325 research outputs found
Asymmetric quantum cloning machines in any dimension
A family of asymmetric cloning machines for -dimensional quantum states is
introduced. These machines produce two imperfect copies of a single state that
emerge from two distinct Heisenberg channels. The tradeoff between the quality
of these copies is shown to result from a complementarity akin to Heisenberg
uncertainty principle. A no-cloning inequality is derived for isotropic
cloners: if and are the depolarizing fractions associated with
the two copies, the domain in -space located
inside a particular ellipse representing close-to-perfect cloning is forbidden.
More generally, a no-cloning uncertainty relation is discussed, quantifying the
impossibility of copying imposed by quantum mechanics. Finally, an asymmetric
Pauli cloning machine is defined that makes two approximate copies of a quantum
bit, while the input-to-output operation underlying each copy is a (distinct)
Pauli channel. The class of symmetric Pauli cloning machines is shown to
provide an upper bound on the quantum capacity of the Pauli channel of
probabilities , and .Comment: 18 pages RevTeX, 3 Postscript figures; new discussion on no-cloning
uncertainty relations, several corrections, added reference
Understanding entanglement as resource: locally distinguishing unextendible product bases
It is known that the states in an unextendible product basis (UPB) cannot be
distinguished perfectly when the parties are restricted to local operations and
classical communication (LOCC). Previous discussions of such bases have left
open the following question: What entanglement resources are necessary and/or
sufficient for this task to be possible with LOCC? In this paper, I present
protocols which use entanglement more efficiently than teleportation to
distinguish certain classes of UPB's. The ideas underlying my approach to this
problem offer rather general insight into why entanglement is useful for such
tasks.Comment: Final, published version. Many revisions following very useful
suggestions of the referee have been added. In particular, Appendix A has
been completely rewritte
Detection and prevention of financial abuse against elders
This article is made available through the Brunel Open Access Publishing Fund. Copyright @ The Authors. This article is published under the Creative Commons Attribution (CC BY 3.0) licence. Anyone
may reproduce, distribute, translate and create derivative works of this article (for both
commercial and non-commercial purposes), subject to full attribution to the original publication
and authors. The full terms of this licence may be seen at http://creativecommons.org/licences/
by/3.0/legalcode.Purpose â This paper reports on banking and finance professionals' decision making in the context of elder financial abuse. The aim was to identify the case features that influence when abuse is identified and when action is taken.
Design/methodology/approach â Banking and finance professionals (n=70) were shown 35 financial abuse case scenarios and were asked to judge how certain they were that the older person was being abused and the likelihood of taking action.
Findings â Three case features significantly influenced certainty of financial abuse: the nature of the financial problem presented, the older person's level of mental capacity and who was in charge of the client's money. In cases where the older person was more confused and forgetful, there was increased suspicion that financial abuse was taking place. Finance professionals were less certain that financial abuse was occurring if the older person was in charge of his or her own finances.
Originality/value â The research findings have been used to develop freely available online training resources to promote professionals' decision making capacity (www.elderfinancialabuse.co.uk). The resources have been advocated for use by Building Societies Association as well as CIFAS, the UK's Fraud Prevention Service.The research reported here was funded by the UK cross council New Dynamicsof Ageing Programme, ESRC Reference No. RES-352-25-0026, with Mary L.M. Gilhooly asPrincipal Investigator. Web-based training tools, developed from the research findings, weresubsequently funded by the ESRC follow-on fund ES/J001155/1 with Priscilla A. Harries asPrincipal Investigator
Singlet states and the estimation of eigenstates and eigenvalues of an unknown Controlled-U gate
We consider several problems that involve finding the eigenvalues and
generating the eigenstates of unknown unitary gates. We first examine
Controlled-U gates that act on qubits, and assume that we know the eigenvalues.
It is then shown how to use singlet states to produce qubits in the eigenstates
of the gate. We then remove the assumption that we know the eigenvalues and
show how to both find the eigenvalues and produce qubits in the eigenstates.
Finally, we look at the case where the unitary operator acts on qutrits and has
eigenvalues of 1 and -1, where the eigenvalue 1 is doubly degenerate. The
eigenstates are unknown. We are able to use a singlet state to produce a qutrit
in the eigenstate corresponding to the -1 eigenvalue.Comment: Latex, 10 pages, no figure
Optimal probabilistic cloning and purification of quantum states
We investigate the probabilistic cloning and purification of quantum states.
The performance of these probabilistic operations is quantified by the average
fidelity between the ideal and actual output states. We provide a simple
formula for the maximal achievable average fidelity and we explictly show how
to construct a probabilistic operation that achieves this fidelity. We
illustrate our method on several examples such as the phase covariant cloning
of qubits, cloning of coherent states, and purification of qubits transmitted
via depolarizing channel and amplitude damping channel. Our examples reveal
that the probabilistic cloner may yield higher fidelity than the best
deterministic cloner even when the states that should be cloned are linearly
dependent and are drawn from a continuous set.Comment: 9 pages, 2 figure
Several experimental realizations of symmetric phase-covariant quantum cloner of single-photon qubits
We compare several optical implementations of phase-covariant cloning
machines. The experiments are based on copying of the polarization state of a
single photon in bulk optics by special unbalanced beam splitter or by balanced
beam splitter accompanied by a state filtering. Also the all-fiber based setup
is discussed, where the information is encoded into spatial modes, i.e., the
photon can propagate through two optical fibers. Each of the four
implementations possesses some advantages and disadvantages that are discussed.Comment: 8 pages, 11 figure
A Topological Study of Contextuality and Modality in Quantum Mechanics
Kochen-Specker theorem rules out the non-contextual assignment of values to
physical magnitudes. Here we enrich the usual orthomodular structure of quantum
mechanical propositions with modal operators. This enlargement allows to refer
consistently to actual and possible properties of the system. By means of a
topological argument, more precisely in terms of the existence of sections of
sheaves, we give an extended version of Kochen-Specker theorem over this new
structure. This allows us to prove that contextuality remains a central feature
even in the enriched propositional system.Comment: 10 pages, no figures, submitted to I. J. Th. Phy
Experimental reversion of the optimal quantum cloning and flipping processes
The quantum cloner machine maps an unknown arbitrary input qubit into two
optimal clones and one optimal flipped qubit. By combining linear and
non-linear optical methods we experimentally implement a scheme that, after the
cloning transformation, restores the original input qubit in one of the output
channels, by using local measurements, classical communication and feedforward.
This significant teleportation-like method demonstrates how the information is
preserved during the cloning process. The realization of the reversion process
is expected to find useful applications in the field of modern multi-partite
quantum cryptography.Comment: 10 pages, 3 figure
Strategies and Networks for State-Dependent Quantum Cloning
State-dependent cloning machines that have so far been considered either
deterministically copy a set of states approximately, or probablistically copy
them exactly. In considering the case of two equiprobable pure states, we
derive the maximum global fidelity of approximate clones given initial
exact copies, where . We also consider strategies which interpolate
between approximate and exact cloning. A tight inequality is obtained which
expresses a trade-off between the global fidelity and success probability. This
inequality is found to tend, in the limit as , to a known
inequality which expresses the trade-off between error and inconclusive result
probabilities for state-discrimination measurements. Quantum-computational
networks are also constructed for the kinds of cloning machine we describe. For
this purpose, we introduce two gates: the distinguishability transfer and state
separation gates. Their key properties are describedComment: 12 pages, 6 eps figures, submitted to Phys. Rev.
Helstrom Theorem by No-Signaling Condition
We prove a special case of Helstrom theorem by using no-signaling condition
in the special theory of relativity that faster-than-light communication is
impossible.Comment: Minor corrections (A reference added, discussion part deleted, typos
in equations corrected), 2 pages, RevTe
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