250 research outputs found
Nonequilibrium Langevin Approach to Quantum Optics in Semiconductor Microcavities
Recently the possibility of generating nonclassical polariton states by means
of parametric scattering has been demonstrated. Excitonic polaritons propagate
in a complex interacting environment and contain real electronic excitations
subject to scattering events and noise affecting quantum coherence and
entanglement. Here we present a general theoretical framework for the realistic
investigation of polariton quantum correlations in the presence of coherent and
incoherent interaction processes. The proposed theoretical approach is based on
the {\em nonequilibrium quantum Langevin approach for open systems} applied to
interacting-electron complexes described within the dynamics controlled
truncation scheme. It provides an easy recipe to calculate multi-time
correlation functions which are key-quantities in quantum optics. As a first
application, we analyze the build-up of polariton parametric emission in
semiconductor microcavities including the influence of noise originating from
phonon induced scattering.Comment: some corrections in the presentation mad
The parity-violating asymmetry in the 3He(n,p)3H reaction
The longitudinal asymmetry induced by parity-violating (PV) components in the
nucleon-nucleon potential is studied in the charge-exchange reaction 3He(n,p)3H
at vanishing incident neutron energies. An expression for the PV observable is
derived in terms of T-matrix elements for transitions from the {2S+1}L_J=1S_0
and 3S_1 states in the incoming n-3He channel to states with J=0 and 1 in the
outgoing p-3H channel. The T-matrix elements involving PV transitions are
obtained in first-order perturbation theory in the hadronic weak-interaction
potential, while those connecting states of the same parity are derived from
solutions of the strong-interaction Hamiltonian with the
hyperspherical-harmonics method. The coupled-channel nature of the scattering
problem is fully accounted for. Results are obtained corresponding to realistic
or chiral two- and three-nucleon strong-interaction potentials in combination
with either the DDH or pionless EFT model for the weak-interaction potential.
The asymmetries, predicted with PV pion and vector-meson coupling constants
corresponding (essentially) to the DDH "best values" set, range from -9.44 to
-2.48 in units of 10^{-8}, depending on the input strong-interaction
Hamiltonian. This large model dependence is a consequence of cancellations
between long-range (pion) and short-range (vector-meson) contributions, and is
of course sensitive to the assumed values for the PV coupling constants.Comment: 19 pages, 15 tables, revtex
Dynamics-Controlled Truncation Scheme for Nonlinear Dynamics in Semiconductor Microcavities
We present a systematic theory of Coulomb-induced correlation effects in the
nonlinear optical processes within the strong-coupling regime. In this paper we
shall set a dynamics controlled truncation scheme \cite{Axt Stahl} microscopic
treatment of nonlinear parametric processes in SMCs including the
electromagnetic field quantization. It represents the starting point for the
microscopic approach to quantum optics experiments in the strong coupling
regime without any assumption on the quantum statistics of electronic
excitations (excitons) involved. We exploit a previous technique, used in the
semiclassical context, which, once applied to four-wave mixing in quantum
wells, allowed to understand a wide range of observed phenomena \cite{Sham
PRL95}. We end up with dynamical equations for exciton and photon operators
which extend the usual semiclassical description of Coulomb interaction
effects, in terms of a mean-field term plus a genuine non-instantaneous
four-particle correlation, to quantum optical effects.Comment: preprint version, no figures an entire section adde
Decoherence-Free Emergence of Macroscopic Local Realism for entangled photons in a cavity
We investigate the influence of environmental noise on polarization entangled
light generated by parametric emission in a cavity. By adopting a recently
developed separability criterion, we show that: i) self-stimulation may
suppress the detrimental influence of noise on entanglement; ii) when
self-stimulation becomes effective, a classical model of parametric emission
incorporating noise provides the same results of quantum theory for the
expectation values involved in the separability criterion. Moreover we show
that, in the macroscopic limit, it is impossible to observe violations of local
realism with measurements of -particle correlations, whatever n but finite.
These results provide an interesting example of the emergence of macroscopic
local realism in the presence of strong entanglement even in the absence of
decoherence.Comment: 1 figur
Abnormal plasticity of sensorimotor circuits extends beyond the affected body part in focal dystonia
Objective: To test whether abnormal sensorimotor plasticity in focal hand dystonia is a primary abnormality or is merely a consequence of the dystonic posture. Methods: This study used the paired associative stimulation (PAS) paradigm, an experimental intervention, capable of producing long term potentiation (LTP) like changes in the sensorimotor system in humans. PAS involves transcranial magnetic stimulation combined with median nerve stimulation. 10 patients with cranial and cervical dystonia, who showed no dystonic symptoms in the hand, and nine patients with hemifacial spasm (HFS), a non-dystonic condition, were compared with 10 healthy age matched controls. Motor evoked potential amplitudes and cortical silent period (CSP) duration were measured at baseline before PAS and for up to 60 min (T0, T30 and T60) after PAS in the abductor pollicis brevis and the first dorsal interosseus muscles. Results: Patients with dystonia showed a stronger increase in corticospinal excitability than healthy controls and patients with HFS. In addition, patients with dystonia showed a loss of topographical specificity of PAS induced effects, with a facilitation in both the median and ulnar innervated muscles. While PAS conditioning led to a prolonged CSP in healthy controls and patients with HFS, it had no effect on the duration of the CSP in patients with cranial and cervical dystonia. Conclusion: The data suggests that excessive motor cortex plasticity is not restricted to the circuits clinically affected by dystonia but generalises across the entire sensorimotor system, possibly representing an endophenotypic trait of the disease
Botulinum toxin A treatment for primary hemifacial spasm - A 10-year multicenter study
BACKGROUND:
Botulinum toxin A (BTX) is the currently preferred symptomatic treatment for primary hemifacial spasm (HFS), but its long-term efficacy and safety are not known.
OBJECTIVE:
To assess the long-term effectiveness and safety of BTX in the treatment of primary HFS.
DESIGN:
Retrospective review of medical records of the 1st and 10th years of treatment.
SETTING:
Outpatient clinics of 4 Italian university centers in the Italian Movement Disorders Study Group.
PARTICIPANTS:
A series of 65 patients with primary HFS who had received BTX injections regularly for at least 10 years.
MAIN OUTCOME MEASURES:
Mean duration of improvement and quality of the effect induced by the preceding treatment (measured using a patient self-evaluation scale) and occurrence and duration of adverse effects in the 1st and 10th years of treatment.
RESULTS:
Using a mean BTX dose per treatment session similar to that used by others, we obtained a 95% response rate and an overall mean duration of improvement of 12.6 weeks during year 1. The effectiveness of BTX in relieving the symptoms of primary HFS, as measured by the response rate and average duration of improvement, remained unchanged in the 1st and 10th years. Patients needed statistically similar BTX doses in the 1st and 10th years. The rate of local adverse effects (including upper lid ptosis, facial weakness, and diplopia) diminished significantly in the 10th year of treatment.
CONCLUSION:
Treatment with BTX effectively induces sustained relief from symptoms of HFS in the long term, with only minimal and transient adverse reactions
Spatial Integration of Somatosensory Inputs during Sensory-Motor Plasticity Phenomena Is Normal in Focal Hand Dystonia.
Background: Surround inhibition is a system that sharpens sensation by creating an inhibitory zone around the central core of activation. In the motor system, this mechanism probably contributes to the selection of voluntary movements, and it seems to be lost in dystonia. Objectives. To explore if sensory information is abnormally processed and integrated in focal hand dystonia (FHD) and if surround inhibition phenomena are operating during sensory-motor plasticity and somatosensory integration in normal humans and in patients with FHD. Methods. We looked at the MEP facilitation obtained after 5 Hz repetitive paired associative stimulation of median (PAS M), ulnar (PAS U), and median + ulnar nerve (PAS MU) stimulation in 8 normal subjects and 8 FHD. We evaluated the ratio MU/(M + U) ∗ 100 and the spatial and temporal somatosensory integration recording the somatosensory evoked potentials (SEPs) evoked by a dual nerve input. Results: FHD had two main abnormalities: first, the amount of facilitation was larger than normal subjects; second, the spatial specificity was lost. The MU/(M + U) ∗ 100 ratio was similar in healthy subjects and in FHD patients, and the somatosensory integration was normal in this subset of patients. Conclusions. The inhibitory integration of somatosensory inputs and the somatosensory inhibition are normal in patients with focal dystonia as well as lateral surrounding inhibition phenomena during sensory-motor plasticity in FHD
Possible risk factors for primary adult onset dystonia: a case-control investigation by the Italian Movement Disorders Study Group.
Signatures of three-nucleon interactions in few-nucleon systems
Recent experimental results in three-body systems have unambiguously shown
that calculations based only on nucleon-nucleon forces fail to accurately
describe many experimental observables and one needs to include effects which
are beyond the realm of the two-body potentials. This conclusion owes its
significance to the fact that experiments and calculations can both be
performed with a high accuracy. In this review, both theoretical and
experimental achievements of the past decade will be underlined. Selected
results will be presented. The discussion on the effects of the three-nucleon
forces is, however, limited to the hadronic sector. It will be shown that
despite the major successes in describing these seemingly simple systems, there
are still clear discrepancies between data and the state-of-the-art
calculations.Comment: accepted for publication in Rep. Prog. Phy
Variational Calculation on A=3 and 4 Nuclei with Non-Local Potentials
The application of the hyperspherical harmonic approach to the case of
non-local two-body potentials is described. Given the properties of the
hyperspherical harmonic functions, there are no difficulties in considering the
approach in both coordinate and momentum space. The binding energies and other
ground state properties of A=3 and 4 nuclei are calculated using the CD Bonn
2000 and N3LO two-body potentials. The results are shown to be in excellent
agreement with corresponding ones obtained by other accurate techniques.Comment: 12 pages, 6 tables, RevTex
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