223 research outputs found
Ultracoherence and Canonical Transformations
The (in)finite dimensional symplectic group of homogeneous canonical
transformations is represented on the bosonic Fock space by the action of the
group on the ultracoherent vectors, which are generalizations of the coherent
states.Comment: 24 page
Applications of Canonical Transformations
Canonical transformations are defined and discussed along with the
exponential, the coherent and the ultracoherent vectors. It is shown that the
single-mode and the -mode squeezing operators are elements of the group of
canonical transformations. An application of canonical transformations is made,
in the context of open quantum systems, by studying the effect of squeezing of
the bath on the decoherence properties of the system. Two cases are analyzed.
In the first case the bath consists of a massless bosonic field with the bath
reference states being the squeezed vacuum states and squeezed thermal states
while in the second case a system consisting of a harmonic oscillator
interacting with a bath of harmonic oscillators is analyzed with the bath being
initially in a squeezed thermal state.Comment: 14 page
A Feynman-Kac Formula for Anticommuting Brownian Motion
Motivated by application to quantum physics, anticommuting analogues of
Wiener measure and Brownian motion are constructed. The corresponding Ito
integrals are defined and the existence and uniqueness of solutions to a class
of stochastic differential equations is established. This machinery is used to
provide a Feynman-Kac formula for a class of Hamiltonians. Several specific
examples are considered.Comment: 21 page
Initial correlations effects on decoherence at zero temperature
We consider a free charged particle interacting with an electromagnetic bath
at zero temperature. The dipole approximation is used to treat the bath
wavelengths larger than the width of the particle wave packet. The effect of
these wavelengths is described then by a linear Hamiltonian whose form is
analogous to phenomenological Hamiltonians previously adopted to describe the
free particle-bath interaction. We study how the time dependence of decoherence
evolution is related with initial particle-bath correlations. We show that
decoherence is related to the time dependent dressing of the particle. Moreover
because decoherence induced by the T=0 bath is very rapid, we make some
considerations on the conditions under which interference may be experimentally
observed.Comment: 16 pages, 1 figur
Phase Diffusion in Quantum Dissipative Systems
We study the dynamics of the quantum phase distribution associated with the
reduced density matrix of a system for a number of situations of practical
importance, as the system evolves under the influence of its environment,
interacting via a quantum nondemoliton type of coupling, such that there is
decoherence without dissipation, as well as when it interacts via a dissipative
interaction, resulting in decoherence as well as dissipation. The system is
taken to be either a two-level atom (or equivalently, a spin-1/2 system) or a
harmonic oscillator, and the environment is modeled as a bath of harmonic
oscillators, starting out in a squeezed thermal state. The impact of the
different environmental parameters on the dynamics of the quantum phase
distribution for the system starting out in various initial states, is
explicitly brought out. An interesting feature that emerges from our work is
that the relationship between squeezing and temperature effects depends on the
type of system-bath interaction. In the case of quantum nondemolition type of
interaction, squeezing and temperature work in tandem, producing a diffusive
effect on the phase distribution. In contrast, in case of a dissipative
interaction, the influence of temperature can be counteracted by squeezing,
which manifests as a resistence to randomization of phase. We make use of the
phase distributions to bring out a notion of complementarity in atomic systems.
We also study the dispersion of the phase using the phase distributions
conditioned on particular initial states of the system.Comment: Accepted for publication in Physical Review A; changes in section V;
20 pages, 12 figure
Euclidean Supergravity in Terms of Dirac Eigenvalues
It has been recently shown that the eigenvalues of the Dirac operator can be
considered as dynamical variables of Euclidean gravity. The purpose of this
paper is to explore the possiblity that the eigenvalues of the Dirac operator
might play the same role in the case of supergravity. It is shown that for this
purpose some primary constraints on covariant phase space as well as secondary
constraints on the eigenspinors must be imposed. The validity of primary
constraints under covariant transport is further analyzed. It is show that in
the this case restrictions on the tanget bundle and on the spinor bundle of
spacetime arise. The form of these restrictions is determined under some
simplifying assumptions. It is also shown that manifolds with flat curvature of
tangent bundle and spinor bundle and spinor bundle satisfy these restrictons
and thus they support the Dirac eigenvalues as global observables.Comment: Misprints and formulae corrected; to appear in Phys. Rev.
The Gisin-Percival stochastic Schrödinger equation from standard quantum filtering theory
We show that the quantum state diffusion equation of Gisin and Percival,
driven by complex Wiener noise, is equivalent up to a global stochastic phase
to quantum trajectory models. With an appropriate feedback scheme, we set up an
analogue continuous measurement model with exactly simulates the Gisin-Percival
quantum state diffusion.Comment: Originally submitted to a Theoretical Physics journal but rejected
with the re-submission instructions to drop my discussion and references to
the papers of Gisin and Percival, which I consider unethical. To be now
submitted to an appropriate Mathematical Physics journal instea
Dystonia in neurodegeneration with brain iron accumulation: outcome of bilateral pallidal stimulation
Neurodegeneration with brain iron accumulation encompasses a heterogeneous group of rare neurodegenerative disorders that are characterized by iron accumulation in the brain. Severe generalized dystonia is frequently a prominent symptom and can be very disabling, causing gait impairment, difficulty with speech and swallowing, pain and respiratory distress. Several case reports and one case series have been published concerning therapeutic outcome of pallidal deep brain stimulation in dystonia caused by neurodegeneration with brain iron degeneration, reporting mostly favourable outcomes. However, with case studies, there may be a reporting bias towards favourable outcome. Thus, we undertook this multi-centre retrospective study to gather worldwide experiences with bilateral pallidal deep brain stimulation in patients with neurodegeneration with brain iron accumulation. A total of 16 centres contributed 23 patients with confirmed neurodegeneration with brain iron accumulation and bilateral pallidal deep brain stimulation. Patient details including gender, age at onset, age at operation, genetic status, magnetic resonance imaging status, history and clinical findings were requested. Data on severity of dystonia (Burke Fahn Marsden Dystonia Rating Scale—Motor Scale, Barry Albright Dystonia Scale), disability (Burke Fahn Marsden Dystonia Rating Scale—Disability Scale), quality of life (subjective global rating from 1 to 10 obtained retrospectively from patient and caregiver) as well as data on supportive therapy, concurrent pharmacotherapy, stimulation settings, adverse events and side effects were collected. Data were collected once preoperatively and at 2–6 and 9–15 months postoperatively. The primary outcome measure was change in severity of dystonia. The mean improvement in severity of dystonia was 28.5% at 2–6 months and 25.7% at 9–15 months. At 9–15 months postoperatively, 66.7% of patients showed an improvement of 20% or more in severity of dystonia, and 31.3% showed an improvement of 20% or more in disability. Global quality of life ratings showed a median improvement of 83.3% at 9–15 months. Severity of dystonia preoperatively and disease duration predicted improvement in severity of dystonia at 2–6 months; this failed to reach significance at 9–15 months. The study confirms that dystonia in neurodegeneration with brain iron accumulation improves with bilateral pallidal deep brain stimulation, although this improvement is not as great as the benefit reported in patients with primary generalized dystonias or some other secondary dystonias. The patients with more severe dystonia seem to benefit more. A well-controlled, multi-centre prospective study is necessary to enable evidence-based therapeutic decisions and better predict therapeutic outcomes
Real-Time CARS Imaging Reveals a Calpain-Dependent Pathway for Paranodal Myelin Retraction during High-Frequency Stimulation
High-frequency electrical stimulation is becoming a promising therapy for neurological disorders, however the response of the central nervous system to stimulation remains poorly understood. The current work investigates the response of myelin to electrical stimulation by laser-scanning coherent anti-Stokes Raman scattering (CARS) imaging of myelin in live spinal tissues in real time. Paranodal myelin retraction at the nodes of Ranvier was observed during 200 Hz electrical stimulation. Retraction was seen to begin minutes after the onset of stimulation and continue for up to 10 min after stimulation was ceased, but was found to reverse after a 2 h recovery period. The myelin retraction resulted in exposure of Kv 1.2 potassium channels visualized by immunofluorescence. Accordingly, treating the stimulated tissue with a potassium channel blocker, 4-aminopyridine, led to the appearance of a shoulder peak in the compound action potential curve. Label-free CARS imaging of myelin coupled with multiphoton fluorescence imaging of immuno-labeled proteins at the nodes of Ranvier revealed that high-frequency stimulation induced paranodal myelin retraction via pathologic calcium influx into axons, calpain activation, and cytoskeleton degradation through spectrin break-down
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