10,395 research outputs found
Constrained fitting of three-point functions
We determine matrix elements for semileptonic decay. The use of the
constrained fitting method and multiple smearings for both two- and three-point
correlators allows an improved calculation of the form factors.Comment: Talk given at Lattice2001(heavyquark), 3 pages, 4 figure
Efficient estimation of Banach parameters in semiparametric models
Consider a semiparametric model with a Euclidean parameter and an
infinite-dimensional parameter, to be called a Banach parameter. Assume: (a)
There exists an efficient estimator of the Euclidean parameter. (b) When the
value of the Euclidean parameter is known, there exists an estimator of the
Banach parameter, which depends on this value and is efficient within this
restricted model. Substituting the efficient estimator of the Euclidean
parameter for the value of this parameter in the estimator of the Banach
parameter, one obtains an efficient estimator of the Banach parameter for the
full semiparametric model with the Euclidean parameter unknown. This hereditary
property of efficiency completes estimation in semiparametric models in which
the Euclidean parameter has been estimated efficiently. Typically, estimation
of both the Euclidean and the Banach parameter is necessary in order to
describe the random phenomenon under study to a sufficient extent. Since
efficient estimators are asymptotically linear, the above substitution method
is a particular case of substituting asymptotically linear estimators of a
Euclidean parameter into estimators that are asymptotically linear themselves
and that depend on this Euclidean parameter. This more general substitution
case is studied for its own sake as well, and a hereditary property for
asymptotic linearity is proved.Comment: Published at http://dx.doi.org/10.1214/009053604000000913 in the
Annals of Statistics (http://www.imstat.org/aos/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Fidelity threshold for long-range entanglement in quantum networks
We present a strategy to generate long-range entanglement in noisy quantum
networks. We consider a cubic lattice whose bonds are partially entangled mixed
states of two qubits, and where quantum operations can be applied perfectly at
the nodes. In contrast to protocols designed for one- or two-dimensional
regular lattices, we find that entanglement can be created between arbitrarily
distant qubits if the fidelity of the bonds is higher than a critical value,
independent of the system size. Therefore, we show that a constant overhead of
local resources, together with connections of finite fidelity, is sufficient to
achieve long-distance quantum communication in noisy networks.Comment: published versio
Asymptotically conical Calabi-Yau manifolds, I
This is the first part in a two-part series on complete Calabi-Yau manifolds
asymptotic to Riemannian cones at infinity. We begin by proving general
existence and uniqueness results. The uniqueness part relaxes the decay
condition needed in earlier work to ,
relying on some new ideas about harmonic functions. We then look at a few
examples: (1) Crepant resolutions of cones. This includes a new class of
Ricci-flat small resolutions associated with flag manifolds. (2) Affine
deformations of cones. One focus here is the question of the precise rate of
decay of the metric to its tangent cone. We prove that the optimal rate for the
Stenzel metric on is .Comment: 27 pages, various corrections, final versio
Entanglement properties of multipartite entangled states under the influence of decoherence
We investigate entanglement properties of multipartite states under the
influence of decoherence. We show that the lifetime of (distillable)
entanglement for GHZ-type superposition states decreases with the size of the
system, while for a class of other states -namely all graph states with
constant degree- the lifetime is independent of the system size. We show that
these results are largely independent of the specific decoherence model and are
in particular valid for all models which deal with individual couplings of
particles to independent environments, described by some quantum optical master
equation of Lindblad form. For GHZ states, we derive analytic expressions for
the lifetime of distillable entanglement and determine when the state becomes
fully separable. For all graph states, we derive lower and upper bounds on the
lifetime of entanglement. To this aim, we establish a method to calculate the
spectrum of the partial transposition for all mixed states which are diagonal
in a graph state basis. We also consider entanglement between different groups
of particles and determine the corresponding lifetimes as well as the change of
the kind of entanglement with time. This enables us to investigate the behavior
of entanglement under re-scaling and in the limit of large (infinite) number of
particles. Finally we investigate the lifetime of encoded quantum superposition
states and show that one can define an effective time in the encoded system
which can be orders of magnitude smaller than the physical time. This provides
an alternative view on quantum error correction and examples of states whose
lifetime of entanglement (between groups of particles) in fact increases with
the size of the system.Comment: 27 pages, 11 figure
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