3,807 research outputs found
Catalytic Conversion Probabilities for Bipartite Pure States
For two given bipartite-entangled pure states, an expression is obtained for
the least upper bound of conversion probabilities using catalysis. The
attainability of the upper bound can also be decided if that bound is less than
one.Comment: 4 pages; comments appreciated; the article is a modified version of
this preprint combined with arXiv:0707.044
Optimal discrimination of quantum operations
We address the problem of discriminating with minimal error probability two
given quantum operations. We show that the use of entangled input states
generally improves the discrimination. For Pauli channels we provide a complete
comparison of the optimal strategies where either entangled or unentangled
input states are used.Comment: 4 pages, no figure
Quantum Phase Transitions in Anti-ferromagnetic Planar Cubic Lattices
Motivated by its relation to an -hard problem, we analyze the
ground state properties of anti-ferromagnetic Ising-spin networks embedded on
planar cubic lattices, under the action of homogeneous transverse and
longitudinal magnetic fields. This model exhibits a quantum phase transition at
critical values of the magnetic field, which can be identified by the
entanglement behavior, as well as by a Majorization analysis. The scaling of
the entanglement in the critical region is in agreement with the area law,
indicating that even simple systems can support large amounts of quantum
correlations. We study the scaling behavior of low-lying energy gaps for a
restricted set of geometries, and find that even in this simplified case, it is
impossible to predict the asymptotic behavior, with the data allowing equally
good fits to exponential and power law decays. We can therefore, draw no
conclusion as to the algorithmic complexity of a quantum adiabatic ground-state
search for the system.Comment: 7 pages, 13 figures, final version (accepted for publication in PRA
Hitting Time of Quantum Walks with Perturbation
The hitting time is the required minimum time for a Markov chain-based walk
(classical or quantum) to reach a target state in the state space. We
investigate the effect of the perturbation on the hitting time of a quantum
walk. We obtain an upper bound for the perturbed quantum walk hitting time by
applying Szegedy's work and the perturbation bounds with Weyl's perturbation
theorem on classical matrix. Based on the definition of quantum hitting time
given in MNRS algorithm, we further compute the delayed perturbed hitting time
(DPHT) and delayed perturbed quantum hitting time (DPQHT). We show that the
upper bound for DPQHT is actually greater than the difference between the
square root of the upper bound for a perturbed random walk and the square root
of the lower bound for a random walk.Comment: 9 page
Computable bounds for the discrimination of Gaussian states
By combining the Minkowski inequality and the quantum Chernoff bound, we
derive easy-to-compute upper bounds for the error probability affecting the
optimal discrimination of Gaussian states. In particular, these bounds are
useful when the Gaussian states are unitarily inequivalent, i.e., they differ
in their symplectic invariants.Comment: 8 pages, 1 figure. REVTe
Graphical determination of pK values of the active-site groups of enzymes. An analysis of the bell-shaped curves
Information geometry of Gaussian channels
We define a local Riemannian metric tensor in the manifold of Gaussian
channels and the distance that it induces. We adopt an information-geometric
approach and define a metric derived from the Bures-Fisher metric for quantum
states. The resulting metric inherits several desirable properties from the
Bures-Fisher metric and is operationally motivated from distinguishability
considerations: It serves as an upper bound to the attainable quantum Fisher
information for the channel parameters using Gaussian states, under generic
constraints on the physically available resources. Our approach naturally
includes the use of entangled Gaussian probe states. We prove that the metric
enjoys some desirable properties like stability and covariance. As a byproduct,
we also obtain some general results in Gaussian channel estimation that are the
continuous-variable analogs of previously known results in finite dimensions.
We prove that optimal probe states are always pure and bounded in the number of
ancillary modes, even in the presence of constraints on the reduced state input
in the channel. This has experimental and computational implications: It limits
the complexity of optimal experimental setups for channel estimation and
reduces the computational requirements for the evaluation of the metric:
Indeed, we construct a converging algorithm for its computation. We provide
explicit formulae for computing the multiparametric quantum Fisher information
for dissipative channels probed with arbitrary Gaussian states, and provide the
optimal observables for the estimation of the channel parameters (e.g. bath
couplings, squeezing, and temperature).Comment: 19 pages, 4 figure
Small-scale dynamo in cool main sequence stars. II. The effect of metallicity
All cool main sequence stars including our Sun are thought to have magnetic
fields. Observations of the Sun revealed that even in quiet regions small-scale
turbulent magnetic fields are present. Simulations further showed that such
magnetic fields affect the subsurface and photospheric structure, and thus the
radiative transfer and emergent flux. Since small-scale turbulent magnetic
fields on other stars cannot be directly observed, it is imperative to study
their effects on the near surface layers numerically. Until recently
comprehensive three-dimensional simulations capturing the effect of small-scale
turbulent magnetic fields only exists for the solar case. A series of
investigations extending SSD simulations for other stars has been started. Here
we aim to examine small-scale turbulent magnetic fields in stars of solar
effective temperature but different metallicity. We investigate the properties
of three-dimensional simulations of the magneto-convection in boxes covering
the upper convection zone and photosphere carried out with the MURaM code for
metallicity values of with and without a
small-scale-dynamo. We find that small-scale turbulent magnetic fields enhanced
by a small-scale turbulent dynamo noticeably affect the subsurface dynamics and
significantly change the flow velocities in the photosphere. Moreover,
significantly stronger magnetic field strengths are present in the convection
zone for low metallicity. Whereas, at the optical surface the averaged vertical
magnetic field ranges from 64G for M/H = 0.5 to 85G for M/H = -1.0.Comment: 13 pages, 18 figures, submitted to A&
CHIANTI - an Atomic Database for Emission Lines. Paper VI: Proton Rates and Other Improvements
The CHIANTI atomic database contains atomic energy levels, wavelengths,
radiative transition probabilities and electron excitation data for a large
number of ions of astrophysical interest. Version 4 has been released, and
proton excitation data is now included, principally for ground configuration
levels that are close in energy. The fitting procedure for excitation data,
both electrons and protons, has been extended to allow 9 point spline fits in
addition to the previous 5 point spline fits. This allows higher quality fits
to data from close-coupling calculations where resonances can lead to
significant structure in the Maxwellian-averaged collision strengths. The
effects of photoexcitation and stimulated emission by a blackbody radiation
field in a spherical geometry on the level balance equations of the CHIANTI
ions can now be studied following modifications to the CHIANTI software. With
the addition of H I, He I and N I, the first neutral species have been added to
CHIANTI. Many updates to existing ion data-sets are described, while several
new ions have been added to the database, including Ar IV, Fe VI and Ni XXI.
The two-photon continuum is now included in the spectral synthesis routines,
and a new code for calculating the relativistic free-free continuum has been
added. The treatment of the free-bound continuum has also been updated.Comment: CHIANTI is available at http://wwwsolar.nrl.navy.mil/chianti.htm
Noncommutative effects in astrophysical objects: a survey
The main implications of noncommutativity over astrophysical objects are
examined. Noncommutativity is introduced through a deformed dispersion relation
and the relevant
thermodynamical quantities are calculated using the grand canonical ensemble
formalism. These results are applied to simple physical models describing
main-sequence stars, white-dwarfs and neutron stars. The stability of
main-sequence stars and white dwarfs is discussed.Comment: 10 pages. Talk presented by C. Z. at the "First Mediterranean
Conference on Classical and Quantum Gravity", Kolymbari (Crete, Greece),
September 14-18, 2009. To appear in the Proceeding
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