2,858 research outputs found
Compensating fictitious magnetic field gradients in optical microtraps by using elliptically polarized dipole light
Tightly focused optical dipole traps induce vector light shifts ("fictitious
magnetic fields") which complicate their use for single-atom trapping and
manipulation. The problem can be mitigated by adding a larger, real magnetic
field, but this solution is not always applicable; in particular, it precludes
fast switching to a field-free configuration. Here we show that this issue can
be addressed elegantly by deliberately adding a small elliptical polarization
component to the dipole beam. In our experiments with single Rb atoms in
a chopped trap, we observe improvements up to a factor 11 of the trap lifetime
compared to the standard, seemingly ideal linear polarization. This effect
results from a modification of heating processes via spin-state diffusion in
state-dependent trapping potentials. We develop Monte-Carlo simulations of the
evolution of the atom's internal and motional states and find that they agree
quantitatively with the experimental data. The method is general and can be
applied in all experiments where the longitudinal polarization component is
non-negligible.Comment: 6 pages, 5 figure
Bond Liquidity Premia
Recent asset pricing models of limits to arbitrage emphasize the role of funding conditions faced by financial intermediaries. In the US, the repo market is the key funding market. Then, the premium of on-the-run U.S. Treasury bonds should share a common component with risk premia in other markets. This observation leads to the following identification strategy. We measure the value of funding liquidity from the cross-section of on-the-run premia by adding a liquidity factor to an arbitrage-free term structure model. As predicted, we find that funding liquidity explains the cross-section of risk premia. An increase in the value of liquidity predicts lower risk premia for on-the-run and off-the-run bonds but higher risk premia on LIBOR loans, swap contracts and corporate bonds. Moreover, the impact is large and pervasive through crisis and normal times. We check the interpretation of the liquidity factor. It varies with transaction costs, S&P500 valuation ratios and aggregate uncertainty. More importantly, the liquidity factor varies with narrow measures of monetary aggregates and measures of bank reserves. Overall, the results suggest that different securities serve, in part, and to varying degrees, to fulfill investors' uncertain future needs for cash depending on the ability of intermediaries to provide immediacy.Financial markets; Financial stability
Overlapping two standing-waves in a microcavity for a multi-atom photon interface
We develop a light-matter interface enabling strong and uniform coupling
between a chain of cold atoms and photons of an optical cavity. This interface
is a fiber Fabry-Perot cavity, doubly resonant for both the wavelength of the
atomic transition and for a geometrically commensurate red-detuned intracavity
trapping lattice. Fulfilling the condition of a strong and uniform atom-photon
coupling requires optimization of the spatial overlap between the two standing
waves in the cavity. In a strong-coupling cavity, where the mode waists and
Rayleigh range are small, we derive the expression of the optimal trapping
wavelength taking into account the Gouy phase. The main parameter controlling
the overlap of the standing waves is the relative phase shift at the reflection
on the cavity mirrors between the two wavelengths, for which we derive the
optimal value. We have built a microcavity optimized according to these
results, employing custom-made mirrors with engineered reflection phase for
both wavelengths. We present a method to measure with high precision the
relative phase shift at reflection, which allows us to determine the spatial
overlap of the two modes in this cavity.Comment: 14 pages, 7 figure
Economie de la diarrhée virale bovine ( BVD) : une meta-analyse
La diarrhĂ©e virale bovine (BVD) est une maladie endĂ©mique et cosmopolite des ruminants. De nombreux pays ont mis en place des plans dâĂ©radication. Plusieurs travaux de synthĂšse ont Ă©valuĂ© les stratĂ©gies autours de cette maladie, mais aucune synthĂšse sur son Ă©valuation Ă©conomique nâest disponible Ă ce jour. Lâobjectif est de synthĂ©tiser les donnĂ©es sur le coĂ»t total de la BVD et la baisse de ce coĂ»t lors de la mise en place de mesures de contrĂŽle, grĂące Ă deux mĂ©ta-analyses.Les mĂ©ta-analyses ont Ă©tĂ© conduites selon les procĂ©dures classiques. Les rĂ©sultats des travaux sont particuliĂšrement pertinents. Le coĂ»t annuel moyen par vache est Ă©valuĂ© Ă 42,14 ⏠(se=8,83). Ce coĂ»t est de 73,7 ⏠lors dâune circulation virale dĂ©butant rapidement, intense et persistante. La vaccination et la biosĂ©curitĂ© permettent une baisse du coĂ»t de la BVD, comparĂ© Ă leur absence, de lâordre de 8-13% et de 29-30% respectivement. Toutefois, lâeffet des mesures de biosĂ©curitĂ© et de vaccination sur le coĂ»t de la BVD est quasiment annulĂ© par la prĂ©sence de contacts avec des animaux tiers ou lors dâintroduction dâanimaux sans contrĂŽle sanitaire prĂ©alable
Highly-efficient, diffraction-limited laser emission from a Vertical External Cavity Surface-emitting Organic Laser
We report on a solid-state laser structure being the organic counterpart of
the Vertical External-Cavity Surface-Emitting Laser (VECSEL) design. The gain
medium is a poly (methyl methacrylate) film doped with Rhodamine 640,
spin-casted onto the High-Reflectivity mirror of a plano-concave resonator.
Upon pumping by 7-ns pulses at 532 nm, a diffraction-limited beam (M^2=1) was
obtained, with a conversion efficiency of 43%; higher peak powers (2kW) could
be attained when resorting to shorter (0.5 ns) pump pulses. The spectrum was
controlled by the thickness of the active layer playing the role of an
intracavity etalon; tunability is demonstrated over up to 20 nm
EFFECTS OF ONE-HOUR ROAD RUNNING AND SHOE ON TIBIAL ACCELERATIONS IN RECREATIONAL RUNNERS
The purpose of this study was twofold. Firstly, we evaluated the effect of one-hour running on tibial acceleration parameters. Secondly, we determined whether the shoe fatigue after one-hour running would influence these tibial acceleration parameters. Ten runners ran at a constant preferred speed with a standard running shoe. They also ran one bout of 5 min with another identical but fresh running shoe before and after the one-hour run. Tibial acceleration amplitudes and wavelet analyses demonstrated a significant reduction of the tibial impact parameters after one-hour running, but no significant shoe effect. These reductions could be attributable to the slight increase in stride frequency from the beginning to the end of the one-hour running. Noteworthy, the tibial acceleration reduction was observable along the anteroposterior axis, not along the vertical axis. Considering the tibial acceleration as a potential risk factor for developing running-related injuries, these parameters did not change in a harmful way after one-hour of running at the preferred running speed. The shoe modification during the one-hour run did not seem to influence running biomechanics
Real time 3D US-tagging combined with 3D phase-based motion estimation
International audienceBy contrast with 2D imaging, quantitative analysis of 3D motion from ultrasound images can provide improved information in several applications, such as arterial mechanical assessment, heart motion and blood flow. Unfortunately, it remains difficult to obtain a high definition of the motion estimate in the lateral and elevation directions (i.e. perpendicular to the beam axis). To increase the definition in both these directions, this paper presents a 3D extension of a the transverse oscillations method that enables one to obtain ultrasound fields featuring oscillations along the 3 spatial dimensions, using a single apodization function. The 3D motion method is estimated using the phases of the images. Simulation results show that a 3D trajectory can be followed with a relative mean error smaller than 8%
Source localization and identification with a compact array of digital mems microphones
International audienceA compact microphone array was developed for source localization and identification. This planar array consists of an arrangement of 32 digital MEMS microphones, concentrated in an aperture of fewer than 10 centimeters, and connected to a computer by Ethernet (AVB protocol). 3D direction of arrival (DOA) localization is performed using the pressure and the particle velocity estimated at the center of the array. The pressure is estimated by averaging the signals of multiple microphones. We compare high order pressure finite differences to the Phase and Amplitude Gradient Estimation (PAGE) method for particle velocity estimation. This paper also aims at presenting a method for UAV detection using the developed sensor and supervised binary classification
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