3,549 research outputs found
Measured Quantum Dynamics of a Trapped Ion
The measurement process is taken into account in the dynamics of trapped ions
prepared in nonclassical motional states. The induced decoherence is shown to
manifest itself both in the inhibition of the internal population dynamics and
in a damping of the vibrational motion without classical counterpart.
Quantitative comparison with present experimental capabilities is discussed,
leading to a proposal for the verification of the predicted effects.Comment: 5 Pages, no figures; Plain REVTeX; to be published in the 1st May
issue of Phys. Rev. A, Rap. Commun. (1997
Independent component analysis of interictal fMRI in focal epilepsy: comparison with general linear model-based EEG-correlated fMRI
The general linear model (GLM) has been used to analyze simultaneous EEG–fMRI to reveal BOLD changes linked to interictal epileptic discharges (IED) identified on scalp EEG. This approach is ineffective when IED are not evident in the EEG. Data-driven fMRI analysis techniques that do not require an EEG derived model may offer a solution in these circumstances. We compared the findings of independent components analysis (ICA) and EEG-based GLM analyses of fMRI data from eight patients with focal epilepsy. Spatial ICA was used to extract independent components (IC) which were automatically classified as either BOLD-related, motion artefacts, EPI-susceptibility artefacts, large blood vessels, noise at high spatial or temporal frequency. The classifier reduced the number of candidate IC by 78%, with an average of 16 BOLD-related IC. Concordance between the ICA and GLM-derived results was assessed based on spatio-temporal criteria. In each patient, one of the IC satisfied the criteria to correspond to IED-based GLM result. The remaining IC were consistent with BOLD patterns of spontaneous brain activity and may include epileptic activity that was not evident on the scalp EEG. In conclusion, ICA of fMRI is capable of revealing areas of epileptic activity in patients with focal epilepsy and may be useful for the analysis of EEG–fMRI data in which abnormalities are not apparent on scalp EEG
Multiple-time correlation functions for non-Markovian interaction: Beyond the Quantum Regression Theorem
Multiple time correlation functions are found in the dynamical description of
different phenomena. They encode and describe the fluctuations of the dynamical
variables of a system. In this paper we formulate a theory of non-Markovian
multiple-time correlation functions (MTCF) for a wide class of systems. We
derive the dynamical equation of the {\it reduced propagator}, an object that
evolve state vectors of the system conditioned to the dynamics of its
environment, which is not necessarily at the vacuum state at the initial time.
Such reduced propagator is the essential piece to obtain multiple-time
correlation functions. An average over the different environmental histories of
the reduced propagator permits us to obtain the evolution equations of the
multiple-time correlation functions. We also study the evolution of MTCF within
the weak coupling limit and it is shown that the multiple-time correlation
function of some observables satisfy the Quantum Regression Theorem (QRT),
whereas other correlations do not. We set the conditions under which the
correlations satisfy the QRT. We illustrate the theory in two different cases;
first, solving an exact model for which the MTCF are explicitly given, and
second, presenting the results of a numerical integration for a system coupled
with a dissipative environment through a non-diagonal interaction.Comment: Submitted (04 Jul 04
Spatial fluctuations in an optical parametric oscillator below threshold with an intracavity photonic crystal
We show how to control spatial quantum correlations in a multimode degenerate
optical parametric oscillator type I below threshold by introducing a spatially
inhomogeneous medium, such as a photonic crystal, in the plane perpendicular to
light propagation. We obtain the analytical expressions for all the
correlations in terms of the relevant parameters of the problem and study the
number of photons, entanglement, squeezing, and twin beams. Considering
different regimes and configurations we show the possibility to tune the
instability thresholds as well as the quantumness of correlations by breaking
the translational invariance of the system through a photonic crystal
modulation.Comment: 12 pages, 7 figure
Psychological functioning in adolescents referred to specialist gender identity clinics across Europe : a clinical comparison study between four clinics
Adolescents seeking professional help with their gender identity development often present with psychological difficulties. Existing literature on psychological functioning of gender diverse young people is limited and mostly bound to national chart reviews. This study examined the prevalence of psychological functioning and peer relationship problems in adolescents across four European specialist gender services (The Netherlands, Belgium, the UK, and Switzerland), using the Child Behavioural Checklist (CBCL) and the Youth Self-Report (YSR). Differences in psychological functioning and peer relationships were found in gender diverse adolescents across Europe. Overall, emotional and behavioural problems and peer relationship problems were most prevalent in adolescents from the UK, followed by Switzerland and Belgium. The least behavioural and emotional problems and peer relationship problems were reported by adolescents from The Netherlands. Across the four clinics, a similar pattern of gender differences was found. Birth-assigned girls showed more behavioural problems and externalising problems in the clinical range, as reported by their parents. According to self-report, internalising problems in the clinical range were more prevalent in adolescent birth-assigned boys. More research is needed to gain a better understanding of the difference in clinical presentations in gender diverse adolescents and to investigate what contextual factors that may contribute to this
Squeezing spectra from s-ordered quasiprobability distributions. Application to dispersive optical bistability
It is well known that the squeezing spectrum of the field exiting a nonlinear
cavity can be directly obtained from the fluctuation spectrum of normally
ordered products of creation and annihilation operators of the cavity mode. In
this article we show that the output field squeezing spectrum can be derived
also by combining the fluctuation spectra of any pair of s-ordered products of
creation and annihilation operators. The interesting result is that the
spectrum obtained in this way from the linearized Langevin equations is exact,
and this occurs in spite of the fact that no s-ordered quasiprobability
distribution verifies a true Fokker-Planck equation, i.e., the Langevin
equations used for deriving the squeezing spectrum are not exact. The
(linearized) intracavity squeezing obtained from any s-ordered distribution is
also exact. These results are exemplified in the problem of dispersive optical
bistability.Comment: 15 pages, no figures, to be published in Journal of Modern Optic
Re-flaring of a Post-Flare Loop System Driven by Flux Rope Emergence and Twisting
In this letter, we study in detail the evolution of the post-flare loops on
2005 January 15 that occurred between two consecutive solar eruption events,
both of which generated a fast halo CME and a major flare. The post-flare loop
system, formed after the first CME/flare eruption, evolved rapidly, as
manifested by the unusual accelerating rise motion of the loops. Through
nonlinear force-free field (NLFFF) models, we obtain the magnetic structure
over the active region. It clearly shows that the flux rope below the loops
also kept rising accompanied with increasing twist and length. Finally, the
post-flare magnetic configuration evolved to a state that resulted in the
second CME/flare eruption. This is an event in which the post-flare loops can
re-flare in a short period of 16 hr following the first CME/flare
eruption. The observed re-flaring at the same location is likely driven by the
rapid evolution of the flux rope caused by the magnetic flux emergence and the
rotation of the sunspot. This observation provides valuable information on
CME/flare models and their prediction.Comment: 11 pages, 4 figures, 1 table, accepted for publication in ApJ Lette
Steady state fluctuations of the dissipated heat for a quantum stochastic model
We introduce a quantum stochastic dynamics for heat conduction. A multi-level
subsystem is coupled to reservoirs at different temperatures. Energy quanta are
detected in the reservoirs allowing the study of steady state fluctuations of
the entropy dissipation. Our main result states a symmetry in its large
deviation rate function.Comment: 41 pages, minor changes, published versio
Quantum fluctuation theorem: Can we go from micro to meso?
Quantum extensions of the Gallavotti-Cohen fluctuation theorem (FT) for the
entropy production have been discussed by several authors. There is a practical
gap between microscopic forms of FT and mesoscopic (i.e. not purely
Hamiltonian) forms for open systems. In a microscopic setup, it is easy to
state and to prove FT. In a mesoscopic setup, it is difficult to identify
fluctuations of the entropy production. (This difficulty is absent in the
classical case.) We discuss a particular mesoscopic model: a Lindblad master
equation, in which we state FT and, more importantly, connect it rigorously
with the underlying microscopic FT. We also remark that FT is satisfied by the
Lesovik-Levitov formula for statistics of charge transport.Comment: Conference Proceedings (Brussels, march 2006), 10 pages, to appear in
Comptes rendus - Physiqu
Using sound localization to gain depth perception for the visually impaired through sensory substitution
The visually impaired do not have the visual ability to localize objects in three-dimensional space, rather, they rely on their other senses to gain depth perception. Sensory substitution is the concept of substituting one sense for another, normally substituting an impaired sense with a functioning sense. Visual-to-auditory sensory substitution substitutes an impaired visual sense with a functioning auditory sense. This research aimed to investigate and develop techniques for visualto-auditory sensory substitution – using sound localization as a sensory substitution for depth perception. The research started by investigating the characteristics of human audition with a particular focus on how humans localize sounds. It then looked at existing visual-to-auditory sensory substitution systems and the techniques they used. From the existing systems, a system known as MeloSee was chosen as a baseline for developing and evaluating further sensory substitution prototypes. The baseline prototype (0) was then implemented and a preliminary study performed. Based on the knowledge gained from the preliminary study, baseline implementation and the background research, a set of improvement recommendations were generated. The next iteration – Prototype 1 (1) – was then developed based on the recommendations. A comparative study between 0 and 1 was then performed. Based on the study, another set of improvement recommendations were generated. From the recommendations, a final prototype was developed – Prototype 2 (2). The last comparative study was then performed between 0 and 2, with a third set of recommendations being generated as a result. From the studies it was found that participants using 0 were able to identify when they were approaching large objects such as walls. 1 built on that, improving the ability to identify the quadrant of a nearby isolated object. 2 built on 0 and 1, achieving similar results to 1 for identifying the quadrant of nearby isolated objects, and improving on 0 and 1 with regard to depth discrimination – especially for navigation tasks where there were no obstacles. Based on the three sets of recommendations and what was learnt over the course of the research, a set of visual-to-auditory sensory substitution techniques were presented. The techniques aim to be useful for implementing visual-to-auditory sensory substitution systems, which would provide the visually impaired with the visual ability to localize objects in three-dimensional space through sound
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