9,581 research outputs found
Comparison between S. T. radar and in situ balloon measurements
A campaign for simultaneous in situ and remote observation of both troposphere and stratosphere took place near Aire-sur-l'Adour (in southeastern France) on May 4, 1984. The aim of this campaign was a better understanding of the physics of radar echoes. The backscattered signal obtained with a stratosphere-troposphere radar both at the vertical and 15 deg. off vertical is compared with the velocity and temperature measurements made in the same region (about 10 km north of the radar site) by balloon-borne ionic anenometers and temperature sensors. In situ measurements clearly indicate that the temperature fluctuations are not always consistent with the standard turbulent theory. Nevertheless, the assumptions generally made (isotropy and turbulent field in k) and the classical formulation so derived for radar reflectivity are able to reproduce the shape of the radar return power profiles in oblique directions. Another significant result is the confirmation of the role played by the atmospheric stratification in the vertical echo power. It is important to develop these simultaneous in situ and remote experiments for a better description of the dynamical and thermal structure of the atmosphere and for a better understanding of the mechanisms governing clear-air radar reflectivity
Practical quantum realization of the ampere from the electron charge
One major change of the future revision of the International System of Units
(SI) is a new definition of the ampere based on the elementary charge \emph{e}.
Replacing the former definition based on Amp\`ere's force law will allow one to
fully benefit from quantum physics to realize the ampere. However, a quantum
realization of the ampere from \emph{e}, accurate to within in
relative value and fulfilling traceability needs, is still missing despite many
efforts have been spent for the development of single-electron tunneling
devices. Starting again with Ohm's law, applied here in a quantum circuit
combining the quantum Hall resistance and Josephson voltage standards with a
superconducting cryogenic amplifier, we report on a practical and universal
programmable quantum current generator. We demonstrate that currents generated
in the milliampere range are quantized in terms of
( is the Josephson frequency) with a measurement uncertainty of
. This new quantum current source, able to deliver such accurate
currents down to the microampere range, can greatly improve the current
measurement traceability, as demonstrated with the calibrations of digital
ammeters. Beyond, it opens the way to further developments in metrology and in
fundamental physics, such as a quantum multimeter or new accurate comparisons
to single electron pumps.Comment: 15 pages, 4 figure
Evidence of growing spatial correlations at the glass transition from nonlinear response experiments
The ac nonlinear dielectric response of glycerol was
measured close to its glass transition temperature to investigate the
prediction that supercooled liquids respond in an increasingly non-linear way
as the dynamics slows down (as spin-glasses do). We find that
indeed displays several non trivial features. It is peaked
as a function of the frequency and obeys scaling as a function of
, with the relaxation time of the liquid. The height
of the peak, proportional to the number of dynamically correlated molecules
, increases as the system becomes glassy, and decays as a
power-law of over several decades beyond the peak. These findings
confirm the collective nature of the glassy dynamics and provide the first
direct estimate of the dependence of .Comment: 22 pages, 6 figures. With respect to v1, a few new sentences were
added in the introduction and conclusion, references were updated, some typos
corrected
Probe of Lorentz Invariance Violation effects and determination of the distance of PG 1553+113
The high frequency peaked BL Lac object PG 1553+113 underwent a flaring event
in 2012. The High Energy Stereoscopic System (H.E.S.S.) observed this source
for two consecutive nights at very high energies (VHE, 100~GeV). The data
show an increase of a factor of three of the flux with respect to archival
measurements with the same instrument and hints of intra-night variability. The
data set has been used to put constraints on possible Lorentz invariance
violation (LIV), manifesting itself as an energy dependence of the velocity of
light in vacuum, and to set limits on the energy scale at which Quantum Gravity
effects causing LIV may arise. With a new method to combine H.E.S.S. and Fermi
large area telescope data, the previously poorly known redshift of PG 1555+113
has been determined to be close to the value derived from optical measurements.Comment: 2014 Fermi Symposium proceedings - eConf C14102.
The power of random measurements: measuring Tr(\rho^n) on single copies of \rho
While it is known that Tr(\rho^n) can be measured directly (i.e., without
first reconstructing the density matrix) by performing joint measurements on n
copies of the same state rho, it is shown here that random measurements on
single copies suffice, too. Averaging over the random measurements directly
yields estimates of Tr(\rho^n), even when it is not known what measurements
were actually performed (so that one cannot reconstruct \rho)
Proximity effect between two superconductors spatially resolved by scanning tunneling spectroscopy
We present a combined experimental and theoretical study of the proximity
effect in an atomic-scale controlled junction between two different
superconductors. Elaborated on a Si(111) surface, the junction comprises a Pb
nanocrystal with an energy gap of 1.2 meV, connected to a crystalline atomic
monolayer of lead with a gap of 0.23 meV. Using in situ scanning tunneling
spectroscopy we probe the local density of states of this hybrid system both in
space and in energy, at temperatures below and above the critical temperature
of the superconducting monolayer. Direct and inverse proximity effects are
revealed with high resolution. Our observations are precisely explained with
the help of a self-consistent solution of the Usadel equations. In particular,
our results demonstrate that in the vicinity of the Pb islands, the Pb
monolayer locally develops a finite proximity-induced superconducting order
parameter, well above its own bulk critical temperature. This leads to a giant
proximity effect where the superconducting correlations penetrate inside the
monolayer a distance much larger than in a non-superconducting metal.Comment: 13 pages, 14 figures, accepted for publication in Physical Review
Mg II Absorber Number Density at z~0.05: Implications for Omega_DLA Evolution
An unbiased sample of 147 quasar/AGN spectra, obtained with the FOS/HST, has
been searched for intervening MgII absorbers over the redshift range 0<z<0.15.
The total redshift path searched is 18.8, with the survey being 80% complete to
a 5-sigma rest-frame equivalent width, W_r(2796), of 0.6 Ang. Main results of
this work are: [1] Four systems were found, with a mean redshift of =0.06,
yielding a redshift number density dN/dz=0.22(+0.12)(-0.09) for absorbers with
W_r(2796)>0.6 Ang. This is consistent with the value expected if these systems
do not evolve from higher redshifts (z=2.2). [2] No systems with W_r(2796)<0.6
Ang were found. It is a 2-sigma result to have a null detection of smaller
W_r(2796) systems. If this implies a turnover in the low W_r(2796) region of
the equivalent width distribution at z~0, then there is at least a 25%
reduction in the average galaxy gas cross section from z<0.2 galaxies. [3]
These systems have strong FeII absorption and are good candidates for damped
Ly-alpha absorbers DLAs (see Rao & Turnshek 2000, ApJS, 130, 1). This
translates to a redshift number density of dN/dz=0.08(+0.09)(-0.05) for DLAs at
z~0. In tandem with the data analyzed by Rao & Turnshek, these results indicate
that the redshift number density of DLAs does not evolve from z~4 to z~0. If
the HI mass function does not evolve from z~0.5 to z~0, then the cosmological
HI mass density is also deduced to not evolve from z~4 to z~0. These z~0
results for MgII absorption-selected DLAs are at odds with those based upon
21-cm emission from HI galaxies by a factor of five to six.Comment: 23 pages, 7 Figures, accepted to ApJ. Replaced version includes
additional figures and tables and substantial modifications to the tex
Entanglement Witnesses from Single-Particle Interference
We describe a general method of realizing entanglement witnesses in terms of
the interference pattern of a single quantum probe. After outlining the
principle, we discuss specific realizations both with electrons in mesoscopic
Aharonov-Bohm rings and with photons in standard Young's double-slit or
coherent-backscattering interferometers.Comment: 5 pages, 3 figures, epl2, uses pstricks.st
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