5,106 research outputs found
Quantum random walk of two photons in separable and entangled state
We discuss quantum random walk of two photons using linear optical elements.
We analyze the quantum random walk using photons in a variety of quantum states
including entangled states. We find that for photons initially in separable
Fock states, the final state is entangled. For polarization entangled photons
produced by type II downconverter, we calculate the joint probability of
detecting two photons at a given site. We show the remarkable dependence of the
two photon detection probability on the quantum nature of the state. In order
to understand the quantum random walk, we present exact analytical results for
small number of steps like five. We present in details numerical results for a
number of cases and supplement the numerical results with asymptotic analytical
results
The geometry of a naked singularity created by standing waves near a Schwarzschild horizon, and its application to the binary black hole problem
The most promising way to compute the gravitational waves emitted by binary
black holes (BBHs) in their last dozen orbits, where post-Newtonian techniques
fail, is a quasistationary approximation introduced by Detweiler and being
pursued by Price and others. In this approximation the outgoing gravitational
waves at infinity and downgoing gravitational waves at the holes' horizons are
replaced by standing waves so as to guarantee that the spacetime has a helical
Killing vector field. Because the horizon generators will not, in general, be
tidally locked to the holes' orbital motion, the standing waves will destroy
the horizons, converting the black holes into naked singularities that resemble
black holes down to near the horizon radius. This paper uses a spherically
symmetric, scalar-field model problem to explore in detail the following BBH
issues: (i) The destruction of a horizon by the standing waves. (ii) The
accuracy with which the resulting naked singularity resembles a black hole.
(iii) The conversion of the standing-wave spacetime (with a destroyed horizon)
into a spacetime with downgoing waves by the addition of a ``radiation-reaction
field''. (iv) The accuracy with which the resulting downgoing waves agree with
the downgoing waves of a true black-hole spacetime (with horizon). The model
problem used to study these issues consists of a Schwarzschild black hole
endowed with spherical standing waves of a scalar field. It is found that the
spacetime metric of the singular, standing-wave spacetime, and its
radiation-reaction-field-constructed downgoing waves are quite close to those
for a Schwarzschild black hole with downgoing waves -- sufficiently close to
make the BBH quasistationary approximation look promising for
non-tidally-locked black holes.Comment: 12 pages, 6 figure
Spontaneously generated atomic entanglement in free space: reinforced by incoherent pumping
We study spontaneously generated entanglement (SGE) between two identical
multilevel atoms in free space via vacuum-induced radiative coupling. We show
that the SGE in two-atom systems may initially increase with time but
eventually vanishes in the time scale determined by the excited state lifetime
and radiative coupling strength between the two atoms. We demonstrate that a
steady-state SGE can be established by incoherently pumping the excited states
of the two-atom system. We have shown that an appropriate rate of incoherent
pump can help producing optimal SGE. The multilevel systems offer us more
chanel to establish entanglement. The system under consideration could be
realized in a tight trap or atoms/ions doped in a solid substrate.Comment: have some difference with published version (please see PRA
Comparison of Gravitational Wave Detector Network Sky Localization Approximations
Gravitational waves emitted during compact binary coalescences are a
promising source for gravitational-wave detector networks. The accuracy with
which the location of the source on the sky can be inferred from gravitational
wave data is a limiting factor for several potential scientific goals of
gravitational-wave astronomy, including multi-messenger observations. Various
methods have been used to estimate the ability of a proposed network to
localize sources. Here we compare two techniques for predicting the uncertainty
of sky localization -- timing triangulation and the Fisher information matrix
approximations -- with Bayesian inference on the full, coherent data set. We
find that timing triangulation alone tends to over-estimate the uncertainty in
sky localization by a median factor of for a set of signals from
non-spinning compact object binaries ranging up to a total mass of , and the over-estimation increases with the mass of the system. We
find that average predictions can be brought to better agreement by the
inclusion of phase consistency information in timing-triangulation techniques.
However, even after corrections, these techniques can yield significantly
different results to the full analysis on specific mock signals. Thus, while
the approximate techniques may be useful in providing rapid, large scale
estimates of network localization capability, the fully coherent Bayesian
analysis gives more robust results for individual signals, particularly in the
presence of detector noise.Comment: 11 pages, 7 Figure
Electronic energy spectra and wave functions on the square Fibonacci tiling
We study the electronic energy spectra and wave functions on the square
Fibonacci tiling, using an off-diagonal tight-binding model, in order to
determine the exact nature of the transitions between different spectral
behaviors, as well as the scaling of the total bandwidth as it becomes finite.
The macroscopic degeneracy of certain energy values in the spectrum is invoked
as a possible mechanism for the emergence of extended electronic Bloch wave
functions as the dimension changes from one to two
Generation of entangled photon-pairs from a single quantum dot embedded in a planar photonic-crystal cavity
We present a formal theory of single quantum-dot coupling to a planar
photonic crystal that supports quasi-degenerate cavity modes, and use this
theory to describe, and optimize, entangled-photon-pair generation via the
biexciton-exciton cascade. In the generated photon pairs, either both photons
are spontaneously emitted from the dot, or one photon is emitted from the
biexciton spontaneously and the other is emitted via the leaky-cavity mode. In
the strong-coupling regime, the generated photon pairs can be maximally
entangled, in qualitative agreement with the simple dressed-state predictions
of Johne {\em et al.} [Phys. Rev. Lett. vol. 100, 240404 (2008)]. We derive
useful and physically-intuitive analytical formulas for the spectrum of the
emitted photon pairs in the presence of exciton and biexciton broadening, which
is necessary to connect to experiments, and demonstrate the clear failure of
using a dressed-state approach. We also present a method for calculating and
optimizing the entanglement between the emitted photons, which can account for
post-sample spectral filtering. Pronounced entanglement values of greater than
80% are demonstrated using experimentally achievable parameters, even without
spectral filtering.Comment: 5pages 4 fi
Lateral Effects in Fermion Antibunching
Lateral effects are analyzed in the antibunching of a beam of free
non-interacting fermions. The emission of particles from a source is
dynamically described in a 3D full quantum field-theoretical framework. The
size of the source and the detectors, as well as the temperature of the source
are taken into account and the behavior of the visibility is scrutinized as a
function of these parameters.Comment: 22 pages, 4 figure
Measurements of higher order noise correlations in a quantum dot with a finite bandwidth detector
We present measurements of the fourth and fifth cumulants of the distribution
of transmitted charge in a tunable quantum dot. We investigate how the measured
statistics is influenced by the finite bandwidth of the detector and by the
finite measurement time. By including the detector when modeling the system, we
use the theory of full counting statistics to calculate the noise levels for
the combined system. The predictions of the finite-bandwidth model are in good
agreement with measured data
Single-photon excitation of a coherent state: catching the elementary step of stimulated light emission
When a single quantum of electromagnetic field excitation is added to the
same spatio-temporal mode of a coherent state, a new field state is generated
that exhibits intermediate properties between those of the two parents. Such a
single-photon-added coherent state is obtained by the action of the photon
creation operator on a coherent state and can thus be regarded as the result of
the most elementary excitation process of a classical light field. Here we
present and describe in depth the experimental realization of such states and
their complete analysis by means of a novel ultrafast, time-domain, quantum
homodyne tomography technique clearly revealing their non-classical character.Comment: 9 pages, 9 figures. Accepted for publication in Phys. Rev.
On the Interpretation of Supernova Light Echo Profiles and Spectra
The light echo systems of historical supernovae in the Milky Way and local
group galaxies provide an unprecedented opportunity to reveal the effects of
asymmetry on observables, particularly optical spectra. Scattering dust at
different locations on the light echo ellipsoid witnesses the supernova from
different perspectives and the light consequently scattered towards Earth
preserves the shape of line profile variations introduced by asymmetries in the
supernova photosphere. However, the interpretation of supernova light echo
spectra to date has not involved a detailed consideration of the effects of
outburst duration and geometrical scattering modifications due to finite
scattering dust filament dimension, inclination, and image point-spread
function and spectrograph slit width. In this paper, we explore the
implications of these factors and present a framework for future resolved
supernova light echo spectra interpretation, and test it against Cas A and SN
1987A light echo spectra. We conclude that the full modeling of the dimensions
and orientation of the scattering dust using the observed light echoes at two
or more epochs is critical for the correct interpretation of light echo
spectra. Indeed, without doing so one might falsely conclude that differences
exist when none are actually present.Comment: 18 pages, 22 figures, accepted for publication in Ap
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