26,676 research outputs found
Stochastic dynamics of a Josephson junction threshold detector
We generalize the stochastic path integral formalism by considering
Hamiltonian dynamics in the presence of general Markovian noise. Kramers'
solution of the activation rate for escape over a barrier is generalized for
non-Gaussian driving noise in both the overdamped and underdamped limit. We
apply our general results to a Josephson junction detector measuring the
electron counting statistics of a mesoscopic conductor. Activation rate
dependence on the third current cumulant includes an additional term
originating from the back-action of the measurement circuit.Comment: 5 pages, 2 figures, discussion of experiment added, typos correcte
The structure of chromospheres around late-type giants and supergiants
Observations alpha Tau (K5III) and beta Gru (M2II) made at high resolution are used to confirm line identifications of features blended at low resolution. The high resolution spectra allow selected pairs of lines to be used to find the electron density and the opacity. Methods for determining these factors and the usual emission measure are presented. The electron density and opacity can be used together with the emission measure to place constraints on the structure of the atmosphere. The line formation processes are briefly discussed. Photo-excitation by strong lines appears to be important in these late type atmospheres
Experimental Violation of Two-Party Leggett-Garg Inequalities with Semi-weak Measurements
We generalize the derivation of Leggett-Garg inequalities to systematically
treat a larger class of experimental situations by allowing multi-particle
correlations, invasive detection, and ambiguous detector results. Furthermore,
we show how many such inequalities may be tested simultaneously with a single
setup. As a proof of principle, we violate several such two-particle
inequalities with data obtained from a polarization-entangled biphoton state
and a semi-weak polarization measurement based on Fresnel reflection. We also
point out a non- trivial connection between specific two-party Leggett-Garg
inequality violations and convex sums of strange weak values.Comment: 4 pages, 6 figure
An astronomical search for evidence of new physics: Limits on gravity-induced birefringence from the magnetic white dwarf RE J0317-853
The coupling of the electromagnetic field directly with gravitational gauge
fields leads to new physical effects that can be tested using astronomical
data. Here we consider a particular case for closer scrutiny, a specific
nonminimal coupling of torsion to electromagnetism, which enters into a
metric-affine geometry of space-time. We show that under the assumption of this
nonminimal coupling, spacetime is birefringent in the presence of such a
gravitational field. This leads to the depolarization of light emitted from
extended astrophysical sources. We use polarimetric data of the magnetic white
dwarf to set strong constraints on the essential coupling
constant for this effect, giving k^2 \lsim (19 {m})^2 .Comment: Statements about Moffat's NGT modified. Accepted for publication in
Phys.Rev.
Temporal variations in scattering and dispersion measure in the Crab Pulsar and their effect on timing precision
We have measured variations in scattering time scales in the Crab Pulsar over
a 30-year period, using observations made at 610 MHz with the 42-ft telescope
at Jodrell Bank Observatory. Over more recent years, where regular Lovell
Telescope observations at frequencies around 1400 MHz were available, we have
also determined the dispersion measure variations, after disentangling the
scattering delay from the dispersive delay. We demonstrate a relationship
between scattering and dispersion measure variations, with a correlation
coefficient of . The short time scales over which these quantities
vary, the size of the variations, and the close correlation between scattering
and dispersion measure all suggest that the effects are due to discrete
structures within the Crab Nebula, with size scales of AU
(corresponding to an angular size of mas at an assumed distance of 2200
pc). We mitigate the effects of scattering on the observed pulse shape by using
the measured scattering information to modify the template used for generating
the pulse arrival times, thus improving the precision to which the pulsar can
be timed. We test this on timing data taken during periods of high scattering,
and obtain a factor of two improvement in the root mean square of the timing
residuals.Comment: 10 pages, 7 figures. Accepted for publication in MNRA
Model for Cumulative Solar Heavy Ion Energy and Linear Energy Transfer Spectra
A probabilistic model of cumulative solar heavy ion energy and LET spectra is developed for spacecraft design applications. Spectra are given as a function of confidence level, mission time period during solar maximum and shielding thickness. It is shown that long-term solar heavy ion fluxes exceed galactic cosmic ray fluxes during solar maximum for shielding levels of interest. Cumulative solar heavy ion fluences should therefore be accounted for in single event effects rate calculations and in the planning of space missions
Numerical relativity simulation of GW150914 beyond general relativity
We produce the first astrophysically-relevant numerical binary black hole
gravitational waveform in a higher-curvature theory of gravity beyond general
relativity. We simulate a system with parameters consistent with GW150914, the
first LIGO detection, in order-reduced dynamical Chern-Simons gravity, a theory
with motivations in string theory and loop quantum gravity. We present results
for the leading-order corrections to the merger and ringdown waveforms, as well
as the ringdown quasi-normal mode spectrum. We estimate that such corrections
may be discriminated in detections with signal to noise ratio , with the precise value depending on the dimension of the GR waveform
family used in data analysis.Comment: 7 pages + appendices, 8 figures, Updated to match Phys. D. Rev
articl
Parity meter for charge qubits: an efficient quantum entangler
We propose a realization of a charge parity meter based on two double quantum
dots alongside a quantum point contact. Such a device is a specific example of
the general class of mesoscopic quadratic quantum measurement detectors
previously investigated by Mao et al. [Phys. Rev. Lett. 93, 056803 (2004)]. Our
setup accomplishes entangled state preparation by a current measurement alone,
and allows the qubits to be effectively decoupled by pinching off the parity
meter. Two applications of the parity meter are discussed: the measurement of
Bell's inequality in charge qubits and the realization of a controlled NOT
gate.Comment: 8 pages, 4 figures; v2: discussion of measurement time, references
adde
Analysis of Skylab 2 S193 scatterometer data
SKYLAB II S193 Scatterometer data for the passes of June 5, 1973, over the Gulf of Mexico and June 6, 1973, over Pacific Hurricane AVA were analyzed. The S193 scatterometer measured the radar cross section of the ocean at 13.9 GHz (Ku-band) as a function of incidence angle. The fields-of-view of the scatterometer were known. In the absence of a large body of Ku-band ocean radar data, the results of the NRL experiments at X-band (8.9 GHz) were used for comparison. The S193 data of June 5, 1973, when a practically uniform wind field was present, show reasonable agreement with the NRL empirical and theoretical models
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