5,338 research outputs found
The Effects of Symmetries on Quantum Fidelity Decay
We explore the effect of a system's symmetries on fidelity decay behavior.
Chaos-like exponential fidelity decay behavior occurs in non-chaotic systems
when the system possesses symmetries and the applied perturbation is not tied
to a classical parameter. Similar systems without symmetries exhibit
faster-than-exponential decay under the same type of perturbation. This
counter-intuitive result, that extra symmetries cause the system to behave in a
chaotic fashion, may have important ramifications for quantum error correction.Comment: 5 pages, 3 figures, to be published Phys. Rev. E Rapid Communicatio
Solving the Hamilton-Jacobi equation for gravitationally interacting electromagnetic and scalar fields
The spatial gradient expansion of the generating functional was recently
developed by Parry, Salopek, and Stewart to solve the Hamiltonian constraint in
Einstein-Hamilton-Jacobi theory for gravitationally interacting dust and scalar
fields. This expansion is used here to derive an order-by-order solution of the
Hamiltonian constraint for gravitationally interacting electromagnetic and
scalar fields. A conformal transformation and functional integral are used to
derive the generating functional up to the terms fourth order in spatial
gradients. The perturbations of a flat Friedmann-Robertson-Walker cosmology
with a scalar field, up to second order in spatial gradients, are given. The
application of this formalism is demonstrated in the specific example of an
exponential potential.Comment: 14 pages, uses amsmath,amssymb, referees' suggestions implemented, to
appear in Classical and Quantum Gravit
Optimal distinction between non-orthogonal quantum states
Given a finite set of linearly independent quantum states, an observer who
examines a single quantum system may sometimes identify its state with
certainty. However, unless these quantum states are orthogonal, there is a
finite probability of failure. A complete solution is given to the problem of
optimal distinction of three states, having arbitrary prior probabilities and
arbitrary detection values. A generalization to more than three states is
outlined.Comment: 9 pages LaTeX, one PostScript figure on separate pag
Quantum Fidelity Decay of Quasi-Integrable Systems
We show, via numerical simulations, that the fidelity decay behavior of
quasi-integrable systems is strongly dependent on the location of the initial
coherent state with respect to the underlying classical phase space. In
parallel to classical fidelity, the quantum fidelity generally exhibits
Gaussian decay when the perturbation affects the frequency of periodic phase
space orbits and power-law decay when the perturbation changes the shape of the
orbits. For both behaviors the decay rate also depends on initial state
location. The spectrum of the initial states in the eigenbasis of the system
reflects the different fidelity decay behaviors. In addition, states with
initial Gaussian decay exhibit a stage of exponential decay for strong
perturbations. This elicits a surprising phenomenon: a strong perturbation can
induce a higher fidelity than a weak perturbation of the same type.Comment: 11 pages, 11 figures, to be published Phys. Rev.
Nonlinear Schroedinger Equation in the Presence of Uniform Acceleration
We consider a recently proposed nonlinear Schroedinger equation exhibiting
soliton-like solutions of the power-law form , involving the
-exponential function which naturally emerges within nonextensive
thermostatistics [, with ]. Since
these basic solutions behave like free particles, obeying , and (), it is relevant to investigate how they
change under the effect of uniform acceleration, thus providing the first steps
towards the application of the aforementioned nonlinear equation to the study
of physical scenarios beyond free particle dynamics. We investigate first the
behaviour of the power-law solutions under Galilean transformation and discuss
the ensuing Doppler-like effects. We consider then constant acceleration,
obtaining new solutions that can be equivalently regarded as describing a free
particle viewed from an uniformly accelerated reference frame (with
acceleration ) or a particle moving under a constant force . The latter
interpretation naturally leads to the evolution equation with .
Remarkably enough, the potential couples to , instead of coupling
to , as happens in the familiar linear case ().Comment: 4 pages, no figure
Information-disturbance tradeoff in quantum measurements
We present a simple information-disturbance tradeoff relation valid for any
general measurement apparatus: The disturbance between input and output states
is lower bounded by the information the apparatus provides in distinguishing
these two states.Comment: 4 Pages, 1 Figure. Published version (reference added and minor
changes performed
Non-linear operations in quantum information theory
Quantum information theory is used to analize various non-linear operations
on quantum states. The universal disentanglement machine is shown to be
impossible, and partial (negative) results are obtained in the state-dependent
case. The efficiency of the transformation of non-orthogonal states into
orthogonal ones is discussed.Comment: 11 pages, LaTeX, 3 figures on separate page
Influence of detector motion in entanglement measurements with photons
We investigate how the polarization correlations of entangled photons
described by wave packets are modified when measured by moving detectors. For
this purpose, we analyze the Clauser-Horne-Shimony-Holt Bell inequality as a
function of the apparatus velocity. Our analysis is motivated by future
experiments with entangled photons designed to use satellites. This is a first
step towards the implementation of quantum information protocols in a global
scale
The most probable wave function of a single free moving particle
We develop the most probable wave functions for a single free quantum
particle given its momentum and energy by imposing its quantum probability
density to maximize Shannon information entropy. We show that there is a class
of solutions in which the quantum probability density is self-trapped with
finite-size spatial support, uniformly moving hence keeping its form unchanged.Comment: revtex, 4 page
Distinguishing two single-mode Gaussian states by homodyne detection: An information-theoretic approach
It is known that the quantum fidelity, as a measure of the closeness of two
quantum states, is operationally equivalent to the minimal overlap of the
probability distributions of the two states over all possible POVMs; the POVM
realizing the minimum is optimal. We consider the ability of homodyne detection
to distinguish two single-mode Gaussian states, and investigate to what extent
it is optimal in this information-theoretic sense. We completely identify the
conditions under which homodyne detection makes an optimal distinction between
two single-mode Gaussian states of the same mean, and show that if the Gaussian
states are pure, they are always optimally distinguished.Comment: 6 pages, 4 figures, published version with a detailed discussio
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