5,783 research outputs found
Many-body quantum dynamics of polarisation squeezing in optical fibre
We report new experiments that test quantum dynamical predictions of
polarization squeezing for ultrashort photonic pulses in a birefringent fibre,
including all relevant dissipative effects. This exponentially complex
many-body problem is solved by means of a stochastic phase-space method. The
squeezing is calculated and compared to experimental data, resulting in
excellent quantitative agreement. From the simulations, we identify the
physical limits to quantum noise reduction in optical fibres. The research
represents a significant experimental test of first-principles time-domain
quantum dynamics in a one-dimensional interacting Bose gas coupled to
dissipative reservoirs.Comment: 4 pages, 4 figure
Generating Entangled Microwave Radiation Over Two Transmission Lines
Using a superconducting circuit, the Josephson mixer, we demonstrate the
first experimental realization of spatially separated two-mode squeezed states
of microwave light. Driven by a pump tone, a first Josephson mixer generates,
out of quantum vacuum, a pair of entangled fields at different frequencies on
separate transmission lines. A second mixer, driven by a -phase shifted
copy of the first pump tone, recombines and disentangles the two fields. The
resulting output noise level is measured to be lower than for vacuum state at
the input of the second mixer, an unambiguous proof of entanglement. Moreover,
the output noise level provides a direct, quantitative measure of entanglement,
leading here to the demonstration of 6 Mebit.s (Mega entangled bits per
second) generated by the first mixer.Comment: 5 pages, 4 figures. Supplementary Information can be found here as an
ancillary fil
Thermodynamics of the \phi^4 theory in tadpole approximation
Relying on the Luttinger-Ward theorem we derive a thermodynamically
selfconsistent and scale independent approximation of the thermodynamic
potential for the scalar theory in the tadpole approximation. The
resulting thermodynamic potential as a function of the temperature is similar
to the one of the recently proposed screened perturbation theory.Comment: 6 pages, including 1 eps figur
Fingering convection and cloudless models for cool brown dwarf atmospheres
This work aims to improve the current understanding of the atmospheres of
brown dwarfs, especially cold ones with spectral type T and Y, whose modeling
is a current challenge. Silicate and iron clouds are believed to disappear at
the photosphere at the L/T transition, but cloudless models fail to reproduce
correctly the spectra of T dwarfs, advocating for the addition of more physics,
e.g. other types of clouds or internal energy transport mechanisms. We use a
one-dimensional (1D) radiative/convective equilibrium code ATMO to investigate
this issue. This code includes both equilibrium and out-of-equilibrium
chemistry and solves consistently the PT structure. Included opacity sources
are H2-H2, H2-He, H2O, CO, CO2, CH4, NH3, K, Na, and TiO, VO if they are
present in the atmosphere. We show that the spectra of Y dwarfs can be
accurately reproduced with a cloudless model if vertical mixing and NH3
quenching are taken into account. T dwarf spectra still have some reddening in
e.g. J - H compared to cloudless models. This reddening can be reproduced by
slightly reducing the temperature gradient in the atmosphere. We propose that
this reduction of the stabilizing temperature gradient in these layers, leading
to cooler structures, is due to the onset of fingering convection, triggered by
the destabilizing impact of condensation of very thin dust.Comment: Accepted in ApJ
From correlation functions to scattering amplitudes
We study the correlators of half-BPS protected operators in N=4
super-Yang-Mills theory, in the limit where the positions of the adjacent
operators become light-like separated. We compute the loop corrections by means
of Lagrangian insertions. The divergences resulting from the light-cone limit
are regularized by changing the dimension of the integration measure over the
insertion points. Switching from coordinates to dual momenta, we show that the
logarithm of the correlator is identical with twice the logarithm of the
matching MHV gluon scattering amplitude. We present a number of examples of
this new relation, at one and two loops.Comment: typos corrected, references adde
Effective diffusion constant in a two dimensional medium of charged point scatterers
We obtain exact results for the effective diffusion constant of a two
dimensional Langevin tracer particle in the force field generated by charged
point scatterers with quenched positions. We show that if the point scatterers
have a screened Coulomb (Yukawa) potential and are uniformly and independently
distributed then the effective diffusion constant obeys the
Volgel-Fulcher-Tammann law where it vanishes. Exact results are also obtained
for pure Coulomb scatterers frozen in an equilibrium configuration of the same
temperature as that of the tracer.Comment: 9 pages IOP LaTex, no figure
Continuum Derrida Approach to Drift and Diffusivity in Random Media
By means of rather general arguments, based on an approach due to Derrida
that makes use of samples of finite size, we analyse the effective diffusivity
and drift tensors in certain types of random medium in which the motion of the
particles is controlled by molecular diffusion and a local flow field with
known statistical properties. The power of the Derrida method is that it uses
the equilibrium probability distribution, that exists for each {\em finite}
sample, to compute asymptotic behaviour at large times in the {\em infinite}
medium. In certain cases, where this equilibrium situation is associated with a
vanishing microcurrent, our results demonstrate the equality of the
renormalization processes for the effective drift and diffusivity tensors. This
establishes, for those cases, a Ward identity previously verified only to
two-loop order in perturbation theory in certain models. The technique can be
applied also to media in which the diffusivity exhibits spatial fluctuations.
We derive a simple relationship between the effective diffusivity in this case
and that for an associated gradient drift problem that provides an interesting
constraint on previously conjectured results.Comment: 18 pages, Latex, DAMTP-96-8
Dualities for Loop Amplitudes of N=6 Chern-Simons Matter Theory
In this paper we study the one- and two-loop corrections to the four-point
amplitude of N=6 Chern-Simons matter theory. Using generalized unitarity
methods we express the one- and two-loop amplitudes in terms of dual-conformal
integrals. Explicit integration by using dimensional reduction gives vanishing
one-loop result as expected, while the two-loop result is non-vanishing and
matches with the Wilson loop computation. Furthermore, the two-loop correction
takes the same form as the one-loop correction to the four-point amplitude of
N=4 super Yang-Mills. We discuss possible higher loop extensions of this
correspondence between the two theories. As a side result, we extend the method
of dimensional reduction for three dimensions to five dimensions where dual
conformal symmetry is most manifest, demonstrating significant simplification
to the computation of integrals.Comment: 32 pages and 6 figures. v2: minus sign corrections, ref updated v3:
Published versio
The Resolved Asteroid Program - Size, shape, and pole of (52) Europa
With the adaptive optics (AO) system on the 10 m Keck-II telescope, we
acquired a high quality set of 84 images at 14 epochs of asteroid (52) Europa
on 2005 January 20. The epochs covered its rotation period and, by following
its changing shape and orientation on the plane of sky, we obtained its
triaxial ellipsoid dimensions and spin pole location. An independent
determination from images at three epochs obtained in 2007 is in good agreement
with these results. By combining these two data sets, along with a single epoch
data set obtained in 2003, we have derived a global fit for (52) Europa of
diameters (379x330x249) +/- (16x8x10) km, yielding a volume-equivalent
spherical-diameter of 315 +/- 7 km, and a rotational pole within 7 deg of [RA;
Dec] = [257,+12] in an Equatorial J2000 reference frame (ECJ2000: 255,+35).
Using the average of all mass determinations available forEuropa, we derive a
density of 1.5 +/- 0.4, typical of C-type asteroids. Comparing our images with
the shape model of Michalowski et al. (A&A 416, 2004), derived from optical
lightcurves, illustrates excellent agreement, although several edge features
visible in the images are not rendered by the model. We therefore derived a
complete 3-D description of Europa's shape using the KOALA algorithm by
combining our imaging epochs with 4 stellar occultations and 49 lightcurves. We
use this 3-D shape model to assess these departures from ellipsoidal shape.
Flat facets (possible giant craters) appear to be less distinct on (52) Europa
than on other C-types that have been imaged in detail. We show that fewer giant
craters, or smaller craters, is consistent with its expected impact history.
Overall, asteroid (52) Europa is still well modeled as a smooth triaxial
ellipsoid with dimensions constrained by observations obtained over several
apparitions.Comment: Accepted for publication in Icaru
Quantum many-body simulations using Gaussian phase-space representations
Phase-space representations are of increasing importance as a viable and
successful means to study exponentially complex quantum many-body systems from
first principles. This review traces the background of these methods, starting
from the early work of Wigner, Glauber and Sudarshan. We focus on modern
phase-space approaches using non-classical phase-space representations. These
lead to the Gaussian representation, which unifies bosonic and fermionic
phase-space. Examples treated include quantum solitons in optical fibers,
colliding Bose-Einstein condensates, and strongly correlated fermions on
lattices.Comment: Short Review (10 pages); Corrected typo in eq (14); Added a few more
reference
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