3,299 research outputs found
Reports Of Conferences, Institutes, And Seminars
This quarter\u27s column offers coverage of multiple sessions from the 2016 Electronic Resources & Libraries (ER&L) Conference, held April 3â6, 2016, in Austin, Texas. Topics in serials acquisitions dominate the column, including reports on altmetrics, cost per use, demand-driven acquisitions, and scholarly communications and the use of subscriptions agents; ERMS, access, and knowledgebases are also featured
Classical simulation of noninteracting-fermion quantum circuits
We show that a class of quantum computations that was recently shown to be
efficiently simulatable on a classical computer by Valiant corresponds to a
physical model of noninteracting fermions in one dimension. We give an
alternative proof of his result using the language of fermions and extend the
result to noninteracting fermions with arbitrary pairwise interactions, where
gates can be conditioned on outcomes of complete von Neumann measurements in
the computational basis on other fermionic modes in the circuit. This last
result is in remarkable contrast with the case of noninteracting bosons where
universal quantum computation can be achieved by allowing gates to be
conditioned on classical bits (quant-ph/0006088).Comment: 26 pages, 1 figure, uses wick.sty; references added to recent results
by E. Knil
Zeno Dynamics of von Neumann Algebras
The dynamical quantum Zeno effect is studied in the context of von Neumann
algebras. We identify a localized subalgebra on which the Zeno dynamics acts by
automorphisms. The Zeno dynamics coincides with the modular dynamics of that
subalgebra, if an additional assumption is satisfied. This relates the modular
operator of that subalgebra to the modular operator of the original algebra by
a variant of the Kato-Lie-Trotter product formula.Comment: Revised version; further typos corrected; 9 pages, AMSLaTe
Restoration of rotational invariance of bound states on the light front
We study bound states in a model with scalar nucleons interacting via an
exchanged scalar meson using the Hamiltonian formalism on the light front. In
this approach manifest rotational invariance is broken when the Fock space is
truncated. By considering an effective Hamiltonian that takes into account two
meson exchanges, we find that this breaking of rotational invariance is
decreased from that which occurs when only one meson exchange is included. The
best improvement occurs when the states are weakly bound.Comment: 20 pages, 6 figures, uses feynMF; changed typos, clarified use of
angular momentu
Three-dimensional Simulations of Disk Accretion to an Inclined Dipole: I. Magnetospheric Flow at Different Theta
We present results of fully three-dimensional MHD simulations of disk
accretion to a rotating magnetized star with its dipole moment inclined at an
angle Theta to the rotation axis of the disk. We observed that matter accretes
from the disk to a star in two or several streams depending on Theta. Streams
may precess around the star at small Theta. The inner regions of the disk are
warped. The warping is due to the tendency of matter to co-rotate with inclined
magnetosphere. The accreting matter brings positive angular momentum to the
(slowly rotating) star tending to spin it up. The corresponding torque N_z
depends only weakly on Theta. The angular momentum flux to the star is
transported predominantly by the magnetic field; the matter component
contributes < 1 % of the total flux. Results of simulations are important for
understanding the nature of classical T Tauri stars, cataclysmic variables, and
X-ray pulsars.Comment: 26 pages, 22 figures, LaTeX, macros: emulapj.sty, avi simulations are
available at http://www.astro.cornell.edu/us-rus/inclined.ht
The Generalized Gell-Mann--Low Theorem for Relativistic Bound States
The recently established generalized Gell-Mann--Low theorem is applied in
lowest perturbative order to bound-state calculations in a simple scalar field
theory with cubic couplings. The approach via the generalized Gell-Mann--Low
Theorem retains, while being fully relativistic, many of the desirable features
of the quantum mechanical approaches to bound states. In particular, no
abnormal or unphysical solutions are found in the model under consideration.
Both the non-relativistic and one-body limits are straightforward and
consistent. The results for the spectrum are compared to those of the
Bethe-Salpeter equation (in the ladder approximation) and related equations.Comment: 24 pages, 6 pspicture diagrams, 4 postscript figure
Electromagnetic Meson Form Factors in the Salpeter Model
We present a covariant scheme to calculate mesonic transitions in the
framework of the Salpeter equation for -states. The full Bethe
Salpeter amplitudes are reconstructed from equal time amplitudes which were
obtained in a previous paper\cite{Mue} by solving the Salpeter equation for a
confining plus an instanton induced interaction. This method is applied to
calculate electromagnetic form factors and decay widths of low lying
pseudoscalar and vector mesons including predictions for CEBAF experiments. We
also describe the momentum transfer dependence for the processes
.Comment: 22 pages including 10 figure
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