187 research outputs found
On the generalization of quantum state comparison
We investigate the unambiguous comparison of quantum states in a scenario
that is more general than the one that was originally suggested by Barnett et
al. First, we find the optimal solution for the comparison of two states taken
from a set of two pure states with arbitrary a priori probabilities. We show
that the optimal coherent measurement is always superior to the optimal
incoherent measurement. Second, we develop a strategy for the comparison of two
states from a set of N pure states, and find an optimal solution for some
parameter range when N=3. In both cases we use the reduction method for the
corresponding problem of mixed state discrimination, as introduced by Raynal et
al., which reduces the problem to the discrimination of two pure states only
for N=2. Finally, we provide a necessary and sufficient condition for
unambiguous comparison of mixed states to be possible.Comment: 8 pages, 4 figures, Proposition 1 corrected, appendix adde
Typical local measurements in generalised probabilistic theories: emergence of quantum bipartite correlations
What singles out quantum mechanics as the fundamental theory of Nature? Here
we study local measurements in generalised probabilistic theories (GPTs) and
investigate how observational limitations affect the production of
correlations. We find that if only a subset of typical local measurements can
be made then all the bipartite correlations produced in a GPT can be simulated
to a high degree of accuracy by quantum mechanics. Our result makes use of a
generalisation of Dvoretzky's theorem for GPTs. The tripartite correlations can
go beyond those exhibited by quantum mechanics, however.Comment: 5 pages, 1 figure v2: more details in the proof of the main resul
Optimal inequalities for state-independent contextuality
Contextuality is a natural generalization of nonlocality which does not need
composite systems or spacelike separation and offers a wider spectrum of
interesting phenomena. Most notably, in quantum mechanics there exist scenarios
where the contextual behavior is independent of the quantum state. We show that
the quest for an optimal inequality separating quantum from classical
noncontextual correlations in an state-independent manner admits an exact
solution, as it can be formulated as a linear program. We introduce the
noncontextuality polytope as a generalization of the locality polytope, and
apply our method to identify two different tight optimal inequalities for the
most fundamental quantum scenario with state-independent contextuality.Comment: REVTeX4.1, 5 pages, 1 figure; v2: improved presentation and
significantly extended result
Acceleration Schemes for Ab-Initio Molecular Dynamics and Electronic Structure Calculations
We study the convergence and the stability of fictitious dynamical methods
for electrons. First, we show that a particular damped second-order dynamics
has a much faster rate of convergence to the ground-state than first-order
steepest descent algorithms while retaining their numerical cost per time step.
Our damped dynamics has efficiency comparable to that of conjugate gradient
methods in typical electronic minimization problems. Then, we analyse the
factors that limit the size of the integration time step in approaches based on
plane-wave expansions. The maximum allowed time step is dictated by the highest
frequency components of the fictitious electronic dynamics. These can result
either from the large wavevector components of the kinetic energy or from the
small wavevector components of the Coulomb potential giving rise to the so
called {\it charge sloshing} problem. We show how to eliminate large wavevector
instabilities by adopting a preconditioning scheme that is implemented here for
the first-time in the context of Car-Parrinello ab-initio molecular dynamics
simulations of the ionic motion. We also show how to solve the charge-sloshing
problem when this is present. We substantiate our theoretical analysis with
numerical tests on a number of different silicon and carbon systems having both
insulating and metallic character.Comment: RevTex, 9 figures available upon request, to appear in Phys. Rev.
Relativistic analysis of the 208Pb(e,e'p)207Tl reaction at high momentum
The recent 208Pb(e,e'p)207Tl data from NIKHEF-K at high missing momentum
(p_m>300 MeV/c) are compared to theoretical results obtained with a fully
relativistic formalism previously applied to analyze data on the low missing
momentum (p_m < 300 MeV/c) region. The same relativistic optical potential and
mean field wave functions are used in the two p_m-regions. The spectroscopic
factors of the various shells are extracted from the analysis of the low-p_m
data and then used in the high-p_m region. In contrast to previous analyses
using a nonrelativistic mean field formalism, we do not find a substantial
deviation from the mean field predictions other than that of the spectroscopic
factors, which appear to be consistent with both low- and high-p_m data. We
find that the difference between results of relativistic and nonrelativistic
formalisms is enhanced in the p_m<0 region that will be interesting to explore
experimentally.Comment: 12 pages, LaTeX+Revtex, included 3 postscript figures. To appear in
the Physical Review C (Rapid Communications
Coexisting conical bipolar and equatorial outflows from a high-mass protostar
The BN/KL region in the Orion molecular cloud is an archetype in the study of
the formation of stars much more massive than the Sun. This region contains
luminous young stars and protostars, but it is difficult to study because of
overlying dust and gas. Our basic expectations are shaped to some extent by the
present theoretical picture of star formation, the cornerstone of which is that
protostars acrete gas from rotating equatorial disks, and shed angular momentum
by ejecting gas in bipolar outflows. The main source of the outflow in the
BN/KL region may be an object known as radio source I, which is commonly
believed to be surrounded by a rotating disk of molecular material. Here we
report high-resolution observations of silicon monoxide (SiO) and water maser
emission from the gas surrounding source I; we show that within 60 AU (about
the size of the Solar System), the region is dominated by a conical bipolar
outflow, rather than the expected disk. A slower outflow, close to the
equatorial plane of the protostellar system, extends to radii of 1,000 AU.Comment: 10 pages, 2 figures. Accepted by Nature. To appear December 199
Ten Million Degree Gas in M 17 and the Rosette Nebula: X-ray Flows in Galactic H II Regions
We present the first high-spatial-resolution X-ray images of two high-mass
star forming regions, the Omega Nebula (M 17) and the Rosette Nebula (NGC
2237--2246), obtained with the Chandra X-ray Observatory Advanced CCD Imaging
Spectrometer (ACIS) instrument. The massive clusters powering these H II
regions are resolved at the arcsecond level into >900 (M 17) and >300 (Rosette)
stellar sources similar to those seen in closer young stellar clusters.
However, we also detect soft diffuse X-ray emission on parsec scales that is
spatially and spectrally distinct from the point source population. The diffuse
emission has luminosity L_x ~ 3.4e33 ergs/s in M~17 with plasma energy
components at kT ~0.13 and ~0.6 keV (1.5 and 7 MK), while in Rosette it has L_x
\~6e32 ergs/s with plasma energy components at kT ~0.06 and ~0.8 keV (0.7 and 9
MK). This extended emission most likely arises from the fast O-star winds
thermalized either by wind-wind collisions or by a termination shock against
the surrounding media. We establish that only a small portion of the wind
energy and mass appears in the observed diffuse X-ray plasma; in these blister
H II regions, we suspect that most of it flows without cooling into the
low-density interstellar medium. These data provide compelling observational
evidence that strong wind shocks are present in H II regions.Comment: 35 pages, including 11 figures; to appear in ApJ, August 20, 2003. A
version with high-resolution figures is available at
ftp://ftp.astro.psu.edu/pub/townsley/diffuse.ps.g
NGC6240: Merger-Induced Star Formation & Gas Dynamics
We present spatially resolved integral field spectroscopic K-band data at a
resolution of 0.13" (60pc) and interferometric CO(2-1) line observations of the
prototypical merging system NGC6240. Despite the clear rotational signature,
the stellar kinematics in the two nuclei are dominated by dispersion. We use
Jeans modelling to derive the masses and the mass-to-light ratios of the
nuclei. Combining the luminosities with the spatially resolved Br-gamma
equivalent width shows that only 1/3 of the K-band continuum from the nuclei is
associated with the most recent star forming episode; and that less than 30% of
the system's bolometric luminosity and only 9% of its stellar mass is due to
this starburst. The star formation properties, calculated from typical merger
star formation histories, demonstrate the impact of different assumptions about
the star formation history. The properties of the nuclei, and the existence of
a prominent old stellar population, indicate that the nuclei are remnants of
the progenitor galaxies' bulges.Comment: 18 pages, 14 figures. Accepted for publication in A&
Why Do Situational Interviews Predict Performance? Is it Saying How You Would Behave or Knowing How You Should Behave?
Purpose: The present study examined two theoretical explanations for why situational interviews predict work-related performance, namely (a) that they are measures of intervieweesâ behavioral intentions or (b) that they are measures of intervieweesâ ability to correctly decipher situational demands. Design/Methodology/Approach: We tested these explanations with 101 students, who participated in a 2-day selection simulation. Findings: In line with the first explanation, there was considerable similarity between what participants said they would do and their actual behavior in corresponding work-related situations. However, the underlying postulated mechanism was not supported by the data. In line with the second explanation, participantsâ ability to correctly decipher situational demands was related to performance in both the interview and work-related situations. Furthermore, the relationship between the interview and performance in the work-related situations was partially explained by this ability to decipher situational demands. Implications: Assessing intervieweesâ ability to identify criteria might be of additional value for making selection decisions, particularly for jobs where it is essential to assess situational demands. Originality/Value: The present study made an effort to open the âblack boxâ of situational interview validity by examining two explanations for their validity. The results provided only moderate support for the first explanation. However, the second explanation was fully supported by these results
First determination of the dynamical mass of a binary L dwarf
We present here the results of astrometric, photometric and spectroscopic
observations leading to the determination of the orbit and dynamical masses of
the binary L dwarf 2MASSW J0746425+2000321. High angular resolution
observations spread over almost 4 years and obtained with the Hubble Space
Telescope (HST), the ESO Very Large Telescope (VLT), and a the W. M. Keck
Observatory (Keck) allow us to cover 36% of the period, corresponding to 60% of
the orbit, and, for the first time, to derive a precise estimate of the total
and individual masses of such a late-type object. We find an orbital period of
3850.9 days. The corresponding total mass is
0.146 M_{\sun}, with uncertainties depending on the
distance. Spatially resolved low resolution optical (550--1025 nm) spectra have
been obtained with HST/STIS, allowing us to measure the spectral types of the
two components (L00.5 for the primary and L1.50.5 for the secondary).
We also present precise photometry of the individual components measured on the
high angular resolution images obtained with HST/ACS and WFPC2 (visible),
VLT/NACO (J, H and Ks bands) and Keck I (Ks) band). These spectral and
photometric measurements enable us to estimate their effective temperatures and
mass ratio, and to place the object accurately in a H--R diagram. The binary
system is most likely formed by a primary with a mass of 0.0850.010
M_{\sun} and a secondary with a mass of 0.0660.006 M_{\sun}, thus
clearly substellar, for an age of approximately 300150 Myr. H
variability indicates chromospheric and/or magnetic activity.Comment: accepted for publication in A&A 16 pages, 7 figures, 6 table
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