5,363 research outputs found
Damping and decoherence of a nanomechanical resonator due to a few two level systems
We consider a quantum model of a nanomechanical flexing beam resonator
interacting with a bath comprising a few damped tunneling two level systems
(TLS's). In contrast with a resonator interacting bilinearly with an ohmic free
oscillator bath (modeling clamping loss, for example), the mechanical resonator
damping is amplitude dependent, while the decoherence of quantum superpositions
of mechanical position states depends only weakly on their spatial separation
Giant enhancement of quantum decoherence by frustrated environments
This Letter studies the decoherence in a system of two antiferromagnetically
coupled spins that interact with a spin bath environment. Systems are
considered that range from the rotationally invariant to highly anisotropic
spin models, have different topologies and values of parameters that are fixed
or are allowed to fluctuate randomly. We explore the conditions under which the
two-spin system clearly shows an evolution from the initial spin-up - spin-down
state towards the maximally entangled singlet state. We demonstrate that
frustration and, especially, glassiness of the spin environment strongly
enhances the decoherence of the two-spin system
Planilha eletrônica para auxilio à tomada de decisão em manejo de irrigação localizada: aplicação no cultivo da videira.
Decoherence in an Interacting Quantum Field Theory: The Vacuum Case
We apply the decoherence formalism to an interacting scalar field theory. In
the spirit of the decoherence literature, we consider a "system field" and an
"environment field" that interact via a cubic coupling. We solve for the
propagator of the system field, where we include the self-energy corrections
due to the interaction with the environment field. In this paper, we consider
an environment in the vacuum state (T=0). We show that neglecting inaccessible
non-Gaussian correlators increases the entropy of the system as perceived by
the observer. Moreover, we consider the effect of a changing mass of the system
field in the adiabatic regime, and we find that at late times no additional
entropy has been generated.Comment: 40 pages, published versio
Quantum-classical correspondence on compact phase space
We propose to study the -norm distance between classical and quantum
phase space distributions, where for the latter we choose the Wigner function,
as a global phase space indicator of quantum-classical correspondence. For
example, this quantity should provide a key to understand the correspondence
between quantum and classical Loschmidt echoes. We concentrate on fully chaotic
systems with compact (finite) classical phase space. By means of numerical
simulations and heuristic arguments we find that the quantum-classical fidelity
stays at one up to Ehrenfest-type time scale, which is proportional to the
logarithm of effective Planck constant, and decays exponentially with a maximal
classical Lyapunov exponent, after that time.Comment: 26 pages. 9 figures (31 .epz files), submitted to Nonlinearit
Non-Equilibrium Quantum Fields in the Large N Expansion
An effective action technique for the time evolution of a closed system
consisting of one or more mean fields interacting with their quantum
fluctuations is presented. By marrying large expansion methods to the
Schwinger-Keldysh closed time path (CTP) formulation of the quantum effective
action, causality of the resulting equations of motion is ensured and a
systematic, energy conserving and gauge invariant expansion about the
quasi-classical mean field(s) in powers of developed. The general method
is exposed in two specific examples, symmetric scalar \l\F^4 theory
and Quantum Electrodynamics (QED) with fermion fields. The \l\F^4 case is
well suited to the numerical study of the real time dynamics of phase
transitions characterized by a scalar order parameter. In QED the technique may
be used to study the quantum non-equilibrium effects of pair creation in strong
electric fields and the scattering and transport processes in a relativistic
plasma. A simple renormalization scheme that makes practical the
numerical solution of the equations of motion of these and other field theories
is described.Comment: 43 pages, LA-UR-94-783 (PRD, in press), uuencoded PostScrip
Generalized Uncertainty Relations and Long Time Limits for Quantum Brownian Motion Models
We study the time evolution of the reduced Wigner function for a class of
quantum Brownian motion models. We derive two generalized uncertainty
relations. The first consists of a sharp lower bound on the uncertainty
function, , after evolution for time in the
presence of an environment. The second, a stronger and simpler result, consists
of a lower bound at time on a modified uncertainty function, essentially
the area enclosed by the contour of the Wigner function. In both
cases the minimizing initial state is a non-minimal Gaussian pure state. These
generalized uncertainty relations supply a measure of the comparative size of
quantum and thermal fluctuations. We prove two simple inequalites, relating
uncertainty to von Neumann entropy, and the von Neumann entropy to linear
entropy. We also prove some results on the long-time limit of the Wigner
function for arbitrary initial states. For the harmonic oscillator the Wigner
function for all initial states becomes a Gaussian at large times (often, but
not always, a thermal state). We derive the explicit forms of the long-time
limit for the free particle (which does not in general go to a Gaussian), and
also for more general potentials in the approximation of high temperature.Comment: 35 pages (plain Tex, revised to avoid corruption during file
transmission), Imperial College preprint 92-93/25 (1994
Contractive Schroedinger cat states for a free mass
Contractive states for a free quantum particle were introduced by Yuen [Yuen
H P 1983 Phys. Rev. Lett. 51, 719] in an attempt to evade the standard quantum
limit for repeated position measurements. We show how appropriate families of
two- and three component ``Schroedinger cat states'' are able to support
non-trivial correlations between the position and momentum observables leading
to contractive behavior. The existence of contractive Schroedinger cat states
is suggestive of potential novel roles of non-classical states for precision
measurement schemes.Comment: 24 pages, 7 encapsulated eps color figures, REVTeX4 style. Published
online in New Journal of Physics 5 (2003) 5.1-5.21. Higher-resolution figures
available in published version. (accessible at http://www.njp.org/
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