23,345 research outputs found
Surprises in the suddenly-expanded infinite well
I study the time-evolution of a particle prepared in the ground state of an
infinite well after the latter is suddenly expanded. It turns out that the
probability density shows up quite a surprising behaviour:
for definite times, {\it plateaux} appear for which is
constant on finite intervals for . Elements of theoretical explanation are
given by analyzing the singular component of the second derivative
. Analytical closed expressions are obtained for some
specific times, which easily allow to show that, at these times, the density
organizes itself into regular patterns provided the size of the box in large
enough; more, above some critical time-dependent size, the density patterns are
independent of the expansion parameter. It is seen how the density at these
times simply results from a construction game with definite rules acting on the
pieces of the initial density.Comment: 24 pages, 14 figure
Superconductor-proximity effect in chaotic and integrable billiards
We explore the effects of the proximity to a superconductor on the level
density of a billiard for the two extreme cases that the classical motion in
the billiard is chaotic or integrable. In zero magnetic field and for a uniform
phase in the superconductor, a chaotic billiard has an excitation gap equal to
the Thouless energy. In contrast, an integrable (rectangular or circular)
billiard has a reduced density of states near the Fermi level, but no gap. We
present numerical calculations for both cases in support of our analytical
results. For the chaotic case, we calculate how the gap closes as a function of
magnetic field or phase difference.Comment: 4 pages, RevTeX, 2 Encapsulated Postscript figures. To be published
by Physica Scripta in the proceedings of the "17th Nordic Semiconductor
Meeting", held in Trondheim, June 199
Multiple jet impingement heat transfer characteristic: Experimental investigation of in-line and staggered arrays with crossflow
Heat transfer characteristics were obtained for configurations designed to model the impingement cooled midchord region of air cooled gas turbine airfoils. The configurations tested were inline and staggered two-dimensional arrays of circular jets with ten spanwise rows of holes. The cooling air was constrained to exit in the chordwise direction along the channel formed by the jet orifice plate and the heat transfer surface. Tests were run for chordwise jet hole spacings of five, ten, and fifteen hole diameters; spanwise spacings of four, six, and eight diameters; and channel heights of one, two, three, and six diameters. Mean jet Reynolds numbers ranged from 5000 to 50,000. The thermal boundary condition at the heat transfer test surface was isothermal. Tests were run for sets of geometrically similar configurations of different sizes. Mean and chordwise resolved Nusselt numbers were determined utilizing a specially constructed test surface which was segmented in the chordwise direction
Quantum to classical transition in a system with a mixed classical dynamics
We study how decoherence rules the quantum-classical transition of the Kicked
Harmonic Oscillator (KHO). When the amplitude of the kick is changed the system
presents a classical dynamics that range from regular to a strong chaotic
behavior. We show that for regular and mixed classical dynamics, and in the
presence of noise, the distance between the classical and the quantum phase
space distributions is proportional to a single parameter which relates the effective Planck constant
, the kick amplitude and the diffusion constant . This
is valid when , a case that is always attainable in the semiclassical
regime independently of the value of the strength of noise given by . Our
results extend a recent study performed in the chaotic regime.Comment: 10 pages, 7 figure
Entanglement-enhanced measurement of a completely unknown phase
The high-precision interferometric measurement of an unknown phase is the
basis for metrology in many areas of science and technology. Quantum
entanglement provides an increase in sensitivity, but present techniques have
only surpassed the limits of classical interferometry for the measurement of
small variations about a known phase. Here we introduce a technique that
combines entangled states with an adaptive algorithm to precisely estimate a
completely unspecified phase, obtaining more information per photon that is
possible classically. We use the technique to make the first ab initio
entanglement-enhanced optical phase measurement. This approach will enable
rapid, precise determination of unknown phase shifts using interferometry.Comment: 6 pages, 4 figure
Observation of a Chiral State in a Microwave Cavity
A microwave experiment has been realized to measure the phase difference of
the oscillating electric field at two points inside the cavity. The technique
has been applied to a dissipative resonator which exhibits a singularity --
called exceptional point -- in its eigenvalue and eigenvector spectrum. At the
singularity, two modes coalesce with a phase difference of We
conclude that the state excited at the singularity has a definitiv chirality.Comment: RevTex 4, 5 figure
Adaptive Measurements in the Optical Quantum Information Laboratory
Adaptive techniques make practical many quantum measurements that would
otherwise be beyond current laboratory capabilities. For example: they allow
discrimination of nonorthogonal states with a probability of error equal to the
Helstrom bound; they allow measurement of the phase of a quantum oscillator
with accuracy approaching (or in some cases attaining) the Heisenberg limit;
and they allow estimation of phase in interferometry with a variance scaling at
the Heisenberg limit, using only single qubit measurement and control. Each of
these examples has close links with quantum information, in particular
experimental optical quantum information: the first is a basic quantum
communication protocol; the second has potential application in linear optical
quantum computing; the third uses an adaptive protocol inspired by the quantum
phase estimation algorithm. We discuss each of these examples, and their
implementation in the laboratory, but concentrate upon the last, which was
published most recently [Higgins {\em et al.}, Nature vol. 450, p. 393, 2007].Comment: 12 pages, invited paper to be published in IEEE Journal of Selected
Topics in Quantum Electronics: Quantum Communications and Information Scienc
Induced superconductivity distinguishes chaotic from integrable billiards
Random-matrix theory is used to show that the proximity to a superconductor
opens a gap in the excitation spectrum of an electron gas confined to a
billiard with a chaotic classical dynamics. In contrast, a gapless spectrum is
obtained for a non-chaotic rectangular billiard, and it is argued that this is
generic for integrable systems.Comment: 4 pages RevTeX, 2 eps-figures include
Decoherence and the quantum-classical limit in the presence of chaos
We investigate how decoherence affects the short-time separation between
quantum and classical dynamics for classically chaotic systems, within the
framework of a specific model. For a wide range of parameters, the distance
between the corresponding phase-space distributions depends on a single
parameter that relates an effective Planck constant ,
the Lyapunov coeffficient, and the diffusion constant. This distance peaks at a
time that depends logarithmically on , in agreement with
previous estimations of the separation time for Hamiltonian systems. However,
for , the separation remains small, going down with , so the concept of separation time loses its meaning.Comment: 5 pages, 4 figures (in 6 postscript files) two of them are color
figure
Classical and quantum chaos in a circular billiard with a straight cut
We study classical and quantum dynamics of a particle in a circular billiard
with a straight cut. This system can be integrable, nonintegrable with soft
chaos, or nonintegrable with hard chaos, as we vary the size of the cut. We use
a quantum web to show differences in the quantum manifestations of classical
chaos for these three different regimes.Comment: LaTeX2e, 8 pages including 3 Postscript figures and 4 GIF figures,
submitted to Phys. Rev.
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