758 research outputs found
Multilevel Holstein-Primakoff approximation and its application to atomic spin squeezing and ensemble quantum memories
We show that an ensemble of identical d-level atoms can be efficiently
described by d-1 collective oscillator degrees of freedom in the vicinity of a
product state with all atoms in the same, but otherwise arbitrary
single-particle state. We apply our description to two different kinds of spin
squeezing: (i) when each spin-F atom is individually squeezed without creating
interatomic entanglement and (ii) when a particular collective atomic
oscillator mode is squeezed via quantum non-demolition (QND) measurement and
feedback. When combined in sequence, the order of the two methods is relevant
in the final degree of squeezing. We also discuss the role of the two kinds of
squeezing when multi-sublevel atoms are used as quantum memories for light.Comment: 12 pages, 3 figure
Quantized recurrence time in iterated open quantum dynamics
The expected return time to the original state is a key concept
characterizing systems obeying both classical or quantum dynamics. We consider
iterated open quantum dynamical systems in finite dimensional Hilbert spaces, a
broad class of systems that includes classical Markov chains and unitary
discrete time quantum walks on networks. Starting from a pure state, the time
evolution is induced by repeated applications of a general quantum channel, in
each timestep followed by a measurement to detect whether the system has
returned to the original state. We prove that if the superoperator is unital in
the relevant Hilbert space (the part of the Hilbert space explored by the
system), then the expectation value of the return time is an integer, equal to
the dimension of this relevant Hilbert space. We illustrate our results on
partially coherent quantum walks on finite graphs. Our work connects the
previously known quantization of the expected return time for bistochastic
Markov chains and for unitary quantum walks, and shows that these are special
cases of a more general statement. The expected return time is thus a
quantitative measure of the size of the part of the Hilbert space available to
the system when the dynamics is started from a certain state
Continuous variable remote state preparation
We extend exact deterministic remote state preparation (RSP) with minimal
classical communication to quantum systems of continuous variables. We show
that, in principle, it is possible to remotely prepare states of an ensemble
that is parameterized by infinitely many real numbers, i.e., by a real
function, while the classical communication cost is one real number only. We
demonstrate continuous variable RSP in three examples using (i) quadrature
measurement and phase space displacement operations, (ii) measurement of the
optical phase and unitaries shifting the same, and (iii) photon counting and
photon number shift.Comment: 7 pages, RevTeX
Observable Signatures of Planet Accretion in Red Giant Stars I: Rapid Rotation and Light Element Replenishment
The orbital angular momentum of a close-orbiting giant planet can be
sufficiently large that, if transferred to the envelope of the host star during
the red giant branch (RGB) evolution, it can spin-up the star's rotation to
unusually large speeds. This spin-up mechanism is one possible explanation for
the rapid rotators detected among the population of generally slow-rotating red
giant stars. These rapid rotators thus comprise a unique stellar sample
suitable for searching for signatures of planet accretion in the form of
unusual stellar abundances due to the dissemination of the accreted planet in
the stellar envelope. In this study, we look for signatures of replenishment in
the Li abundances and (to a lesser extent) 12C/13C, which are both normally
lowered during RGB evolution. Accurate abundances were measured from high
signal-to-noise echelle spectra for samples of both slow and rapid rotator red
giant stars. We find that the rapid rotators are on average enriched in lithium
compared to the slow rotators, but both groups of stars have identical
distributions of 12C/13C within our measurement precision. Both of these
abundance results are consistent with the accretion of planets of only a few
Jupiter masses. We also explore alternative scenarios for understanding the
most Li-rich stars in our sample---particularly Li regeneration during various
stages of stellar evolution. Finally, we find that our stellar samples show
non-standard abundances even at early RGB stages, suggesting that initial
protostellar Li abundances and 12C/13C may be more variable than originally
thought.Comment: Accepted for publication in the Astrophysical Journal. 29 pages in
emulateapj format, including 16 figures and 12 tables. Tables 4 and 8 are
provided in their entirety as plain text ancillary files (and will also be
available in the electronic edition of ApJ
Hot DQ White Dwarfs: Something Different
We present a detailed analysis of all the known Hot DQ white dwarfs in the
Fourth Data Release of the Sloan Digital Sky Survey (SDSS) recently found to
have carbon dominated atmospheres. Our spectroscopic and photometric analysis
reveals that these objects all have effective temperatures between ~18,000 and
24,000 K. The surface composition is found to be completely dominated by
carbon, as revealed by the absence of Hbeta and HeI 4471 lines (or
determination of trace amount in a few cases). We find that the surface gravity
of all objects but one seems to be ''normal'' and around log g = 8.0 while one
is likely near log g = 9.0. The presence of a weak magnetic field is directly
detected by spectropolarimetry in one object and is suspected in two others. We
propose that these strange stars could be cooled down versions of the weird
PG1159 star H1504+65 and form a new family of hydrogen and helium deficient
objects following the post-AGB phase. Finally, we present the results of full
nonadiabatic calculations dedicated specifically to each of the Hot DQ that
show that only SDSS J142625.70+575218.4 is expected to exhibit luminosity
variations. This result is in excellent agreement with recent observations by
Montgomery et al. who find that J142625.70+575218.4 is the only pulsator among
6 Hot DQ white dwarfs surveyed in February 2008.Comment: 33 pages, 7 figures, accepted for publication in Ap
Screened thermonuclear reactions and predictive stellar evolution of detached double-lined eclipsing binaries
The low energy fusion cross sections of charged-particle nuclear reactions
(and the respective reaction rates) in stellar plasmas are enhanced due to
plasma screening effects. We study the impact of those effects on predictive
stellar evolution simulations for detached double-lined eclipsing binaries. We
follow the evolution of binary systems (pre-main sequence or main sequence
stars) with precisely determined radii and masses from 1.1Mo to 23Mo (from
their birth until their present state). The results indicate that all the
discrepancies between the screened and unscreened models (in terms of
luminosity, stellar radius, and effective temperature) are within the
observational uncertainties. Moreover, no nucleosynthetic or compositional
variation was found due to screening corrections. Therefore all thermonuclear
screening effects on the charged-particle nuclear reactions that occur in the
binary stars considered in this work (from their birth until their present
state) can be totally disregarded. In other words, all relevant
charged-particle nuclear reactions can be safely assumed to take place in a
vacuum, thus simplifying and accelerating the simulation processes.Comment: 5 RevTex pages,no figures. Accepted for publication in Phys.Rev.
Spectroscopic properties of inhomogeneously broadened spin ensembles in a cavity
In large ensembles of identical atoms or spins, the interaction with a mode
of the electromagnetic radiation field concentrates in a single superradiant
degree of freedom with a collectively enhanced coupling. Given a controllable
inhomogeneous broadening, such ensembles may be used for multi-mode storage of
quantum states of the radiation field with applications in quantum
communication networks and quantum computers. In this paper we analyze how the
width and shape of the inhomogeneous broadening influence the collective
enhancement and the dynamics of the cavity-ensemble system with focus on the
consequences for the ensemble's applicability for quantum information
processing tasks.Comment: 12 pages, 5 figure
HST/FOS Time-resolved spectral mapping of IP Pegasi at the end of an outburst
We report an eclipse mapping analysis of time-resolved ultraviolet
spectroscopy covering three eclipses of the dwarf nova IP Pegasi on the late
decline of the 1993 May outburst. The eclipse maps of the first run show
evidence of one spiral arm, suggesting that spiral structures may still be
present in the accretion disc 9 days after the onset of the outburst. In the
spatially resolved spectra the most prominent lines appear in emission at any
radius, being stronger in the inner disc regions. The spectrum of the gas
stream is clearly distinct from the disc spectrum in the intermediate and outer
disc regions, suggesting the occurrence of gas stream overflow. The full width
half maximum of C IV is approximately constant with radius, in contrast to the
expected law for a gas in Keplerian orbits. This line
probably originates in a vertically extended region (chromosphere + disc wind).
The uneclipsed component contributes % of the flux in C IV in the
first run, and becomes negligible in the remaining runs. We fit stellar
atmosphere models to the spatially resolved spectra. The radial run of the disc
color temperature for the three runs is flatter than the expected
law for steady-state optically thick discs models, with
K in the inner regions and K in the outer disc
regions. The solid angles that result from the fits are smaller than expected
from the parameters of the system. The radial run of the solid angle suggests
that the disc is flared in outburst, and decreases in thickness toward the end
of the outburst.Comment: 14 pages, 14 figures, in press in Astronomy & Astrophysic
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