686 research outputs found
Strong CP Violation in External Magnetic Fields
We study the response of the QCD vacuum to an external magnetic field, in the
presence of strong CP violation. Using chiral perturbation theory and large N_c
expansion, we show that the external field would polarize quantum fluctuations
and induce an electric dipole moment of the vacuum, along the direction of the
magnetic field. We estimate the magnitude of this effect in different physical
scenarios. In particular, we find that the polarization induced by the magnetic
field of a magnetar could accelerate electric charges up to energies of the
order \theta 10^3 TeV. We also suggest a connection with the possible existence
of "hot-spots" on the surface of neutron stars.Comment: 4 pages, 1 figure. Major revision. Phenomenological analysis extende
Ultra-Low Noise Microwave Extraction from Fiber-Based Optical Frequency Comb
In this letter, we report on all-optical fiber approach to the generation of
ultra-low noise microwave signals. We make use of two erbium fiber mode-locked
lasers phase locked to a common ultra-stable laser source to generate an 11.55
GHz signal with an unprecedented relative phase noise of -111 dBc/Hz at 1 Hz
from the carrier.The residual frequency instability of the microwave signals
derived from the two optical frequency combs is below 2.3 10^(-16) at 1s and
about 4 10^(-19) at 6.5 10^(4)s (in 5 Hz bandwidth, three days continuous
operation).Comment: 12 pages, 3 figure
Periodic negative differential conductance in a single metallic nano-cage
We report a bi-polar multiple periodic negative differential conductance
(NDC) effect on a single cage-shaped Ru nanoparticle measured using scanning
tunneling spectroscopy. This phenomenon is assigned to the unique
multiply-connected cage architecture providing two (or more) defined routes for
charge flow through the cage. This, in turn, promotes a self- gating effect,
where electron charging of one route affects charge transport along a
neighboring channel, yielding a series of periodic NDC peaks. This picture is
established and analyzed here by a theoretical model
Ultra-low phase noise all-optical microwave generation setup based on commercial devices
In this paper, we present a very simple design based on commercial devices
for the all-optical generation of ultra-low phase noise microwave signals. A
commercial, fibered femtosecond laser is locked to a laser that is stabilized
to a commercial ULE Fabry-Perot cavity. The 10 GHz microwave signal extracted
from the femtosecond laser output exhibits a single sideband phase noise
at 1 Hz Fourier frequency, at
the level of the best value obtained with such "microwave photonics" laboratory
experiments \cite{Fortier2011}. Close-to-the-carrier ultra-low phase noise
microwave signals will now be available in laboratories outside the frequency
metrology field, opening up new possibilities in various domains.Comment: 8 pages, 3 figures. To be published in Applied Optics, early posting
version available at
http://www.opticsinfobase.org/ao/upcoming_pdf.cfm?id=23114
Laparoscopic Roux-en-Y Gastric Bypass After Failed Vertical Banded Gastroplasty: a Multicenter Experience with 203 Patients
Background: Vertical banded gastroplasty (VBG) has long been the main restrictive procedure for morbid obesity but has many long-term complications for which conversion to Roux-en-Y gastric bypass (RYGBP) is often considered the best option. Methods: This series regroups patients operated on by three different surgeons in four different centers. All data were collected prospectively, then pooled and analyzed retrospectively. Results: Out of 2,522 RYGBP performed between 1998 and 2010, 538 were reoperations, including 203 laparoscopic RYGBP after VBG. There were 175 women and 28 men. The mean BMI before VBG was 43.2 ± 6.3, and the mean BMI before reoperation was 37.4 ± 8.3. Most patients had more than one indication for reoperation and/or had regained significant weight. There was no conversion to open surgery. A total of 24 patients (11.8%) developed complications, including nine (4.5%) who required reoperation and one death. With a follow-up of 88.9% after 8years, the mean BMI after 1, 3, 5, 7, and 9years was 29.1, 28.8, 28.7, 29.9, and 28.8, respectively. Conclusions: On the basis of this experience, the largest with laparoscopic reoperative RYGBP after failed VBG, we conclude that this procedure can safely be performed in experienced hands, with weight loss results similar to those observed after primary RYGBP. In patients with too difficult an anatomy below the cardia, dividing the esophagus just above the esophago-gastric junction and performing an esophagojejunostomy may be a safe alternative to converting to a Scopinaro-type BPD, obviating the additional long-term risks associated with malabsorptio
Amplitude to phase conversion of InGaAs pin photo-diodes for femtosecond lasers microwave signal generation
When a photo-diode is illuminated by a pulse train from a femtosecond laser,
it generates microwaves components at the harmonics of the repetition rate
within its bandwidth. The phase of these components (relative to the optical
pulse train) is known to be dependent on the optical energy per pulse. We
present an experimental study of this dependence in InGaAs pin photo-diodes
illuminated with ultra-short pulses generated by an Erbium-doped fiber based
femtosecond laser. The energy to phase dependence is measured over a large
range of impinging pulse energies near and above saturation for two typical
detectors, commonly used in optical frequency metrology with femtosecond laser
based optical frequency combs. When scanning the optical pulse energy, the
coefficient which relates phase variations to energy variations is found to
alternate between positive and negative values, with many (for high harmonics
of the repetition rate) vanishing points. By operating the system near one of
these vanishing points, the typical amplitude noise level of commercial-core
fiber-based femtosecond lasers is sufficiently low to generate state-of-the-art
ultra-low phase noise microwave signals, virtually immune to amplitude to phase
conversion related noise.Comment: 7 pages, 6 figures, submitted to Applied Physics
Ultralow phase noise microwave generation with an Er:fiber-based optical frequency divider
We present an optical frequency divider based on a 200 MHz repetition rate
Er:fiber mode-locked laser that, when locked to a stable optical frequency
reference, generates microwave signals with absolute phase noise that is equal
to or better than cryogenic microwave oscillators. At 1 Hz offset from a 10 GHz
carrier, the phase noise is below -100 dBc/Hz, limited by the optical
reference. For offset frequencies > 10 kHz, the phase noise is shot noise
limited at -145 dBc/Hz. An analysis of the contribution of the residual noise
from the Er:fiber optical frequency divider is also presented.Comment: 4 pages, 3 figure
An Ultra-Stable Referenced Interrogation System in the Deep Ultraviolet for a Mercury Optical Lattice Clock
We have developed an ultra-stable source in the deep ultraviolet, suitable to
fulfill the interrogation requirements of a future fully-operational lattice
clock based on neutral mercury. At the core of the system is a Fabry-P\'erot
cavity which is highly impervious to temperature and vibrational perturbations.
The mirror substrate is made of fused silica in order to exploit the
comparatively low thermal noise limits associated with this material. By
stabilizing the frequency of a 1062.6 nm Yb-doped fiber laser to the cavity,
and including an additional link to LNE-SYRTE's fountain primary frequency
standards via an optical frequency comb, we produce a signal which is both
stable at the 1E-15 level in fractional terms and referenced to primary
frequency standards. The signal is subsequently amplified and frequency-doubled
twice to produce several milliwatts of interrogation signal at 265.6 nm in the
deep ultraviolet.Comment: 7 pages, 6 figure
Computing the Effective Hamiltonian of Low-Energy Vacuum Gauge Fields
A standard approach to investigate the non-perturbative QCD dynamics is
through vacuum models which emphasize the role played by specific gauge field
fluctuations, such as instantons, monopoles or vortexes. The effective
Hamiltonian describing the dynamics of the low-energy degrees of freedom in
such approaches is usually postulated phenomenologically, or obtained through
uncontrolled approximations. In a recent paper, we have shown how lattice field
theory simulations can be used to rigorously compute the effective Hamiltonian
of arbitrary vacuum models by stochastically performing the path integral over
all the vacuum field fluctuations which are not explicitly taken into account.
In this work, we present the first illustrative application of such an approach
to a gauge theory and we use it to compute the instanton size distribution in
SU(2) gluon-dynamics in a fully model independent and parameter-free way.Comment: 10 pages, 4 figure
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