208 research outputs found
Transfer of quantum states using finite resources
We discuss the problem of transfering a qubit from Alice to Bob using a noisy
quantum channel and only finite resources. As the basic protocol for the
transfer we apply quantum teleportation. It turns out that for a certain
quality of the channel direct teleportation combined with qubit purification is
superior to entanglement purification of the channel. If, however, the quality
of the channel is rather low one should simply apply an estimation-preparation
scheme.Comment: 9 pages RevTeX including 5 figures, replaced with revised version, to
appear in Phys. Rev.
Interaction-free generation of entanglement
In this paper, we study how to generate entanglement by interaction-free
measurement. Using Kwiat et al.'s interferometer, we construct a two-qubit
quantum gate that changes a particle's trajectory according to the other
particle's trajectory. We propose methods for generating the Bell state from an
electron and a positron and from a pair of photons by this gate. We also show
that using this gate, we can carry out the Bell measurement with the
probability of 3/4 at the maximum and execute a controlled-NOT operation by the
method proposed by Gottesman and Chuang with the probability of 9/16 at the
maximum. We estimate the success probability for generating the Bell state by
our procedure under imperfect interaction.Comment: 18 pages, Latex2e, 11 eps figures, v2: minor corrections and one
reference added, v3: a minor correctio
Optimization approach to entanglement distillation
We put forward a method for optimized distillation of partly entangled pairs
of qubits into a smaller number of more entangled pairs by recurrent local
unitary operations and projections. Optimized distillation is achieved by
minimization of a cost function with up to 30 real parameters, which is chosen
to be sensitive to the fidelity and the projection probability at each step. We
show that in many cases this approach can significantly improve the
distillation efficiency in comparison to the present methods.Comment: 6 pages, 5 figures, REVTEX. New examples, figures and references,
more detailed explanations. To appear in Phys. Rev.
Experimental preparation of Werner state via spontaneous parametric down-conversion
We present an experiment of preparing Werner state via spontaneous parametric
down-conversion and controlled decoherence of photons in this paper. In this
experiment two independent BBO (beta-barium borate) crystals are used to
produce down-conversion light beams, which are mixed to prepare Werner state.Comment: 6 pages, 4 figures and 2 table
Dynamical description of quantum computing: generic nonlocality of quantum noise
We develop dynamical non-Markovian description of quantum computing in weak
coupling limit, in lowest order approximation. We show that long range memory
of quantum reservoir produces strong interrelation between structure of noise
and quantum algorithm, implying nonlocal attacks of noise. We then argue that
the quantum error correction method fails to protect quantum computation
against electromagnetic or phonon vacuum which exhibit memory. This
shows that the implicit assumption of quantum error correction theory --
independence of noise and self-dynamics -- fails in long time regimes. We also
use our approach to present {\it pure} decoherence and decoherence accompanied
by dissipation in terms of spectral density of reservoir. The so-called {\it
dynamical decoupling} method is discussed in this context. Finally, we propose
{\it minimal decoherence model}, in which the only source of decoherence is
vacuum. We optimize fidelity of quantum information processing under the
trade-off between speed of gate and strength of decoherence.Comment: 12 pages, minor corrections, softened interpretation of the result
A Geometric Picture of Entanglement and Bell Inequalities
We work in the real Hilbert space H_s of hermitian Hilbert-Schmid operators
and show that the entanglement witness which shows the maximal violation of a
generalized Bell inequality (GBI) is a tangent functional to the convex set S
subset H_s of separable states. This violation equals the euclidean distance in
H_s of the entangled state to S and thus entanglement, GBI and tangent
functional are only different aspects of the same geometric picture. This is
explicitly illustrated in the example of two spins, where also a comparison
with familiar Bell inequalities is presented.Comment: 17 pages, 5 figures, 4 references adde
Separability and entanglement in 2x3xN composite quantum systems
The separability and entanglement of quantum mixed states in \Cb^2 \otimes
\Cb^3 \otimes \Cb^N composite quantum systems are investigated. It is shown
that all quantum states with positive partial transposes and rank
are separable.Comment: Latex, 15 page
Quantum computing with mixed states
We discuss a model for quantum computing with initially mixed states.
Although such a computer is known to be less powerful than a quantum computer
operating with pure (entangled) states, it may efficiently solve some problems
for which no efficient classical algorithms are known. We suggest a new
implementation of quantum computation with initially mixed states in which an
algorithm realization is achieved by means of optimal basis independent
transformations of qubits.Comment: 2 figures, 52 reference
A Quantum-mechanical Approach for Constrained Macromolecular Chains
Many approaches to three-dimensional constrained macromolecular chains at
thermal equilibrium, at about room temperatures, are based upon constrained
Classical Hamiltonian Dynamics (cCHDa). Quantum-mechanical approaches (QMa)
have also been treated by different researchers for decades. QMa address a
fundamental issue (constraints versus the uncertainty principle) and are
versatile: they also yield classical descriptions (which may not coincide with
those from cCHDa, although they may agree for certain relevant quantities).
Open issues include whether QMa have enough practical consequences which differ
from and/or improve those from cCHDa. We shall treat cCHDa briefly and deal
with QMa, by outlining old approaches and focusing on recent ones.Comment: Expands review published in The European Physical Journal (Special
Topics) Vol. 200, pp. 225-258 (2011
Utilizing image texture to detect land-cover change in Mediterranean coastal wetlands
Land-use/cover change dynamics were investigated in a Mediterranean coastal wetland. Change Vector Analysis (CVA) without and with image texture derived from the co-occurrence matrix and variogram were evaluated for detecting land-use/cover change. Three Landsat Thematic Mapper (TM) scenes recorded on July 1985, 1993 and 2005 were used, minimizing change detection error caused by seasonal differences. Images were geometrically, atmospherically and radiometrically corrected. CVA without and with texture measures were implemented and assessed using reference images generated by object-based supervised classification. These outputs were used for cross-classification to determine the ‘from–to’ change used to compare between techniques. The Landsat TM image bands together with the variogram yielded the most accurate change detection results, with Kappa statistics of 0.7619 and 0.7637 for the 1985–1993 and 1993–2005 image pairs, respectively
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