421 research outputs found
Decoherence and Initial Correlations in Quantum Brownian Motion
We analyze the evolution of a quantum Brownian particle starting from an
initial state that contains correlations between this system and its
environment. Using a path integral approach, we obtain a master equation for
the reduced density matrix of the system finding relatively simple expressions
for its time dependent coefficients. We examine the evolution of delocalized
initial states (Schr\"odinger's cats) investigating the effectiveness of the
decoherence process. Analytic results are obtained for an ohmic environment
(Drude's model) at zero temperature.Comment: 15 pages, RevTex, 5 figures included. Submitted to Phys. Rev.
Quantum measurement as driven phase transition: An exactly solvable model
A model of quantum measurement is proposed, which aims to describe
statistical mechanical aspects of this phenomenon, starting from a purely
Hamiltonian formulation. The macroscopic measurement apparatus is modeled as an
ideal Bose gas, the order parameter of which, that is, the amplitude of the
condensate, is the pointer variable. It is shown that properties of
irreversibility and ergodicity breaking, which are inherent in the model
apparatus, ensure the appearance of definite results of the measurement, and
provide a dynamical realization of wave-function reduction or collapse. The
measurement process takes place in two steps: First, the reduction of the state
of the tested system occurs over a time of order , where
is the temperature of the apparatus, and is the number of its degrees of
freedom. This decoherence process is governed by the apparatus-system
interaction. During the second step classical correlations are established
between the apparatus and the tested system over the much longer time-scale of
equilibration of the apparatus. The influence of the parameters of the model on
non-ideality of the measurement is discussed. Schr\"{o}dinger kittens, EPR
setups and information transfer are analyzed.Comment: 35 pages revte
Surgical Aspects of Dissecting Aortic Aneurysms
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/66511/2/10.1177_000331975400500313.pd
Exact Master Equation and Non-Markovian Decoherence for Quantum Dot Quantum Computing
In this article, we report the recent progress on decoherence dynamics of
electrons in quantum dot quantum computing systems using the exact master
equation we derived recently based on the Feynman-Vernon influence functional
approach. The exact master equation is valid for general nanostructure systems
coupled to multi-reservoirs with arbitrary spectral densities, temperatures and
biases. We take the double quantum dot charge qubit system as a specific
example, and discuss in details the decoherence dynamics of the charge qubit
under coherence controls. The decoherence dynamics risen from the entanglement
between the system and the environment is mainly non-Markovian. We further
discuss the decoherence of the double-dot charge qubit induced by quantum point
contact (QPC) measurement where the master equation is re-derived using the
Keldysh non-equilibrium Green function technique due to the non-linear coupling
between the charge qubit and the QPC. The non-Markovian decoherence dynamics in
the measurement processes is extensively discussed as well.Comment: 15 pages, Invited article for the special issue "Quantum Decoherence
and Entanglement" in Quantum Inf. Proces
Universality of the Lyapunov regime for the Loschmidt echo
The Loschmidt echo (LE) is a magnitude that measures the sensitivity of
quantum dynamics to perturbations in the Hamiltonian. For a certain regime of
the parameters, the LE decays exponentially with a rate given by the Lyapunov
exponent of the underlying classically chaotic system. We develop a
semiclassical theory, supported by numerical results in a Lorentz gas model,
which allows us to establish and characterize the universality of this Lyapunov
regime. In particular, the universality is evidenced by the semiclassical limit
of the Fermi wavelength going to zero, the behavior for times longer than
Ehrenfest time, the insensitivity with respect to the form of the perturbation
and the behavior of individual (non-averaged) initial conditions. Finally, by
elaborating a semiclassical approximation to the Wigner function, we are able
to distinguish between classical and quantum origin for the different terms of
the LE. This approach renders an understanding for the persistence of the
Lyapunov regime after the Ehrenfest time, as well as a reinterpretation of our
results in terms of the quantum--classical transition.Comment: 33 pages, 17 figures, uses Revtex
The Coherent State Representation of Quantum Fluctuations in the Early Universe
Using the squeezed state formalism the coherent state representation of
quantum fluctuations in an expanding universe is derived. It is shown that this
provides a useful alternative to the Wigner function as a phase space
representation of quantum fluctuations. The quantum to classical transition of
fluctuations is naturally implemented by decohering the density matrix in this
representation. The entropy of the decohered vacua is derived. It is shown that
the decoherence process breaks the physical equivalence between vacua that
differ by a coordinate dependent phase generated by a surface term in the
Lagrangian. In particular, scale invariant power spectra are only obtained for
a special choice of surface term.Comment: 25 pages in revtex 3. This version is completely revised with
corrections and significant new calculation
Breakdown of the Landauer bound for information erasure in the quantum regime
A known aspect of the Clausius inequality is that an equilibrium system
subjected to a squeezing \d S of its entropy must release at least an amount
|\dbarrm Q|=T|\d S| of heat. This serves as a basis for the Landauer
principle, which puts a lower bound for the heat generated by erasure
of one bit of information. Here we show that in the world of quantum
entanglement this law is broken. A quantum Brownian particle interacting with
its thermal bath can either generate less heat or even {\it adsorb} heat during
an analogous squeezing process, due to entanglement with the bath. The effect
exists even for weak but fixed coupling with the bath, provided that
temperature is low enough. This invalidates the Landauer bound in the quantum
regime, and suggests that quantum carriers of information can be much more
efficient than assumed so far.Comment: 13 pages, revtex, 2 eps figure
Output spectrum of a detector measuring quantum oscillations
We consider a two-level quantum system (qubit) which is continuously measured
by a detector and calculate the spectral density of the detector output. In the
weakly coupled case the spectrum exhibits a moderate peak at the frequency of
quantum oscillations and a Lorentzian-shape increase of the detector noise at
low frequency. With increasing coupling the spectrum transforms into a single
Lorentzian corresponding to random jumps between two states. We prove that the
Bayesian formalism for the selective evolution of the density matrix gives the
same spectrum as the conventional master equation approach, despite the
significant difference in interpretation. The effects of the detector
nonideality and the finite-temperature environment are also discussed.Comment: 8 pages, 6 figure
Analytical and numerical investigation of escape rate for a noise driven bath
We consider a system-reservoir model where the reservoir is modulated by an
external noise. Both the internal noise of the reservoir and the external noise
are stationary, Gaussian and are characterized by arbitrary decaying
correlation functions. Based on a relation between the dissipation of the
system and the response function of the reservoir driven by external noise we
numerically examine the model using a full bistable potential to show that one
can recover the turn-over features of the usual Kramers' dynamics when the
external noise modulates the reservoir rather than the system directly. We
derive the generalized Kramers' rate for this nonequilibrium open system. The
theoretical results are verified by numerical simulation.Comment: Revtex, 25 pages, 5 figures. To appear in Phys. Rev.
Generalized quantum Fokker-Planck, diffusion and Smoluchowski equations with true probability distribution functions
Traditionally, the quantum Brownian motion is described by Fokker-Planck or
diffusion equations in terms of quasi-probability distribution functions, e.g.,
Wigner functions. These often become singular or negative in the full quantum
regime. In this paper a simple approach to non-Markovian theory of quantum
Brownian motion using {\it true probability distribution functions} is
presented. Based on an initial coherent state representation of the bath
oscillators and an equilibrium canonical distribution of the quantum mechanical
mean values of their co-ordinates and momenta we derive a generalized quantum
Langevin equation in -numbers and show that the latter is amenable to a
theoretical analysis in terms of the classical theory of non-Markovian
dynamics. The corresponding Fokker-Planck, diffusion and the Smoluchowski
equations are the {\it exact} quantum analogues of their classical
counterparts. The present work is {\it independent} of path integral
techniques. The theory as developed here is a natural extension of its
classical version and is valid for arbitrary temperature and friction
(Smoluchowski equation being considered in the overdamped limit).Comment: RevTex, 16 pages, 7 figures, To appear in Physical Review E (minor
revision
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