702 research outputs found
Exact solution of the Hu-Paz-Zhang master equation
The Hu-Paz-Zhang equation is a master equation for an oscillator coupled to a
linear passive bath. It is exact within the assumption that the oscillator and
bath are initially uncoupled . Here an exact general solution is obtained in
the form of an expression for the Wigner function at time t in terms of the
initial Wigner function. The result is applied to the motion of a Gaussian wave
packet and to that of a pair of such wave packets. A serious divergence arising
from the assumption of an initially uncoupled state is found to be due to the
zero-point oscillations of the bath and not removed in a cutoff model. As a
consequence, worthwhile results for the equation can only be obtained in the
high temperature limit, where zero-point oscillations are neglected. In that
limit closed form expressions for wave packet spreading and attenuation of
coherence are obtained. These results agree within a numerical factor with
those appearing in the literature, which apply for the case of a particle at
zero temperature that is suddenly coupled to a bath at high temperature. On the
other hand very different results are obtained for the physically consistent
case in which the initial particle temperature is arranged to coincide with
that of the bath
Density matrix operatorial solution of the non--Markovian Master Equation for Quantum Brownian Motion
An original method to exactly solve the non-Markovian Master Equation
describing the interaction of a single harmonic oscillator with a quantum
environment in the weak coupling limit is reported. By using a superoperatorial
approach we succeed in deriving the operatorial solution for the density matrix
of the system. Our method is independent of the physical properties of the
environment. We show the usefulness of our solution deriving explicit
expressions for the dissipative time evolution of some observables of physical
interest for the system, such as, for example, its mean energy.Comment: 16 pages, 1 figur
Complex Numbers, Quantum Mechanics and the Beginning of Time
A basic problem in quantizing a field in curved space is the decomposition of
the classical modes in positive and negative frequency. The decomposition is
equivalent to a choice of a complex structure in the space of classical
solutions. In our construction the real tunneling geometries provide the link
between the this complex structure and analytic properties of the classical
solutions in a Riemannian section of space. This is related to the Osterwalder-
Schrader approach to Euclidean field theory.Comment: 27 pages LATEX, UCSBTH-93-0
Decoherence scenarios from micro- to macroscopic superpositions
Environment induced decoherence entails the absence of quantum interference
phenomena from the macroworld. The loss of coherence between superposed wave
packets depends on their separation. The precise temporal course depends on the
relative size of the time scales for decoherence and other processes taking
place in the open system and its environment. We use the exactly solvable model
of an harmonic oscillator coupled to a bath of oscillators to illustrate
various decoherence scenarios: These range from exponential golden-rule decay
for microscopic superpositions, system-specific decay for larger separations in
a crossover regime, and finally universal interaction-dominated decoherence for
ever more macroscopic superpositions.Comment: 11 pages, 7 figures, accompanying paper to quant-ph/020412
Fluctuations of an evaporating black hole from back reaction of its Hawking radiation: Questioning a premise in earlier work
This paper delineates the first steps in a systematic quantitative study of
the spacetime fluctuations induced by quantum fields in an evaporating black
hole. We explain how the stochastic gravity formalism can be a useful tool for
that purpose within a low-energy effective field theory approach to quantum
gravity. As an explicit example we apply it to the study of the
spherically-symmetric sector of metric perturbations around an evaporating
black hole background geometry. For macroscopic black holes we find that those
fluctuations grow and eventually become important when considering sufficiently
long periods of time (of the order of the evaporation time), but well before
the Planckian regime is reached. In addition, the assumption of a simple
correlation between the fluctuations of the energy flux crossing the horizon
and far from it, which was made in earlier work on spherically-symmetric
induced fluctuations, is carefully analyzed and found to be invalid. Our
analysis suggests the existence of an infinite amplitude for the fluctuations
of the horizon as a three-dimensional hypersurface. We emphasize the need for
understanding and designing operational ways of probing quantum metric
fluctuations near the horizon and extracting physically meaningful information.Comment: 10 pages, REVTeX; minor changes, a few references added and a brief
discussion of their relevance included. To appear in the proceedings of the
10th Peyresq meeting. Dedicated to Rafael Sorkin on the occasion of his 60th
birthda
Kcne2 deletion causes early-onset nonalcoholic fatty liver disease via iron deficiency anemia
Nonalcoholic fatty liver disease (NAFLD) is an increasing health problem worldwide, with genetic, epigenetic, and environmental components. Here, we describe the first example of NAFLD caused by genetic disruption of a mammalian potassium channel subunit. Mice with germline deletion of the KCNE2 potassium channel {beta} subunit exhibited NAFLD as early as postnatal day 7. Using mouse genetics, histology, liver damage assays and transcriptomics we discovered that iron deficiency arising from KCNE2-dependent achlorhydria is a major factor in early-onset NAFLD in Kcne2(-/-) mice, while two other KCNE2-dependent defects did not initiate NAFLD. The findings uncover a novel genetic basis for NAFLD and an unexpected potential factor in human KCNE2-associated cardiovascular pathologies, including atherosclerosis
Tension term, interchange symmetry, and the analogy of energy and tension laws of the AdS soliton solution
In this paper, we reconsider the energy and tension laws of the Ricci flat
black hole by taking the contribution of the tension term into account. After
this considering and inspired by the interchange symmetry between the Ricci
flat black hole and the AdS soliton solution which arises from the double
analytic continuation of the time and compact spatial direction, we find out
the analogy of the energy and tension laws of the AdS soliton solution.
Moreover, we also investigate the energy and tension laws of the boosted Ricci
flat black hole, and discuss the boosted AdS soliton solution. However,
although there is the same interchange symmetry between the boosted Ricci flat
black hole and boosted AdS soliton, the analogy of laws of the boosted AdS
soliton solution may be of no sense for the existence of the closed timelike
curves and conical singularity. In spite of that, the conserved charges such as
the energy and momentum of the boosted AdS soliton are well-defined, and an
interesting result is that its energy is lower than that of the static AdS
soliton. On the other hand, note that although the laws obtained above are the
same as those of the asymptotically flat case, the underlying deduced contents
are different. Thus, our results could also be considered as a simple
generalization to the asymptotically AdS case. Moreover, during the
calculation, we find that there may be a new way to define the gravitational
tension which can come from the quasi-local stress tensor of the counter-term
method.Comment: V4: 15 pages, no figure, version to appear in JHE
A review on conductive common-mode EMI suppression methods in inverter fed motor drives
The impact of electromagnetic interference (EMI) is an increasingly important aspect of the performance of switching inverters. The challenges of managing EMI continue to grow with the emergence of wide bandgap (WBG) devices, the trend towards ever-greater integration and higher power rating. This paper reviews suppression methods for the conductive common-mode (CM) EMI in inverter fed motor drives. In order to span EMI suppression across the full system design process, the review considers both mitigation from the sources and suppression along the conduction paths. Furthermore, the shortcomings and merits of the reviewed publications are discussed, and their attenuation frequency range and attenuation level are compared. It is demonstrated that the CM EMI at low frequency is mainly determined by the PWM strategies and can be reduced or even theoretically eliminated through zero common-mode control. On the other hand, the CM EMI at high frequency is markedly influenced by the switching transients of the power devices. Thus, various drive circuits are reviewed which improve the switching behavior. Finally, the deployment of passive and active filters to suppress or compensate for the EMI is discussed
A review of modeling and mitigation techniques for bearing currents in electrical machines with variable-frequency drives
The converter switching in variable-frequency drives can generate high frequency common mode voltage between the machine winding and the converter ground, leading to high frequency parasitic currents which can flow through the machine bearings unless precautions are taken in design and installation. These parasitic and unintended currents in the bearings cause deterioration of the lubrication film and surface damage to the rolling elements of the bearings. These problems will be exacerbated as wide-bandgap semiconductors with faster switching rise times start becoming more widespread in variable-frequency drives. This paper reviews the modelling and mitigation techniques of high frequency bearing currents in inverter fed AC drives. It aims to provide a solid base for the research community to further understanding the bearing currents phenomenon and helping to find novel improved technique for their mitigation and measurement
Dynamics of barrier penetration in thermal medium: exact result for inverted harmonic oscillator
Time evolution of quantum tunneling is studied when the tunneling system is
immersed in thermal medium. We analyze in detail the behavior of the system
after integrating out the environment. Exact result for the inverted harmonic
oscillator of the tunneling potential is derived and the barrier penetration
factor is explicitly worked out as a function of time. Quantum mechanical
formula without environment is modifed both by the potential renormalization
effect and by a dynamical factor which may appreciably differ from the
previously obtained one in the time range of 1/(curvature at the top of
potential barrier).Comment: 30 pages, LATEX file with 11 PS figure
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