13,960 research outputs found
Numerical Analysis of the Capacities for Two-Qubit Unitary Operations
We present numerical results on the capacities of two-qubit unitary
operations for creating entanglement and increasing the Holevo information of
an ensemble. In all cases tested, the maximum values calculated for the
capacities based on the Holevo information are close to the capacities based on
the entanglement. This indicates that the capacities based on the Holevo
information, which are very difficult to calculate, may be estimated from the
capacities based upon the entanglement, which are relatively straightforward to
calculate.Comment: 9 pages, 10 figure
Non-Supersymmetric Attractors in String Theory
We find examples of non-supersymmetric attractors in Type II string theory
compactified on a Calabi Yau three-fold. For a non-supersymmetric attractor the
fixed values to which the moduli are drawn at the horizon must minimise an
effective potential. For Type IIA at large volume, we consider a configuration
carrying D0, D2, D4 and D6 brane charge. When the D6 brane charge is zero, we
find for some range of the other charges, that a non-supersymmetric attractor
solution exists. When the D6 brane charge is non-zero, we find for some range
of charges, a supersymmetry breaking extremum of the effective potential.
Closer examination reveals though that it is not a minimum of the effective
potential and hence the corresponding black hole solution is not an attractor.
Away from large volume, we consider the specific case of the quintic in CP^4.
Working in the mirror IIB description we find non-supersymmetric attractors
near the Gepner point.Comment: Added a few clarification
Mechanical Mixing in Nonlinear Nanomechanical Resonators
Nanomechanical resonators, machined out of Silicon-on-Insulator wafers, are
operated in the nonlinear regime to investigate higher-order mechanical mixing
at radio frequencies, relevant to signal processing and nonlinear dynamics on
nanometer scales. Driven by two neighboring frequencies the resonators generate
rich power spectra exhibiting a multitude of satellite peaks. This nonlinear
response is studied and compared to -order perturbation theory and
nonperturbative numerical calculations.Comment: 5 pages, 7 figure
Rigid invariance as derived from BRS invariance: The abelian Higgs model
Consequences of a symmetry, e.g.\ relations amongst Green functions, are
renormalization scheme independently expressed in terms of a rigid Ward
identity. The corresponding local version yields information on the respective
current. In the case of spontaneous breakdown one has to define the theory via
the BRS invariance and thus to construct rigid and current Ward identity
non-trivially in accordance with it. We performed this construction to all
orders of perturbation theory in the abelian Higgs model as a prelude to the
standard model. A technical tool of interest in itself is the use of a doublet
of external scalar ``background'' fields. The Callan-Symanzik equation has an
interesting form and follows easily once the rigid invariance is established.Comment: 33 pages, Plain Te
Multiplicity at the Stellar/Substellar Boundary in Upper Scorpius
We present the results of a high-resolution imaging survey of 12 brown dwarfs
and very low mass stars in the closest (~145 pc) young (~5 Myr) OB association,
Upper Scorpius. We obtained images with the Advanced Camera for Surveys/High
Resolution Camera on HST through the F555W (V), F775W (i'), and F850LP (z')
filters. This survey discovered three new binary systems, including one
marginally resolved pair with a projected separation of only 4.9 AU, resulting
in an observed binary fraction of 25+/-14% at separations >4 AU. After
correcting for detection biases assuming a uniform distribution of mass ratios
for q>0.6, the estimated binary fraction is 33+/-17%. The binary fraction is
consistent with that inferred for higher-mass stars in Upper Sco, but the
separation and mass ratio distributions appear to be different. All three
low-mass binary systems in Upper Sco are tight (<18 AU) and of similar mass
(q>0.6), consistent with expectations based on previous multiplicity studies of
brown dwarfs and very low mass stars in the field and in open clusters. The
implication is that the distinct separation and mass ratio distributions of
low-mass systems are set in the formation process or at very young ages, rather
than by dynamical disruption of wide systems at ages >5 Myr. Finally, we
combine the survey detection limits with the models of Burrows et al. (1997) to
show that there are no planets or very low-mass brown dwarfs with masses >10
M_J at projected separations >20 AU, or masses >5 M_J at projected separations
>40 AU orbiting any of the low-mass (0.04-0.10 M_sun) objects in our sample.Comment: Accepted for publication in ApJ; 10 pages, 4 figures in emulateapj
forma
Noncommutative Lorentz Symmetry and the Origin of the Seiberg-Witten Map
We show that the noncommutative Yang-Mills field forms an irreducible
representation of the (undeformed) Lie algebra of rigid translations, rotations
and dilatations. The noncommutative Yang-Mills action is invariant under
combined conformal transformations of the Yang-Mills field and of the
noncommutativity parameter \theta. The Seiberg-Witten differential equation
results from a covariant splitting of the combined conformal transformations
and can be computed as the missing piece to complete a covariant conformal
transformation to an invariance of the action.Comment: 20 pages, LaTeX. v2: Streamlined proofs and extended discussion of
Lorentz transformation
Quantum data processing and error correction
This paper investigates properties of noisy quantum information channels. We
define a new quantity called {\em coherent information} which measures the
amount of quantum information conveyed in the noisy channel. This quantity can
never be increased by quantum information processing, and it yields a simple
necessary and sufficient condition for the existence of perfect quantum error
correction.Comment: LaTeX, 20 page
Dyon Spectrum in Generic N=4 Supersymmetric Z_N Orbifolds
We find the exact spectrum of a class of quarter BPS dyons in a generic N=4
supersymmetric Z_N orbifold of type IIA string theory on K3\times T^2 or T^6.
We also find the asymptotic expansion of the statistical entropy to first
non-leading order in inverse power of charges and show that it agrees with the
entropy of a black hole carrying same set of charges after taking into account
the effect of the four derivative Gauss-Bonnet term in the effective action of
the theory.Comment: LaTeX file, 39 pages; minor change
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