25,065 research outputs found
Narrowband Biphoton Generation due to Long-Lived Coherent Population Oscillations
We study the generation of paired photons due to the effect of four-wave
mixing in an ensemble of pumped two-level systems that decay via an
intermediate metastable state. The slow population relaxation of the metastable
state to the ground state is utilized to create long-lived coherent population
oscillation, leading to narrowband nonlinear response of the medium. The
biphotons have a narrow bandwidth, long coherence time and length, which can be
controlled by the pump field. In addition, the biphotons are antibunched, with
antibunching period determined by the dephasing time. During this period,
damped oscillations of the biphoton wavefunction occurs if the pump detuning is
non-zero.Comment: 7 pages, 2 figure
Tkachenko modes of vortex lattices in rapidly rotating Bose-Einstein condensates
We calculate the in-plane modes of the vortex lattice in a rotating Bose
condensate from the Thomas-Fermi to the mean-field quantum Hall regimes. The
Tkachenko mode frequency goes from linear in the wavevector, , for lattice
rotational velocities, , much smaller than the lowest sound wave
frequency in a finite system, to quadratic in in the opposite limit. The
system also supports an inertial mode of frequency . The
calculated frequencies are in good agreement with recent observations of
Tkachenko modes at JILA, and provide evidence for the decrease in the shear
modulus of the vortex lattice at rapid rotation.Comment: 4 pages, 2 figure
Membership of the Orion Nebula Population from the Chandra Orion Ultradeep Project
The Chandra Orion Ultradeep project (COUP) observation described in a
companion paper by Getman et al. provides an exceptionally deep X-ray survey of
the Orion Nebula Cluster and associated embedded young stellar objects.
Membership of the region is important for studies of the stellar IMF, cluster
dynamics, and star formation. The COUP study detected 1616 X-ray sources. In
this study we confirm cloud membership for 1315 stars, identify 16 probable
foreground field stars having optical counterparts with discrepant proper
motions, and classify the remaining 285 X-ray sources, of which 51 are lightly
and 234 heavily obscured. The 51 lightly obscured sources without known
counterparts fall into three groups. (i) Sixteen are likely new members of the
Orion Nebula Cluster. (ii) Two with unusually soft and non-flaring X-ray
emission appear to be associated with nebular shocks, and may be new examples
of X-rays produced at the bow shocks of Herbig-Haro outflows. (iii) The
remaining thirty three are very weak uncertain sources, possibly spurious. Out
of 234 heavily absorbed sources without optical or near-infrared counterparts
75 COUP sources are likely new embedded cloud members (with membership for 42
confirmed by powerful X-ray flares), and the remaining 159 are likely
extragalactic AGN seen through the molecular cloud, as argued by a careful
simulation of the extragalactic background population. Finally, a few new
binary companions to Orion stars may have been found, but most cases of
proximate COUP sources can be attributed to chance superpositions in this
crowded field.Comment: 49 pages, 6 figures, 5 tables. Accepted for publication in ApJS,
special issue dedicated to Chandra Orion Ultradeep Project. A version with
high quality figures can be found at
http://www.astro.psu.edu/users/gkosta/COUP_Membership.pd
Vortex lattice stability and phase coherence in three-dimensional rapidly rotating Bose condensates
We establish the general equations of motion for the modes of a vortex
lattice in a rapidly rotating Bose-Einstein condensate in three dimensions,
taking into account the elastic energy of the lattice and the vortex line
bending energy. As in two dimensions, the vortex lattice supports Tkachenko and
gapped sound modes. In contrast, in three dimensions the Tkachenko mode
frequency at long wavelengths becomes linear in the wavevector for any
propagation direction out of the transverse plane. We compute the correlation
functions of the vortex displacements and the superfluid order parameter for a
homogeneous Bose gas of bounded extent in the axial direction. At zero
temperature the vortex displacement correlations are convergent at large
separation, but at finite temperatures, they grow with separation. The growth
of the vortex displacements should lead to observable melting of vortex
lattices at higher temperatures and somewhat lower particle number and faster
rotation than in current experiments. At zero temperature a system of large
extent in the axial direction maintains long range order-parameter correlations
for large separation, but at finite temperatures the correlations decay with
separation.Comment: 10 pages, 2 figures, Changes include the addition of the particle
density - vortex density coupling and the correct value of the shear modulu
Periodic and Quasi-Periodic Compensation Strategies of Extreme Outages caused by Polarization Mode Dispersion and Amplifier Noise
Effect of birefringent disorder on the Bit Error Rate (BER) in an optical
fiber telecommunication system subject to amplifier noise may lead to extreme
outages, related to anomalously large values of BER. We analyze the Probability
Distribution Function (PDF) of BER for various strategies of Polarization Mode
Dispersion (PMD) compensation. A compensation method is proposed that is
capable of more efficient extreme outages suppression, which leads to
substantial improvement of the fiber system performance.Comment: 3 pages, 1 figure, Submitted to IEEE Photonics Letter
Solid domains in lipid vesicles and scars
The free energy of a crystalline domain coexisting with a liquid phase on a
spherical vesicle may be approximated by an elastic or stretching energy and a
line tension term. The stretching energy generally grows as the area of the
domain, while the line tension term grows with its perimeter. We show that if
the crystalline domain contains defect arrays consisting of finite length grain
boundaries of dislocations (scars) the stretching energy grows linearly with a
characteristic length of the crystalline domain. We show that this result is
critical to understand the existence of solid domains in lipid-bilayers in the
strongly segregated two phase region even for small relative area coverages.
The domains evolve from caps to stripes that become thinner as the line tension
is decreased. We also discuss the implications of the results for other
experimental systems and for the general problem that consists in finding the
ground state of a very large number of particles constrained to move on a fixed
geometry and interacting with an isotropic potential.Comment: 7 pages, 6 eps figure
Energy-momentum balance in quantum dielectrics
We calculate the energy-momentum balance in quantum dielectrics such as
Bose-Einstein condensates. In agreement with the experiment [G. K. Campbell et
al. Phys. Rev. Lett. 94, 170403 (2005)] variations of the Minkowski momentum
are imprinted onto the phase, whereas the Abraham tensor drives the flow of the
dielectric. Our analysis indicates that the Abraham-Minkowski controversy has
its root in the Roentgen interaction of the electromagnetic field in dielectric
media
- …