2,595 research outputs found
Relativistic Doppler effect in quantum communication
When an electromagnetic signal propagates in vacuo, a polarization detector
cannot be rigorously perpendicular to the wave vector because of diffraction
effects. The vacuum behaves as a noisy channel, even if the detectors are
perfect. The ``noise'' can however be reduced and nearly cancelled by a
relative motion of the observer toward the source. The standard definition of a
reduced density matrix fails for photon polarization, because the
transversality condition behaves like a superselection rule. We can however
define an effective reduced density matrix which corresponds to a restricted
class of positive operator-valued measures. There are no pure photon qubits,
and no exactly orthogonal qubit states.Comment: 10 pages LaTe
Evolution of Liouville density of a chaotic system
An area-preserving map of the unit sphere, consisting of alternating twists
and turns, is mostly chaotic. A Liouville density on that sphere is specified
by means of its expansion into spherical harmonics. That expansion initially
necessitates only a finite number of basis functions. As the dynamical mapping
proceeds, it is found that the number of non-negligible coefficients increases
exponentially with the number of steps. This is to be contrasted with the
behavior of a Schr\"odinger wave function which requires, for the analogous
quantum system, a basis of fixed size.Comment: LaTeX 4 pages (27 kB) followed by four short PostScript files (2 kB +
2 kB + 1 kB + 4 kB
Chaotic Evolution in Quantum Mechanics
A quantum system is described, whose wave function has a complexity which
increases exponentially with time. Namely, for any fixed orthonormal basis, the
number of components required for an accurate representation of the wave
function increases exponentially.Comment: 8 pages (LaTeX 16 kB, followed by PostScript 2 kB for figure
Intrinsic Entanglement Degradation by Multi-Mode Detection
Relations between photon scattering, entanglement and multi-mode detection
are investigated. We first establish a general framework in which one- and
two-photon elastic scattering processes can be discussed, then we focus on the
study of the intrinsic entanglement degradation caused by a multi-mode
detection. We show that any multi-mode scattered state cannot maximally violate
the Bell-CHSH inequality because of the momentum spread. The results presented
here have general validity and can be applied to both deterministic and random
scattering processes.Comment: 12 pages, 4 figures, v3: minor changes. Phys. Rev. A (2004), to be
publishe
Simple computer model for the quantum Zeno effect
This paper presents a simple model for repeated measurement of a quantum
system: the evolution of a free particle, simulated by discretising the
particle's position. This model is easily simulated by computer and provides a
useful arena to investigate the effects of measurement upon dynamics, in
particular the slowing of evolution due to measurement (the `quantum Zeno
effect'). The results of this simulation are discussed for two rather different
sorts of measurement process, both of which are (simplified forms of)
measurements used in previous simulations of position measurement. A number of
interesting results due to measurement are found, and the investigation casts
some light on previous disagreements about the presence or absence of the Zeno
effect.Comment: REVTeX; 12 pages including 11 figures; figures reformatted to be more
readable; some small changes made to the description of the mode
Quantum and classical descriptions of a measuring apparatus
A measuring apparatus is described by quantum mechanics while it interacts
with the quantum system under observation, and then it must be given a
classical description so that the result of the measurement appears as
objective reality. Alternatively, the apparatus may always be treated by
quantum mechanics, and be measured by a second apparatus which has such a dual
description. This article examines whether these two different descriptions are
mutually consistent. It is shown that if the dynamical variable used in the
first apparatus is represented by an operator of the Weyl-Wigner type (for
example, if it is a linear coordinate), then the conversion from quantum to
classical terminology does not affect the final result. However, if the first
apparatus encodes the measurement in a different type of operator (e.g., the
phase operator), the two methods of calculation may give different results.Comment: 18 pages LaTeX (including one encapsulated PostScript figure
Hot Plasma Detected in Active Regions by HINODE/XRT and SDO/AIA
Multiple ratios of Hinode/XRT filters showed evidence of a minor very hot emission measure component in active regions. Recently also SDO/AIA detected hot plasma in the core of an active region. Here we provide estimates showing that the amount of emission measure of the hot component detected with SDO is consistent with that detected with Hinode/XRT
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