390 research outputs found
Quantum methods for clock synchronization: Beating the standard quantum limit without entanglement
We introduce methods for clock synchronization that make use of the adiabatic
exchange of nondegenerate two-level quantum systems: ticking qubits. Schemes
involving the exchange of N independent qubits with frequency give a
synchronization accuracy that scales as , i.e., as the
standard quantum limit. We introduce a protocol that makes use of N coherent
exchanges of a single qubit at frequency , leading to an accuracy that
scales as . This protocol beats the standard quantum
limit without the use of entanglement, and we argue that this scaling is the
fundamental limit for clock synchronization allowed by quantum mechanics. We
analyse the performance of these protocols when used with a lossy channel.Comment: 9 pages, 1 figure, published versio
The Return of the Repressed: three examples of how Chinese identity is being reconsolidated for the modern world
After setting the scene for an examination of the changes in culture and self-perception in China today, the authors explore three areas of activity which can be interpreted as illustrating these changes: (1) the current treatment of Confucius, as compared to the recent past; (2) the enthusiasm for the Chinese canon, which has developed from a grassroots movement into government policy; and (3) the way in which the presentation and content of public slogans have changed to, apparently, reflect the substitution of Communist nostrums for Confucian mores. In the first and second cases, the authors suggest that the authorities are acceding to the aspirations and prejudices of the people; rather than leading, they are following, and this has the effect of reinforcing the trend. The third—the gradual abandonment of the use of Marxist shibboleths in propaganda, and their replacement by Confucian adages—is not yet an established fact but, again, the trend is evident. China has revised and modernised its traditional culture and the first fruits of that can be seen in the words and behaviour of its political and intellectual leaders
Choice of Measurement Sets in Qubit Tomography
Optimal generalized measurements for state estimation are well understood.
However, practical quantum state tomography is typically performed using a
fixed set of projective measurements and the question of how to choose these
measurements has been largely unexplored in the literature. In this work we
develop theoretical asymptotic bounds for the average fidelity of pure qubit
tomography using measurement sets whose axes correspond to vertices of Platonic
solids. We also present complete simulations of maximum likelihood tomography
for mixed qubit states using the Platonic solid measurements. We show that
overcomplete measurement sets can be used to improve the accuracy of
tomographic reconstructions.Comment: 13 Pages, 6 figure
The polar ring galaxy AM1934-563 revisited
We report long-slit spectroscopic observations of the dust-lane polar-ring
galaxy AM1934-563 obtained with the Southern African Large Telescope (SALT)
during its performance-verification phase. The observations target the spectral
region of the Ha, [NII] and [SII] emission-lines, but show also deep NaI
stellar absorption lines that we interpret as produced by stars in the galaxy.
We derive rotation curves along the major axis of the galaxy that extend out to
about 8 kpc from the center for both the gaseous and the stellar components,
using the emission and absorption lines. We derive similar rotation curves
along the major axis of the polar ring and point out differences between these
and the ones of the main galaxy. We identify a small diffuse object visible
only in Ha emission and with a low velocity dispersion as a dwarf HII galaxy
and argue that it is probably metal-poor. Its velocity indicates that it is a
fourth member of the galaxy group in which AM1934-563 belongs. We discuss the
observations in the context of the proposal that the object is the result of a
major merger and point out some observational discrepancies from this
explanation. We argue that an alternative scenario that could better fit the
observations may be the slow accretion of cold intergalactic gas, focused by a
dense filament of galaxies in which this object is embedded (abridged).Comment: 19 pages, 13 figures, submitted to MNRAS. Some figures were bitmapped
to reduce the size. Full resolution version is available from
http://www.saao.ac.za/~akniazev/pub/AM1934_563.pd
Rocket Observations of Far-Ultraviolet Dust Scattering in NGC 2023
The reflection nebula NGC 2023 was observed by a rocket-borne long-slit
imaging spectrograph in the 900 -- 1400 Angstrom bandpass on 2000 February 11.
A spectrum of the star, as well as that of the nebular scattered light, was
recorded. Through the use of a Monte Carlo modeling process, the scattering
properties of the dust were derived. The albedo is low, 0.2 -- 0.4, and
decreasing toward shorter wavelengths, while the phase function asymmetry
parameter is consistent with highly forward-scattering grains, g~0.85. The
decrease in albedo, while the optical depth increases to shorter wavelengths,
implies that the far-UV rise in the extinction curve is due to an increase in
absorption efficiency.Comment: 16 pages, 11 figures, accepted for publication in the Astrophysical
Journa
The metallicity extremes of the Sagittarius dSph using SALT spectroscopy of PNe
In this work we present the first spectroscopic results obtained with the
Southern African Large Telescope (SALT) telescope during its
perfomance-verification phase. We find that the Sagittarius dwarf spheroidal
galaxy (Sgr) Sgr contains a youngest stellar population with [O/H] -0.2 and age
t>1 Gyr, and an oldest population with [O/H]=-2.0. The values are based on
spectra of two planetary nebulae (PNe), using empirical abundance
determinations. We calculated abundances for O, N, Ne, Ar, S, Cl, Fe, C and He.
We confirm the high abundances of PN StWr2-21 with 12+log(O/H) = 8.57+/-0.02
dex. The other PN studied, BoBn1, is an extraordinary object in that the neon
abundance exceeds that of oxygen. The abundances of S, Ar and Cl in BoBn1 yield
the original stellar metallicity, corresponding to 12+log(O/H) = 6.72+/-0.16
dex which is 1/110 of the solar value. The actual [O/H] is much higher: third
dredge-up enriched the material by a factor of ~12 in oxygen, ~240 in nitrogen
and ~70 in neon. Neon as well as nitrogen and oxygen content may have been
produced in the intershell of low-mass AGB stars. Well defined broad WR lines
are present in the spectrum of StWr2-21 and absent in the spectrum of BoBn1.
This puts the fraction of [WR]-type central PNe stars to 67% for dSph galaxies.Comment: 14 pages, 4 figures, accepted to MNRA
Rocket and FUSE Observations of IC 405: Differential Extinction and Fluorescent Molecular Hydrogen
We present far-ultraviolet spectroscopy of the emission/reflection nebula IC
405 obtained by a rocket-borne long-slit spectrograph and the Far Ultraviolet
Spectroscopic Explorer. Both data sets show a rise in the ratio of the nebular
surface brightness to stellar flux (S/F_*) of approximately two orders of
magnitude towards the blue end of the far-UV bandpass. Scattering models using
simple dust geometries fail to reproduce the observed S/F_* for realistic grain
properties. The high spectral resolution of the FUSE data reveals a rich
fluorescent molecular hydrogen spectrum ~1000" north of the star that is
clearly distinguished from the steady blue continuum. The S/F_* remains roughly
constant at all nebular pointings, showing that fluorescent molecular hydrogen
is not the dominant cause for the blue rise. We discuss three possible
mechanisms for the ``Blue Dust'': differential extinction of the dominant star
(HD 34078), unusual dust grain properties, and emission from nebular dust. We
conclude that uncertainties in the nebular geometry and the degree of dust
clumping are most likely responsible for the blue rise. As an interesting
consequence of this result, we consider how IC 405 would appear in a spatially
unresolved observation. If IC 405 was observed with a spatial resolution of
less than 0.4 pc, for example, an observer would infer a far-UV flux that was
2.5 times the true value, giving the appearance of a stellar continuum that was
less extinguished than radiation from the surrounding nebula, an effect that is
reminiscent of the observed ultraviolet properties of starburst galaxies.Comment: 8 pages, 9 figures, using emulateapj, ApJ - accepte
Far-Ultraviolet Dust Albedo Measurements in the Upper Scorpius Cloud Using the SPINR Sounding Rocket Experiment
The Spectrograph for Photometric Imaging with Numeric Reconstruction (SPINR)
sounding rocket experiment was launched on 2000 August 4 to record
far-ultraviolet (912-1450 A) spectral and spatial information for the giant
reflection nebula in the Upper Scorpius region. The data were divided into
three arbitrary bandpasses (912-1029 A, 1030-1200 A, and 1235-1450 A) for which
stellar and nebular flux levels were derived. These flux measurements were used
to constrain a radiative transfer model and to determine the dust albedo for
the Upper Scorpius region. The resulting albedos were 0.28+/-0.07 for the
912-1029 A bandpass, 0.33+/-0.07 for the 1030-1200 A bandpass, and 0.77+/-0.13
for the 1235-1450 A bandpass
Observations and modeling of H_2 fluorescence with partial frequency redistribution in giant planet atmospheres
Partial frequency redistribution (PRD), describing the formation of the line
profile, has negligible observational effects for optical depths smaller than
~10^3, at the resolving power of most current instruments. However, when the
spectral resolution is sufficiently high, PRD modeling becomes essential in
interpreting the line shapes and determining the total line fluxes. We
demonstrate the effects of PRD on the H_2 line profiles observed at high
spectral resolution by the Far-Ultraviolet Spectroscopic Explorer (FUSE) in the
atmospheres of Jupiter and Saturn. In these spectra, the asymmetric shapes of
the lines in the Lyman (v"- 6) progression pumped by the solar Ly-beta are
explained by coherent scattering of the photons in the line wings. We introduce
a simple computational approximation to mitigate the numerical difficulties of
radiative transfer with PRD, and show that it reproduces the exact radiative
transfer solution to better than 10%. The lines predicted by our radiative
transfer model with PRD, including the H_2 density and temperature distribution
as a function of height in the atmosphere, are in agreement with the line
profiles observed by FUSE. We discuss the observational consequences of PRD,
and show that this computational method also allows us to include PRD in
modeling the continuum pumped H_2 fluorescence, treating about 4000 lines
simultaneously.Comment: 17 pages, accepted for publication in Ap
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