40,886 research outputs found
Phaseless VLBI mapping of compact extragalactic radio sources
The problem of phaseless aperture synthesis is of current interest in
phase-unstable VLBI with a small number of elements when either the use of
closure phases is not possible (a two-element interferometer) or their quality
and number are not enough for acceptable image reconstruction by standard
adaptive calibration methods. Therefore, we discuss the problem of unique image
reconstruction only from the spectrum magnitude of a source. We suggest an
efficient method for phaseless VLBI mapping of compact extragalactic radio
sources. This method is based on the reconstruction of the spectrum magnitude
for a source on the entire UV plane from the measured visibility magnitude on a
limited set of points and the reconstruction of the sought-for image of the
source by Fienup's method from the spectrum magnitude reconstructed at the
first stage. We present the results of our mapping of the extragalactic radio
source 2200 +420 using astrometric and geodetic observations on a global VLBI
array. Particular attention is given to studying the capabilities of a
two-element interferometer in connection with the putting into operation of a
Russian-made radio interferometer based on Quasar RT-32 radio telescopes.Comment: 21 pages, 6 figure
Multiparticle interference in electronic Mach-Zehnder interferometers
We study theoretically electronic Mach-Zehnder interferometers built from
integer quantum Hall edge states, showing that the results of recent
experiments can be understood in terms of multiparticle interference effects.
These experiments probe the visibility of Aharonov-Bohm (AB) oscillations in
differential conductance as an interferometer is driven out of equilibrium by
an applied bias, finding a lobe pattern in visibility as a function of voltage.
We calculate the dependence on voltage of the visibility and the phase of AB
oscillations at zero temperature, taking into account long range interactions
between electrons in the same edge for interferometers operating at a filling
fraction . We obtain an exact solution via bosonization for models in
which electrons interact only when they are inside the interferometer. This
solution is non-perturbative in the tunneling probabilities at quantum point
contacts. The results match observations in considerable detail provided the
transparency of the incoming contact is close to one-half: the variation in
visibility with bias voltage consists of a series of lobes of decreasing
amplitude, and the phase of the AB-fringes is practically constant inside the
lobes but jumps by at the minima of the visibility. We discuss in
addition the consequences of approximations made in other recent treatments of
this problem. We also formulate perturbation theory in the interaction strength
and use this to study the importance of interactions that are not internal to
the interferometer.Comment: 20 pages, 15 figures, final version as publishe
Advances in Calibration and Imaging Techniques in Radio Interferometry
This paper summarizes some of the major calibration and image reconstruction
techniques used in radio interferometry and describes them in a common
mathematical framework. The use of this framework has a number of benefits,
ranging from clarification of the fundamentals, use of standard numerical
optimization techniques, and generalization or specialization to new
algorithms
The Epistemology of scheduling problems
Scheduling is a knowledge-intensive task spanning over many activities in day-to-day life. It deals with the temporally-bound assignment of jobs to resources. Although scheduling has been extensively researched in the AI community for the past 30 years, efforts have primarily focused on specific applications, algorithms, or 'scheduling shells' and no comprehensive analysis exists on the nature of scheduling problems, which provides a formal account of what scheduling is, independently of the way scheduling problems can be approached. Research on KBS development by reuse makes use of ontologies, to provide knowledge-level specifications of reusable KBS components. In this paper we describe a task ontology, which formally characterises the nature of scheduling problems, independently of particular application domains and in-dependently of how the problems can be solved. Our results provide a comprehensive, domain-independent and formally specified refer-ence model for scheduling applications. This can be used as the ba-sis for further analyses of the class of scheduling problems and also as a concrete reusable resource to support knowledge acquisition and system development in scheduling applications
Observation of the Mott Insulator to Superfluid Crossover of a Driven-Dissipative Bose-Hubbard System
Dissipation is ubiquitous in nature and plays a crucial role in quantum
systems such as causing decoherence of quantum states. Recently, much attention
has been paid to an intriguing possibility of dissipation as an efficient tool
for preparation and manipulation of quantum states. Here we report the
realization of successful demonstration of a novel role of dissipation in a
quantum phase transition using cold atoms. We realize an engineered dissipative
Bose-Hubbard system by introducing a controllable strength of two-body
inelastic collision via photo-association for ultracold bosons in a
three-dimensional optical lattice. In the dynamics subjected to a slow
ramp-down of the optical lattice, we find that strong on-site dissipation
favors the Mott insulating state: the melting of the Mott insulator is delayed
and the growth of the phase coherence is suppressed. The controllability of the
dissipation is highlighted by quenching the dissipation, providing a novel
method for investigating a quantum many-body state and its non-equilibrium
dynamics.Comment: 26 pages, 17 figure
Interactions in Electronic Mach-Zehnder Interferometers with Copropagating Edge Channels
We study Coulomb interactions in the finite bias response of Mach-Zehnder
interferometers, which exploit copropagating edge states in the integer quantum
Hall effect. Here, interactions are particularly important since the coherent
coupling of edge channels is due to a resonant mechanism that is spoiled by
inelastic processes. We find that interactions yield a saturation, as a
function of bias voltage, of the period-averaged interferometer current, which
gives rise to unusual features, such as negative differential conductance,
enhancement of the visibility of the current, and nonbounded or even diverging
visibility of the differential conductance.Comment: 5 pages, 2 figure
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