13,113 research outputs found
Experimental Demonstration of Greenberger-Horne-Zeilinger Correlations Using Nuclear Magnetic Resonance
The Greenberger-Horne-Zeilinger (GHZ) effect provides an example of quantum
correlations that cannot be explained by classical local hidden variables. This
paper reports on the experimental realization of GHZ correlations using nuclear
magnetic resonance (NMR). The NMR experiment differs from the originally
proposed GHZ experiment in several ways: it is performed on mixed states rather
than pure states; and instead of being widely separated, the spins on which it
is performed are all located in the same molecule. As a result, the NMR version
of the GHZ experiment cannot entirely rule out classical local hidden
variables. It nonetheless provides an unambiguous demonstration of the
"paradoxical" GHZ correlations, and shows that any classical hidden variables
must communicate by non-standard and previously undetected forces. The NMR
demonstration of GHZ correlations shows the power of NMR quantum information
processing techniques for demonstrating fundamental effects in quantum
mechanics.Comment: Latex2.09, 8 pages, 1 eps figur
Quantum search without entanglement
Entanglement of quantum variables is usually thought to be a prerequisite for
obtaining quantum speed-ups of information processing tasks such as searching
databases. This paper presents methods for quantum search that give a speed-up
over classical methods, but that do not require entanglement. These methods
rely instead on interference to provide a speed-up. Search without entanglement
comes at a cost: although they outperform analogous classical devices, the
quantum devices that perform the search are not universal quantum computers and
require exponentially greater overhead than a quantum computer that operates
using entanglement. Quantum search without entanglement is compared to
classical search using waves.Comment: 9 pages, TeX, submitted to Physical Review Letter
The Chemical Compositions of the Type II Cepheids -- The BL Her and W Vir Variables
Abundance analyses from high-resolution optical spectra are presented for 19
Type II Cepheids in the Galactic field. The sample includes both short-period
(BL Her) and long-period (W Vir) stars. This is the first extensive abundance
analysis of these variables. The C, N, and O abundances with similar spreads
for the BL Her and W Vir show evidence for an atmosphere contaminated with
-process and CN-cycling products. A notable anomaly of the BL Her
stars is an overabundance of Na by a factor of about five relative to their
presumed initial abundances. This overabundance is not seen in the W Vir stars.
The abundance anomalies running from mild to extreme in W Vir stars but not
seen in the BL Her stars are attributed to dust-gas separation that provides an
atmosphere deficient in elements of high condensation temperature, notably Al,
Ca, Sc, Ti, and -process elements. Such anomalies have previously been seen
among RV Tau stars which represent a long-period extension of the variability
enjoyed by the Type II Cepheids. Comments are offered on how the contrasting
abundance anomalies of BL Her and W Vir stars may be explained in terms of the
stars' evolution from the blue horizontal branch.Comment: 41 pages including 11 figures and 4 tables; Accepted for publication
in Ap
Quantum Simulations on a Quantum Computer
We present a general scheme for performing a simulation of the dynamics of
one quantum system using another. This scheme is used to experimentally
simulate the dynamics of truncated quantum harmonic and anharmonic oscillators
using nuclear magnetic resonance. We believe this to be the first explicit
physical realization of such a simulation.Comment: 4 pages, 2 figures (\documentstyle[prl,aps,epsfig,amscd]{revtex}); to
appear in Phys. Rev. Let
Computational capacity of the universe
Merely by existing, all physical systems register information. And by
evolving dynamically in time, they transform and process that information. The
laws of physics determine the amount of information that a physical system can
register (number of bits) and the number of elementary logic operations that a
system can perform (number of ops). The universe is a physical system. This
paper quantifies the amount of information that the universe can register and
the number of elementary operations that it can have performed over its
history. The universe can have performed no more than ops on
bits.Comment: 17 pages, TeX. submitted to Natur
Quantum computation over continuous variables
This paper provides necessary and sufficient conditions for constructing a
universal quantum computer over continuous variables. As an example, it is
shown how a universal quantum computer for the amplitudes of the
electromagnetic field might be constructed using simple linear devices such as
beam-splitters and phase shifters, together with squeezers and nonlinear
devices such as Kerr-effect fibers and atoms in optical cavities. Such a device
could in principle perform `quantum floating point' computations. Problems of
noise, finite precision, and error correction are discussed.Comment: 9 pages, Te
Simulation of Many-Body Fermi Systems on a Universal Quantum Computer
We provide fast algorithms for simulating many body Fermi systems on a
universal quantum computer. Both first and second quantized descriptions are
considered, and the relative computational complexities are determined in each
case. In order to accommodate fermions using a first quantized Hamiltonian, an
efficient quantum algorithm for anti-symmetrization is given. Finally, a
simulation of the Hubbard model is discussed in detail.Comment: Submitted 11/7/96 to Phys. Rev. Lett. 10 pages, 0 figure
Reduced antimicrobial prescribing during autogenous staphylococcal bacterin therapy: a retrospective study in dogs with pyoderma
Autogenous staphylococcal bacterins are commonly mentioned as treatment for canine recurrent pyoderma but little is known about their efficacy. This retrospective study describes use and assesses efficacy of an autogenous Staphylococcus (pseud)intermedius bacterin in dogs with pyoderma. Frequency and duration of systemic antimicrobial therapy were compared 12 months before and after starting bacterin (Wilcoxon signed-rank test) with data extracted from general practice medical histories
Fundamental parameters of Cepheids. V. Additional photometry and radial velocity for southern Cepheids
I present photometric and radial velocity data for Galactic Cepheids, most of
them being in the southern hemisphere. There are 1250 Geneva 7-color
photometric measurements for 62 Cepheids, the average uncertainty per
measurement is better than 0.01 mag. A total of 832 velocity measurements have
been obtained with the CORAVEL radial velocity spectrograph for 46 Cepheids.
The average accuracy of the radial velocity data is 0.38 km/s. There are 33
stars with both photometry and radial velocity data. I discuss the possible
binarity or period change that these new data reveal. I also present reddenings
for all Cepheids with photometry. The data are available electronically.Comment: To appear in ApJS. Data available electronically at
ftp://cfa-ftp.harvard.edu/pub/dbersier
Proof Relevant Corecursive Resolution
Resolution lies at the foundation of both logic programming and type class
context reduction in functional languages. Terminating derivations by
resolution have well-defined inductive meaning, whereas some non-terminating
derivations can be understood coinductively. Cycle detection is a popular
method to capture a small subset of such derivations. We show that in fact
cycle detection is a restricted form of coinductive proof, in which the atomic
formula forming the cycle plays the role of coinductive hypothesis.
This paper introduces a heuristic method for obtaining richer coinductive
hypotheses in the form of Horn formulas. Our approach subsumes cycle detection
and gives coinductive meaning to a larger class of derivations. For this
purpose we extend resolution with Horn formula resolvents and corecursive
evidence generation. We illustrate our method on non-terminating type class
resolution problems.Comment: 23 pages, with appendices in FLOPS 201
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