5,561 research outputs found
Stellar Populations in the Outskirts of the Small Magellanic Cloud: No Outer Edge Yet
We report the detection of intermediate-age and old stars belonging to the
SMC at 6.5 kpc from the SMC center in the southern direction. We show, from the
analysis of three high quality 34\arcmin 33\arcmin CMDs, that the age
composition of the stellar population is similar at galactocentric distances of
4.7 kpc, 5.6 kpc, and 6.5 kpc. The surface
brightness profile of the SMC follows an exponential law, with no evidence of
truncation, all the way out to 6.5 kpc. These results, taken together, suggest
that the SMC `disk' population is dominating over a possible old Milky Way-like
stellar halo, and that the SMC may be significantly larger than previously
thought.Comment: Accepted for publication in ApJ Letters. High resolution figures are
available at ftp://ftp.iac.es/out/noe
Theory of the Ramsey spectroscopy and anomalous segregation in ultra-cold rubidium
The recent anomalous segregation experiment of Lewandowski et al. (PRL, 88,
070403, 2002) shows dramatic, rapid internal state segregation for two
hyperfine levels of rubidium. We simulate an effective one dimensional model of
the system for experimental parameters and find reasonable agreement with the
data. The Ramsey frequency is found to be insensitive to the decoherence of the
superposition, and is only equivalent to the interaction energy shift for a
pure superposition. A Quantum Boltzmann equation describing collisions is
derived using Quantum Kinetic Theory, taking into account the different
scattering lengths of the internal states. As spin-wave experiments are likely
to be attempted at lower temperatures we examine the effect of degeneracy on
decoherence by considering the recent experiment of Lewandowski et al. where
degeneracy is around 10%. We also find that the segregation effect is only
possible when transport terms are included in the equations of motion, and that
the interactions only directly alter the momentum distributions of the states.
The segregation or spin wave effect is thus entirely due to coherent atomic
motion as foreseen in the experimental reportComment: 26 pages, 4 figures, to be published in J. Phys.
The Morphologies of the Small Magellanic Cloud
We compare the distribution of stars of different spectral types, and hence
mean age, within the central SMC and find that the asymmetric structures are
almost exclusively composed of young main sequence stars. Because of the
relative lack of older stars in these features, and the extremely regular
distribution of red giant and clump stars in the SMC central body, we conclude
that tides alone are not responsible for the irregular appearance of the
central SMC. The dominant physical mechanism in determining the current-day
appearance of the SMC must be star formation triggered by a hydrodynamic
interaction between gaseous components. These results extend the results of
population studies (cf. Gardiner and Hatzidimitriou) inward in radius and also
confirm the suggestion of the spheroidal nature of the central SMC based on
kinematic arguments (Dopita et al; Hardy, Suntzeff & Azzopardi). Finally, we
find no evidence in the underlying older stellar population for a ``bar'' or
``outer arm'', again supporting our classification of the central SMC as a
spheroidal body with highly irregular recent star formation.Comment: 8 pages, accepted for publication in ApJ Letters (higher quality
figures available at http://ngala.as.arizona.edu/dennis/mcsurvey.html
Quantum Zeno Effect for Exponentially Decaying Systems
The quantum Zeno effect -- suppression of decay by frequent measurements --
was believed to occur only when the response of the detector is so quick that
the initial tiny deviation from the exponential decay law is detectable.
However, we show that it can occur even for exactly exponentially decaying
systems, for which this condition is never satisfied, by considering a
realistic case where the detector has a finite energy band of detection. The
conventional theories correspond to the limit of an infinite bandwidth. This
implies that the Zeno effect occurs more widely than expected so far.Comment: 4 pages, 3 figure
Number-Phase Wigner Representation for Efficient Stochastic Simulations
Phase-space representations based on coherent states (P, Q, Wigner) have been
successful in the creation of stochastic differential equations (SDEs) for the
efficient stochastic simulation of high dimensional quantum systems. However
many problems using these techniques remain intractable over long integrations
times. We present a number-phase Wigner representation that can be unraveled
into SDEs. We demonstrate convergence to the correct solution for an anharmonic
oscillator with small dampening for significantly longer than other phase space
representations. This process requires an effective sampling of a non-classical
probability distribution. We describe and demonstrate a method of achieving
this sampling using stochastic weights.Comment: 7 pages, 1 figur
Anharmonic effects on a phonon number measurement of a quantum mesoscopic mechanical oscillator
We generalize a proposal for detecting single phonon transitions in a single
nanoelectromechanical system (NEMS) to include the intrinsic anharmonicity of
each mechanical oscillator. In this scheme two NEMS oscillators are coupled via
a term quadratic in the amplitude of oscillation for each oscillator. One NEMS
oscillator is driven and strongly damped and becomes a transducer for phonon
number in the other measured oscillator. We derive the conditions for this
measurement scheme to be quantum limited and find a condition on the size of
the anharmonicity. We also derive the relation between the phase diffusion
back-action noise due to number measurement and the localization time for the
measured system to enter a phonon number eigenstate. We relate both these time
scales to the strength of the measured signal, which is an induced current
proportional to the position of the readout oscillator.Comment: 13 pages, 2 figure
Quadripartite continuous-variable entanglement via quadruply concurrent downconversion
We investigate an intra-cavity coupled down-conversion scheme to generate
quadripartite entanglement using concurrently resonant nonlinearities. We
verify that quadripartite entanglement is present in this system by calculating
the output fluctuation spectra and then considering violations of optimized
inequalities of the van Loock-Furusawa type. The entanglement characteristics
both above and below the oscillation threshold are considered. We also present
analytic solutions for the quadrature operators and the van Loock-Furusawa
correlations in the undepleted pump approximation.Comment: 9 pages, 5 figure
Intensity fluctuations in steady state superradiance
Alkaline-earth like atoms with ultra-narrow optical transitions enable
superradiance in steady state. The emitted light promises to have an
unprecedented stability with a linewidth as narrow as a few millihertz. In
order to evaluate the potential usefulness of this light source as an
ultrastable oscillator in clock and precision metrology applications it is
crucial to understand the noise properties of this device. In this paper we
present a detailed analysis of the intensity fluctuations by means of
Monte-Carlo simulations and semi-classical approximations. We find that the
light exhibits bunching below threshold, is to a good approximation coherent in
the superradiant regime, and is chaotic above the second threshold.Comment: 8 pages, 5 figure
Statistical mechanics far from equilibrium: prediction and test for a sheared system
We report the complete statistical treatment of a system of particles
interacting via Newtonian forces in continuous boundary-driven flow, far from
equilibrium. By numerically time-stepping the force-balance equations of a
model fluid we measure occupancies and transition rates in simulation. The
high-shear-rate simulation data verify the invariant quantities predicted by
our statistical theory, thus demonstrating that a class of non-equilibrium
steady states of matter, namely sheared complex fluids, is amenable to
statistical treatment from first principles.Comment: 4 pages plus a 3-page pdf supplemen
Spectral Analysis of a Four Mode Cluster State
We theoretically evaluate the squeezed joint operators produced in a single
optical parametric oscillator which generates quadripartite entangled outputs,
as demonstrated experimentally by Pysher et al. \cite{pysher}[Phys. Rev. Lett.
107, 030505 (2011)]. Using a linearized fluctuation analysis we calculate the
squeezing of the joint quadrature operators below threshold for a range of
local oscillator phases and frequencies. These results add to the existing
theoretical understanding of this potentially important system.Comment: 4 pages, 6 figure
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