5,560 research outputs found
Report of the Committee on Communist Activities
Short reports from the Committee on Communist Activities and the Committee on Juvenile Problems, along with the report of the Committee on Resolutions with the text of adopted resolutions
NLTE Strontium and Barium in metal poor red giant stars
We present atmospheric models of red giant stars of various metallicities,
including extremely metal poor (XMP, [Fe/H]<-3.5) models, with many chemical
species, including, significantly, the first two ionization stages of Strontium
(Sr) and Barium (Ba), treated in Non-Local Thermodynamic Equilibrium (NLTE)
with various degrees of realism. We conclude that 1) for all lines that are
useful Sr and Ba abundance diagnostics the magnitude and sense of the computed
NLTE effect on the predicted line strength is metallicity dependent, 2) the
indirect NLTE effect of overlap between Ba and Sr transitions and transitions
of other species that are also treated in NLTE non-negligibly enhances NLTE
abundance corrections for some lines, 3) the indirect NLTE effect of NLTE
opacity of other species on the equilibrium structure of the atmospheric model
is not significant, 4) the computed NLTE line strengths differ negligibly if
collisional b-b and b-f rates are an order of magnitude smaller or larger than
those calculated with standard analytic formulae, and 5) the effect of NLTE
upon the resonance line of Ba II at 4554.03 AA is independent of whether that
line is treated with hyperfine splitting. As a result, the derivation of
abundances of Ba and Sr for metal-poor red giant stars with LTE modeling that
are in the literature should be treated with caution.Comment: 28 pages, 10 figures. Accepted for publication in April 2006
Astrophysical Journa
Actions of the braid group, and new algebraic proofs of results of Dehornoy and Larue
This article surveys many standard results about the braid group with
emphasis on simplifying the usual algebraic proofs.
We use van der Waerden's trick to illuminate the Artin-Magnus proof of the
classic presentation of the algebraic mapping-class group of a punctured disc.
We give a simple, new proof of the Dehornoy-Larue braid-group trichotomy,
and, hence, recover the Dehornoy right-ordering of the braid group.
We then turn to the Birman-Hilden theorem concerning braid-group actions on
free products of cyclic groups, and the consequences derived by Perron-Vannier,
and the connections with the Wada representations. We recall the very simple
Crisp-Paris proof of the Birman-Hilden theorem that uses the Larue-Shpilrain
technique. Studying ends of free groups permits a deeper understanding of the
braid group; this gives us a generalization of the Birman-Hilden theorem.
Studying Jordan curves in the punctured disc permits a still deeper
understanding of the braid group; this gave Larue, in his PhD thesis,
correspondingly deeper results, and, in an appendix, we recall the essence of
Larue's thesis, giving simpler combinatorial proofs.Comment: 51`pages, 13 figure
Full density matrix dynamics for large quantum systems: Interactions, Decoherence and Inelastic effects
We develop analytical tools and numerical methods for time evolving the total
density matrix of the finite-size Anderson model. The model is composed of two
finite metal grains, each prepared in canonical states of differing chemical
potential and connected through a single electronic level (quantum dot or
impurity). Coulomb interactions are either excluded all together, or allowed on
the dot only. We extend this basic model to emulate decoherring and inelastic
scattering processes for the dot electrons with the probe technique. Three
methods, originally developed to treat impurity dynamics, are augmented to
yield global system dynamics: the quantum Langevin equation method, the well
known fermionic trace formula, and an iterative path integral approach. The
latter accommodates interactions on the dot in a numerically exact fashion. We
apply the developed techniques to two open topics in nonequilibrium many-body
physics: (i) We explore the role of many-body electron-electron repulsion
effects on the dynamics of the system. Results, obtained using exact path
integral simulations, are compared to mean-field quantum Langevin equation
predictions. (ii) We analyze aspects of quantum equilibration and
thermalization in large quantum systems using the probe technique, mimicking
elastic-dephasing effects and inelastic interactions on the dot. Here, unitary
simulations based on the fermionic trace formula are accompanied by quantum
Langevin equation calculations
A Global Probe of Cosmic Magnetic Fields to High Redshifts
Faraday rotation (RM) probes of magnetic fields in the universe are sensitive
to cosmological and evolutionary effects as increases beyond 1
because of the scalings of electron density and magnetic fields, and the growth
in the number of expected intersections with galaxy-scale intervenors,
N/. In this new global analysis of an unprecedented large sample of RM's
of high latitude quasars extending out to 3.7 we find that the
distribution of RM broadens with redshift in the 20 80 rad m range
range, despite the (1 +) wavelength dilution expected in the observed
Faraday rotation. Our results indicate that the Universe becomes increasingly
``Faraday-opaque'' to sources beyond 2, that is, as increases
progressively fewer sources are found with a ``small'' RM in the observer's
frame. This is in contrast to sources at z \la1. They suggest that the
environments of galaxies were significantly magnetized at high redshifts, with
magnetic field strengths that were at least as strong within a few Gyr of the
Big Bang as at the current epoch. We separately investigate a simple unevolving
toy model in which the RM is produced by MgII absorber systems, and find that
it can approximately reproduce the observed trend with redshift. An additional
possibility is that the intrinsic RM associated with the radio sources was much
higher in the past, and we show that this is not a trivial consequence of the
higher radio luminosities of the high redshift sources.Comment: 10 pages, 8 figures Astrophysical Jounrnal in press, March 200
Witnessing eigenstates for quantum simulation of Hamiltonian spectra
The efficient calculation of Hamiltonian spectra, a problem often intractable
on classical machines, can find application in many fields, from physics to
chemistry. Here, we introduce the concept of an "eigenstate witness" and
through it provide a new quantum approach which combines variational methods
and phase estimation to approximate eigenvalues for both ground and excited
states. This protocol is experimentally verified on a programmable silicon
quantum photonic chip, a mass-manufacturable platform, which embeds entangled
state generation, arbitrary controlled-unitary operations, and projective
measurements. Both ground and excited states are experimentally found with
fidelities >99%, and their eigenvalues are estimated with 32-bits of precision.
We also investigate and discuss the scalability of the approach and study its
performance through numerical simulations of more complex Hamiltonians. This
result shows promising progress towards quantum chemistry on quantum computers.Comment: 9 pages, 4 figures, plus Supplementary Material [New version with
minor typos corrected.
A generalized quantum microcanonical ensemble
We discuss a generalized quantum microcanonical ensemble. It describes
isolated systems that are not necessarily in an eigenstate of the Hamilton
operator. Statistical averages are obtained by a combination of a time average
and a maximum entropy argument to resolve the lack of knowledge about initial
conditions. As a result, statistical averages of linear observables coincide
with values obtained in the canonical ensemble. Non-canonical averages can be
obtained by taking into account conserved quantities which are non-linear
functions of the microstate.Comment: improved version, new titl
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