104 research outputs found
Physical accessibility of non-completely positive maps
Maps that are not completely positive (CP) are often useful to describe the
dynamics of open systems. An apparent violation of complete positivity can
occur because there are prior correlations of the principal system with the
environment, or if the applied transformation is correlated with the state of
the system. We provide a physically motivated definition of accessible non-CP
maps and derive two necessary conditions for a map to be accessible. We also
show that entanglement between the system and the environment is not necessary
to generate a non-CP dynamics. We describe two simple approximations that may
be sufficient for some problems in process tomography, and then outline what
these methods may be able to tell us in other situations where non-CP dynamics
naturally arise.Comment: 8 pages, 1 eps figure. Final versio
Evanescence in Coined Quantum Walks
In this paper we complete the analysis begun by two of the authors in a
previous work on the discrete quantum walk on the line [J. Phys. A 36:8775-8795
(2003) quant-ph/0303105 ]. We obtain uniformly convergent asymptotics for the
"exponential decay'' regions at the leading edges of the main peaks in the
Schr{\"o}dinger (or wave-mechanics) picture. This calculation required us to
generalise the method of stationary phase and we describe this extension in
some detail, including self-contained proofs of all the technical lemmas
required. We also rigorously establish the exact Feynman equivalence between
the path-integral and wave-mechanics representations for this system using some
techniques from the theory of special functions. Taken together with the
previous work, we can now prove every theorem by both routes.Comment: 32 pages AMS LaTeX, 5 figures in .eps format. Rewritten in response
to referee comments, including some additional references. v3: typos fixed in
equations (131), (133) and (134). v5: published versio
Maximum Power Efficiency and Criticality in Random Boolean Networks
Random Boolean networks are models of disordered causal systems that can
occur in cells and the biosphere. These are open thermodynamic systems
exhibiting a flow of energy that is dissipated at a finite rate. Life does work
to acquire more energy, then uses the available energy it has gained to perform
more work. It is plausible that natural selection has optimized many biological
systems for power efficiency: useful power generated per unit fuel. In this
letter we begin to investigate these questions for random Boolean networks
using Landauer's erasure principle, which defines a minimum entropy cost for
bit erasure. We show that critical Boolean networks maximize available power
efficiency, which requires that the system have a finite displacement from
equilibrium. Our initial results may extend to more realistic models for cells
and ecosystems.Comment: 4 pages RevTeX, 1 figure in .eps format. Comments welcome, v2: minor
clarifications added, conclusions unchanged. v3: paper rewritten to clarify
it; conclusions unchange
The meeting problem in the quantum random walk
We study the motion of two non-interacting quantum particles performing a
random walk on a line and analyze the probability that the two particles are
detected at a particular position after a certain number of steps (meeting
problem). The results are compared to the corresponding classical problem and
differences are pointed out. Analytic formulas for the meeting probability and
its asymptotic behavior are derived. The decay of the meeting probability for
distinguishable particles is faster then in the classical case, but not
quadratically faster. Entangled initial states and the bosonic or fermionic
nature of the walkers are considered
Decomposition of pure states of quantum register
The generalization of Schmidt decomposition due to
Cartelet-Higuchi-Sudbery applied to quantum register (a system of N
qubits) is shown to acquire direct geometrical meaning: any pure
state is canonically associated with a chain of a simplicial
complex. A leading vector method is presented to calculate the
values of the coefficients of appropriate chain
Implementation of NMR quantum computation with para-hydrogen derived high purity quantum states
We demonstrate the first implementation of a quantum algorithm on a liquid
state nuclear magnetic resonance (NMR) quantum computer using almost pure
states. This was achieved using a two qubit device where the initial state is
an almost pure singlet nuclear spin state of a pair of 1H nuclei arising from a
chemical reaction involving para-hydrogen. We have implemented Deutsch's
algorithm for distinguishing between constant and balanced functions with a
single query.Comment: 7 pages RevTex including 6 figures. Figures 4-6 are low quality to
save space. Submitted to Phys Rev
Pairwise entanglement in the XX model with a magnetic impurity
For a 3-qubit Heisenberg model in a uniform magnetic field, the pairwise
thermal entanglement of any two sites is identical due to the exchange symmetry
of sites. In this paper we consider the effect of a non-uniform magnetic field
on the Heisenberg model, modeling a magnetic impurity on one site. Since
pairwise entanglement is calculated by tracing out one of the three sites, the
entanglement clearly depends on which site the impurity is located. When the
impurity is located on the site which is traced out, that is, when it acts as
an external field of the pair, the entanglement can be enhanced to the maximal
value 1; while when the field acts on a site of the pair the corresponding
concurrence can only be increased from 1/3 to 2/3.Comment: 9 Pages, 4 EPS figures, LaTeX 2
Three-tangle for mixtures of generalized GHZ and generalized W states
We give a complete solution for the three-tangle of mixed three-qubit states
composed of a generalized GHZ state, a|000>+b|111>, and a generalized W state,
c|001>+d|010>+f|100>. Using the methods introduced by Lohmayer et al. we
provide explicit expressions for the mixed-state three-tangle and the
corresponding optimal decompositions for this more general case. Moreover, as a
special case we obtain a general solution for a family of states consisting of
a generalized GHZ state and an orthogonal product state
Properties of Entanglement Monotones for Three-Qubit Pure States
Various parameterizations for the orbits under local unitary transformations
of three-qubit pure states are analyzed. The interconvertibility, symmetry
properties, parameter ranges, calculability and behavior under measurement are
looked at. It is shown that the entanglement monotones of any multipartite pure
state uniquely determine the orbit of that state under local unitary
transformations. It follows that there must be an entanglement monotone for
three-qubit pure states which depends on the Kempe invariant defined in [Phys.
Rev. A 60, 910 (1999)]. A form for such an entanglement monotone is proposed. A
theorem is proved that significantly reduces the number of entanglement
monotones that must be looked at to find the maximal probability of
transforming one multipartite state to another.Comment: 14 pages, REVTe
Constraint on teleportation over multipartite pure states
We first define a quantity exhibiting the usefulness of bipartite quantum
states for teleportation, called the quantum teleportation capability, and then
investigate its restricted shareability in multi-party quantum systems. In this
work, we verify that the quantum teleportation capability has a monogamous
property in its shareability for arbitrary three-qutrit pure states by
employing the monogamy inequality in terms of the negativity.Comment: 4 pages, 1 figur
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