence decomposable states, states obtained from BD states via
one parameter and three parameters local operations and classical
communications (LOCC), Werner and isotropic states, and a one
parameter state. We also obtain the optimal decomposition for
multi partite isotropic state. It is shown that in all systems
considered here the average concurrence of the decomposition is equal to the
concurrence. We also show that for some Bell decomposable states
the average concurrence of the decomposition is equal to the lower bound of the
concurrence of state presented recently in [Buchleitner et al,
quant-ph/0302144], so an exact expression for concurrence of these states is
obtained. It is also shown that for isotropic state where
decomposition leads to a separable and an entangled pure state, the average
I-concurrence of the decomposition is equal to the I-concurrence of the state.
Keywords: Quantum entanglement, Optimal Lewenstein-Sanpera decomposition,
Concurrence, Bell decomposable states, LOCC}
PACS Index: 03.65.UdComment: 31 pages, Late
Universality of residence-time distributions in non-adiabatic stochastic resonance
We present mathematically rigorous expressions for the residence-time and
first-passage-time distributions of a periodically forced Brownian particle in
a bistable potential. For a broad range of forcing frequencies and amplitudes,
the distributions are close to periodically modulated exponential ones.
Remarkably, the periodic modulations are governed by universal functions,
depending on a single parameter related to the forcing period. The behaviour of
the distributions and their moments is analysed, in particular in the low- and
high-frequency limits.Comment: 8 pages, 1 figure New version includes distinction between
first-passage-time and residence-time distribution
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