856 research outputs found
Strange Cepheids and RR Lyrae
Strange modes can occur in radiative classical Cepheids and RR Lyrae models.
These are vibrational modes that are trapped near the surface as a result of a
'potential barrier' caused by the sharp hydrogen partial ionization region.
Typically the modal number of the strange mode falls between the 7th and 12th
overtone, depending on the astrophysical parameters of the equilibrium stellar
models (L, M, \Teff, X, Z). Interestingly these modes can be linearly unstable
outside the usual instability strip, in which case they should be observable as
new kinds of variable stars, 'strange Cepheids' or 'strange RR Lyrae' stars.
The present paper reexamines the linear stability properties of the strange
modes by taking into account the effects of an isothermal atmosphere, and of
turbulent convection. It is found that the linear vibrational instability of
the strange modes is resistant to both of these effects. Nonlinear hydrodynamic
calculations indicate that the pulsation amplitude of these modes is likely to
saturate at the millimagnitude level. These modes should therefore be
detectable albeit not without effort.Comment: 6 pages, 7 figures, submitted to Ap
Double-Mode Stellar Pulsations
The status of the hydrodynamical modelling of nonlinear multi-mode stellar
pulsations is discussed. The hydrodynamical modelling of steady double-mode
(DM) pulsations has been a long-standing quest that is finally being concluded.
Recent progress has been made thanks to the introduction of turbulent
convection in the numerical hydrodynamical codes which provide detailed results
for individual models. An overview of the modal selection problem in the HR
diagram can be obtained in the form of bifurcation diagrams with the help of
simple nonresonant amplitude equations that capture the DM phenomenon.Comment: 34 pages, to appear as a chapter in Nonlinear Stellar Pulsation in
the Astrophysics and Space Science Library (ASSL), Editors: M. Takeuti & D.
Sasselov (prints double column with pstops
'2:[email protected](22.0cm,-2cm)[email protected](22.0cm,11.0cm)' in.ps out.ps
An AC Stark Gradient Echo Memory in Cold Atoms
The burgeoning fields of quantum computing and quantum key distribution have
created a demand for a quantum memory. The gradient echo memory scheme is a
quantum memory candidate for light storage that can boast efficiencies
approaching unity, as well as the flexibility to work with either two or three
level atoms. The key to this scheme is the frequency gradient that is placed
across the memory. Currently the three level implementation uses a Zeeman
gradient and warm atoms. In this paper we model a new gradient creation
mechanism - the ac Stark effect - to provide an improvement in the flexibility
of gradient creation and field switching times. We propose this scheme in
concert with a move to cold atoms (~1 mK). These temperatures would increase
the storage times possible, and the small ensemble volumes would enable large
ac Stark shifts with reasonable laser power. We find that memory bandwidths on
the order of MHz can be produced with experimentally achievable laser powers
and trapping volumes, with high precision in gradient creation and switching
times on the order of nanoseconds possible. By looking at the different
decoherence mechanisms present in this system we determine that coherence times
on the order of 10s of milliseconds are possible, as are delay-bandwidth
products of approximately 50 and efficiencies over 90%
Storage and Manipulation of Light Using a Raman Gradient Echo Process
The Gradient Echo Memory (GEM) scheme has potential to be a suitable protocol
for storage and retrieval of optical quantum information. In this paper, we
review the properties of the -GEM method that stores information in
the ground states of three-level atomic ensembles via Raman coupling. The
scheme is versatile in that it can store and re-sequence multiple pulses of
light. To date, this scheme has been implemented using warm rubidium gas cells.
There are different phenomena that can influence the performance of these
atomic systems. We investigate the impact of atomic motion and four-wave mixing
and present experiments that show how parasitic four-wave mixing can be
mitigated. We also use the memory to demonstrate preservation of pulse shape
and the backward retrieval of pulses.Comment: 26 pages, 13 figure
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