39,760 research outputs found
Visibility based angular power spectrum estimation in low frequency radio interferometric observations
We present two estimators to quantify the angular power spectrum of the sky
signal directly from the visibilities measured in radio interferometric
observations. This is relevant for both the foregrounds and the cosmological
21-cm signal buried therein. The discussion here is restricted to the Galactic
synchrotron radiation, the most dominant foreground component after point
source removal. Our theoretical analysis is validated using simulations at 150
MHz, mainly for GMRT and also briefly for LOFAR. The Bare Estimator uses
pairwise correlations of the measured visibilities, while the Tapered Gridded
Estimator uses the visibilities after gridding in the uv plane. The former is
very precise, but computationally expensive for large data. The latter has a
lower precision, but takes less computation time which is proportional to the
data volume. The latter also allows tapering of the sky response leading to
sidelobe suppression, an useful ingredient for foreground removal. Both
estimators avoid the positive bias that arises due to the system noise. We
consider amplitude and phase errors of the gain, and the w-term as possible
sources of errors . We find that the estimated angular power spectrum is
exponentially sensitive to the variance of the phase errors but insensitive to
amplitude errors. The statistical uncertainties of the estimators are affected
by both amplitude and phase errors. The w-term does not have a significant
effect at the angular scales of our interest. We propose the Tapered Gridded
Estimator as an effective tool to observationally quantify both foregrounds and
the cosmological 21-cm signal.Comment: 20 pages, 15 figures, 1 table.One typo corrected in Fig.13. Accepted
for publication in MNRA
Experimental observation of fractional topological phases with photonic qudits
Geometrical and topological phases play a fundamental role in quantum theory.
Geometric phases have been proposed as a tool for implementing unitary gates
for quantum computation. A fractional topological phase has been recently
discovered for bipartite systems. The dimension of the Hilbert space determines
the topological phase of entangled qudits under local unitary operations. Here
we investigate fractional topological phases acquired by photonic entangled
qudits. Photon pairs prepared as spatial qudits are operated inside a Sagnac
interferometer and the two-photon interference pattern reveals the topological
phase as fringes shifts when local operations are performed. Dimensions and were tested, showing the expected theoretical values.Comment: 6 pages, 4 figure
Engineering Art Galleries
The Art Gallery Problem is one of the most well-known problems in
Computational Geometry, with a rich history in the study of algorithms,
complexity, and variants. Recently there has been a surge in experimental work
on the problem. In this survey, we describe this work, show the chronology of
developments, and compare current algorithms, including two unpublished
versions, in an exhaustive experiment. Furthermore, we show what core
algorithmic ingredients have led to recent successes
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