6,394 research outputs found
Impediments to mixing classical and quantum dynamics
The dynamics of systems composed of a classical sector plus a quantum sector
is studied. We show that, even in the simplest cases, (i) the existence of a
consistent canonical description for such mixed systems is incompatible with
very basic requirements related to the time evolution of the two sectors when
they are decoupled. (ii) The classical sector cannot inherit quantum
fluctuations from the quantum sector. And, (iii) a coupling among the two
sectors is incompatible with the requirement of physical positivity of the
theory, i.e., there would be positive observables with a non positive
expectation value.Comment: RevTex, 21 pages. Title slightly modified and summary section adde
The role of infrared divergence for decoherence
Continuous and discrete superselection rules induced by the interaction with
the environment are investigated for a class of exactly soluble Hamiltonian
models. The environment is given by a Boson field. Stable superselection
sectors emerge if and only if the low frequences dominate and the ground state
of the Boson field disappears due to infrared divergence. The models allow
uniform estimates of all transition matrix elements between different
superselection sectors.Comment: 11 pages, extended and simplified proo
Gate stability of GaN-Based HEMTs with P-Type Gate
status: publishe
Incoherent dynamics in neutron-matter interaction
Coherent and incoherent neutron-matter interaction is studied inside a
recently introduced approach to subdynamics of a macrosystem. The equation
describing the interaction is of the Lindblad type and using the Fermi
pseudopotential we show that the commutator term is an optical potential
leading to well-known relations in neutron optics. The other terms, usually
ignored in optical descriptions and linked to the dynamic structure function of
the medium, give an incoherent contribution to the dynamics, which keeps
diffuse scattering and attenuation of the coherent beam into account, thus
warranting fulfilment of the optical theorem. The relevance of this analysis to
experiments in neutron interferometry is briefly discussed.Comment: 15 pages, revtex, no figures, to appear in Phys. Rev.
Fermi's golden rule and exponential decay as a RG fixed point
We discuss the decay of unstable states into a quasicontinuum using models of
the effective Hamiltonian type. The goal is to show that exponential decay and
the golden rule are exact in a suitable scaling limit, and that there is an
associated renormalization group (RG) with these properties as a fixed point.
The method is inspired by a limit theorem for infinitely divisible
distributions in probability theory, where there is a RG with a Cauchy
distribution, i.e. a Lorentz line shape, as a fixed point. Our method of
solving for the spectrum is well known; it does not involve a perturbation
expansion in the interaction, and needs no assumption of a weak interaction. We
use random matrices for the interaction, and show that the ensemble
fluctuations vanish in the scaling limit. Thus the limit is the same for every
model in the ensemble with probability one.Comment: 20 pages, 1 figur
Multiplicity Distributions and Rapidity Gaps
I examine the phenomenology of particle multiplicity distributions, with
special emphasis on the low multiplicities that are a background in the study
of rapidity gaps. In particular, I analyze the multiplicity distribution in a
rapidity interval between two jets, using the HERWIG QCD simulation with some
necessary modifications. The distribution is not of the negative binomial form,
and displays an anomalous enhancement at zero multiplicity. Some useful
mathematical tools for working with multiplicity distributions are presented.
It is demonstrated that ignoring particles with pt<0.2 has theoretical
advantages, in addition to being convenient experimentally.Comment: 24 pages, LaTeX, MSUHEP/94071
Bifurcation Phenomenon in a Spin Relaxation
Spin relaxation in a strong-coupling regime (with respect to the spin system)
is investigated in detail based on the spin-boson model in a stochastic limit.
We find a bifurcation phenomenon in temperature dependence of relaxation
constants, which is never observed in the weak-coupling regime. We also discuss
inequalities among the relaxation constants in our model and show the
well-known relation 2\Gamma_T >= \Gamma_L, for example, for a wider parameter
region than before.Comment: REVTeX4, 5 pages, 5 EPS figure
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
Is the dynamics of open quantum systems always linear?
We study the influence of the preparation of an open quantum system on its
reduced time evolution. In contrast to the frequently considered case of an
initial preparation where the total density matrix factorizes into a product of
a system density matrix and a bath density matrix the time evolution generally
is no longer governed by a linear map nor is this map affine. Put differently,
the evolution is truly nonlinear and cannot be cast into the form of a linear
map plus a term that is independent of the initial density matrix of the open
quantum system. As a consequence, the inhomogeneity that emerges in formally
exact generalized master equations is in fact a nonlinear term that vanishes
for a factorizing initial state. The general results are elucidated with the
example of two interacting spins prepared at thermal equilibrium with one spin
subjected to an external field. The second spin represents the environment. The
field allows the preparation of mixed density matrices of the first spin that
can be represented as a convex combination of two limiting pure states, i.e.
the preparable reduced density matrices make up a convex set. Moreover, the map
from these reduced density matrices onto the corresponding density matrices of
the total system is affine only for vanishing coupling between the spins. In
general, the set of the accessible total density matrices is nonconvex.Comment: 19 pages, 3 figures, minor changes to improve readability, discussion
on Mori's linear regime and references adde
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