3,886 research outputs found
On Geometric Phase from Pure Projections
The geometric phase is usually treated as a quantity modulo 2\pi, a
convention carried over from early work on the subject. The results of a series
of optical interference experiments involving polarization of light, done by
the present author (reviewed in R.Bhandari, Phys. Rep. 281 (1997) p.1) question
the usefulness of such a definition of the geometric phase in that it throws
away useful and measurable information about the system, for example strengths
of singularities giving rise to the geometric phase. Such singularities have
been directly demonstrated by phase-shift measurement in interference
experiments. In this paper, two recent polarization experiments (Hariharan
et.al., J.Mod.Opt. 44 (1997)p.707 and Berry and Klein, J.Mod.Opt. 43
(1996)p.165) are analysed and compared with previous experiments and
potentially detectible singularities in these experiments pointed out.Comment: Latex, 15 pages, 6 figures; ([email protected]
Observable Dirac-type singularities in Berry's phase and the monopole
The physical reality and observability of 2n\pi Berry phases, as opposed to
the usually considered modulo 2\pi topological phases is demonstrated with the
help of computer simulation of a model adiabatic evolution whose parameters are
varied along a closed loop in the parameter space. Using the analogy of Berry's
phase with the Dirac monopole, it is concluded that an interferometer loop
taken around a magnetic monopole of strength n/2 yields an observable 2n\pi
phase shift, where n is an integer. An experiment to observe the effect is
proposed.Comment: 12 pages Latex, 3 postscript figures; submitted to Physical Review
Letters 15 September 2000; revised 19 November 200
The physiological effects of Transcranial Electrical Stimulation do not apply to parameters commonly used in studies of Cognitive Neuromodulation
Transcranial direct current stimulation (tDCS) and transcranial random noise stimulation (tRNS) have been claimed to produce many remarkable enhancements in perception, cognition, learning and numerous clinical conditions. The physiological basis of the claims for tDCS rests on the finding that 1 mA of unilateral anodal stimulation increases cortical excitation and 1 mA of cathodal produces inhibition. Here we show that these classic excitatory and inhibitory effects do not hold for the bilateral stimulation or 2 mA intensity conditions favoured in cognitive enhancement experiments. This is important because many, including some of the most salient claims are based on experiments using 2 mA bilateral stimulation. The claims for tRNS are also based on unilateral stimulation. Here we show that, again the classic excitatory effects of unilateral tRNS do not extend to the bilateral stimulation preferred in enhancement experiments. Further, we show that the effects of unilateral tRNS do not hold when one merely doubles the stimulation duration. We are forced to two conclusions: (i) that even if all the data on TES enhancements are true, the physiological explanations on which the claims are based are at best not established but at worst false, and (ii) that we cannot explain, scientifically at least, how so many experiments can have obtained data consistent with physiological effects that may not exist
Topological properties of Berry's phase
By using a second quantized formulation of level crossing, which does not
assume adiabatic approximation, a convenient formula for geometric terms
including off-diagonal terms is derived. The analysis of geometric phases is
reduced to a simple diagonalization of the Hamiltonian in the present
formulation. If one diagonalizes the geometric terms in the infinitesimal
neighborhood of level crossing, the geometric phases become trivial for any
finite time interval . The topological interpretation of Berry's phase such
as the topological proof of phase-change rule thus fails in the practical
Born-Oppenheimer approximation, where a large but finite ratio of two time
scales is involved.Comment: 9 pages. A new reference was added, and the abstract and the
presentation in the body of the paper have been expanded and made more
precis
The physiological effects of transcranial electrical stimulation do not apply to parameters commonly used in studies of cognitive neuromodulation
Transcranial direct current stimulation (tDCS) and transcranial random noise stimulation (tRNS) have been claimed to produce many remarkable enhancements in perception, cognition, learning and numerous clinical conditions. The physiological basis of the claims for tDCS rests on the finding that 1 mA of unilateral anodal stimulation increases cortical excitation and 1 mA of cathodal produces inhibition. Here we show that these classic excitatory and inhibitory effects do not hold for the bilateral stimulation or 2 mA intensity conditions favoured in cognitive enhancement experiments. This is important because many, including some of the most salient claims are based on experiments using 2 mA bilateral stimulation. The claims for tRNS are also based on unilateral stimulation. Here we show that, again the classic excitatory effects of unilateral tRNS do not extend to the bilateral stimulation preferred in enhancement experiments. Further, we show that the effects of unilateral tRNS do not hold when one merely doubles the stimulation duration. We are forced to two conclusions: (i) that even if all the data on TES enhancements are true, the physiological explanations on which the claims are based are at best not established but at worst false, and (ii) that we cannot explain, scientifically at least, how so many experiments can have obtained data consistent with physiological effects that may not exist
The Geometric Phase and Ray Space Isometries
We study the behaviour of the geometric phase under isometries of the ray
space. This leads to a better understanding of a theorem first proved by
Wigner: isometries of the ray space can always be realised as projections of
unitary or anti-unitary transformations on the Hilbert space. We suggest that
the construction involved in Wigner's proof is best viewed as an use of the
Pancharatnam connection to ``lift'' a ray space isometry to the Hilbert space.Comment: 17 pages, Latex file, no figures, To appear in Pramana J. Phy
Tackling 3D ToF Artifacts Through Learning and the FLAT Dataset
Scene motion, multiple reflections, and sensor noise introduce artifacts in
the depth reconstruction performed by time-of-flight cameras. We propose a
two-stage, deep-learning approach to address all of these sources of artifacts
simultaneously. We also introduce FLAT, a synthetic dataset of 2000 ToF
measurements that capture all of these nonidealities, and allows to simulate
different camera hardware. Using the Kinect 2 camera as a baseline, we show
improved reconstruction errors over state-of-the-art methods, on both simulated
and real data.Comment: ECCV 201
Cosmogenic effects in Mbale chondrite
This article does not have an abstract
Geometric Phases and Multiple Degeneracies in Harmonic Resonators
In a recent experiment Lauber et al. have deformed cyclically a microwave
resonator and have measured the adiabatic normal-mode wavefunctions for each
shape along the path of deformation. The nontrivial observed cyclic phases
around a 3-fold degeneracy were accounted for by Manolopoulos and Child within
an approximate theory. However, open-path geometrical phases disagree with
experiment. By solving exactly the problem, we find unsuspected extra
degeneracies around the multiple one that account for the measured phase
changes throughout the path. It turns out that proliferation of additional
degeneracies around a multiple one is a common feature of quantum mechanics.Comment: 4 pages, 4 figures. Accepted in Phys. Rev. Let
The first interferometric detections of Fast Radio Bursts
We present the first interferometric detections of Fast Radio Bursts (FRBs),
an enigmatic new class of astrophysical transient. In a 180-day survey of the
Southern sky we discovered 3 FRBs at 843 MHz with the UTMOST array, as part of
commissioning science during a major ongoing upgrade. The wide field of view of
UTMOST ( deg) is well suited to FRB searches. The primary beam
is covered by 352 partially overlapping fan-beams, each of which is searched
for FRBs in real time with pulse widths in the range 0.655 to 42 ms, and
dispersion measures 2000 pc cm. Detections of FRBs with the UTMOST
array places a lower limit on their distances of km (limit of
the telescope near-field) supporting the case for an astronomical origin.
Repeating FRBs at UTMOST or an FRB detected simultaneously with the Parkes
radio telescope and UTMOST, would allow a few arcsec localisation, thereby
providing an excellent means of identifying FRB host galaxies, if present. Up
to 100 hours of follow-up for each FRB has been carried out with the UTMOST,
with no repeating bursts seen. From the detected position, we present 3
error ellipses of 15 arcsec x 8.4 deg on the sky for the point of origin for
the FRBs. We estimate an all-sky FRB rate at 843 MHz above a fluence of 11 Jy ms of events sky d at the 95
percent confidence level. The measured rate of FRBs at 843 MHz is of order two
times higher than we had expected, scaling from the FRB rate at the Parkes
radio telescope, assuming that FRBs have a flat spectral index and a uniform
distribution in Euclidean space. We examine how this can be explained by FRBs
having a steeper spectral index and/or a flatter log-log
distribution than expected for a Euclidean Universe.Comment: 13 pages, 8 figures, 2 table
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