57,286 research outputs found

### A Method of Areas for Manipulating the Entanglement Properties of One Copy of a Two-Particle Pure State

We consider the problem of how to manipulate the entanglement properties of a
general two-particle pure state, shared between Alice and Bob, by using only
local operations at each end and classical communication between Alice and Bob.
A method is developed in which this type of problem is found to be equivalent
to a problem involving the cutting and pasting of certain shapes along with a
certain colouring problem. We consider two problems. Firstly we find the most
general way of manipulating the state to obtain maximally entangled states.
After such a manipulation the entangled state |11>+|22>+....|mm> is obtained
with probability p_m. We obtain an expression for the optimal average
entanglement. Also, some results of Lo and Popescu pertaining to this problem
are given simple geometric proofs. Secondly, we consider how to manipulate one
two particle entangled pure state to another with certainty. We derive
Nielsen's theorem (which states the necessary and sufficient condition for this
to be possible) using the method of areas.Comment: 29 pages, 9 figures. Section 2.4 clarified. Error in second colouring
theorem (section 3.2) corrected. Some other minor change

### Fault tolerant quantum key distribution protocol with collective random unitary noise

We propose an easy implementable prepare-and-measure protocol for robust
quantum key distribution with photon polarization. The protocol is fault
tolerant against collective random unitary channel noise. The protocol does not
need any collective quantum measurement or quantum memory. A security proof and
a specific linear optical realization using spontaneous parametric down
conversion are given.Comment: Accepted by PRA as a Rapid Communicatio

### Eavesdropping without quantum memory

In quantum cryptography the optimal eavesdropping strategy requires that the
eavesdropper uses quantum memories in order to optimize her information. What
happens if the eavesdropper has no quantum memory? It is shown that the best
strategy is actually to adopt the simple intercept/resend strategy.Comment: 9 pages LaTeX, 3 figure

### Broadcasting of three qubit entanglement via local copying and entanglement swapping

In this work,We investigate the problem of secretly broadcasting of
three-qubit entangled state between two distant partners. The interesting
feature of this problem is that starting from two particle entangled state
shared between two distant partners we find that the action of local cloner on
the qubits and the measurement on the machine state vector generates
three-qubit entanglement between them. The broadcasting of entanglement is made
secret by sending the measurement result secretly using cryptographic scheme
based on orthogonal states. Further we show that this idea can be extended to
generate three particle entangled state between three distant partners.Comment: 18 pages, 4 figures, Accepted in Physical Review

### Entangled Mixed States and Local Purification

Linden, Massar and Popescu have recently given an optimization argument to
show that a single two-qubit Werner state, or any other mixture of the
maximally entangled Bell states, cannot be purified by local operations and
classical communications. We generalise their result and give a simple
explanation. In particular, we show that no purification scheme using local
operations and classical communications can produce a pure singlet from any
mixed state of two spin-1/2 particles. More generally, no such scheme can
produce a maximally entangled state of any pair of finite-dimensional systems
from a generic mixed state. We also show that the Werner states belong to a
large class of states whose fidelity cannot be increased by such a scheme.Comment: 3 pages, Latex with Revtex. Small clarifications and reference adde

### When only two thirds of the entanglement can be distilled

We provide an example of distillable bipartite mixed state such that, even in
the asymptotic limit, more pure-state entanglement is required to create it
than can be distilled from it. Thus, we show that the irreversibility in the
processes of formation and distillation of bipartite states, recently proved in
[G. Vidal, J.I. Cirac, Phys. Rev. Lett. 86, (2001) 5803-5806], is not limited
to bound-entangled states.Comment: 4 pages, revtex, 1 figur

### Cognitive context and arguments from ontologies for learning

The deployment of learning resources on the web by different experts has resulted in the accessibility of multiple viewpoints about the same topics. In
this work we assume that learning resources are underpinned by ontologies. Different formalizations of domains may result from different contexts, different use of
terminology, incomplete knowledge or conflicting knowledge. We define the notion of cognitive learning context which describes the cognitive context of an agent who refers to multiple and possibly inconsistent ontologies to determine the truth of a proposition. In particular we describe the cognitive states of ambiguity and inconsistency
resulting from incomplete and conflicting ontologies respectively. Conflicts between ontologies can be identified through the derivation of conflicting arguments
about a particular point of view. Arguments can be used to detect inconsistencies between ontologies. They can also be used in a dialogue between a human learner and a software tutor in order to enable the learner to justify her views and detect inconsistencies between her beliefs and the tutor’s own. Two types of arguments are discussed, namely: arguments inferred directly from taxonomic relations
between concepts, and arguments about the necessary an

### The Parity Bit in Quantum Cryptography

An $n$-bit string is encoded as a sequence of non-orthogonal quantum states.
The parity bit of that $n$-bit string is described by one of two density
matrices, $\rho_0^{(n)}$ and $\rho_1^{(n)}$, both in a Hilbert space of
dimension $2^n$. In order to derive the parity bit the receiver must
distinguish between the two density matrices, e.g., in terms of optimal mutual
information. In this paper we find the measurement which provides the optimal
mutual information about the parity bit and calculate that information. We
prove that this information decreases exponentially with the length of the
string in the case where the single bit states are almost fully overlapping. We
believe this result will be useful in proving the ultimate security of quantum
crytography in the presence of noise.Comment: 19 pages, RevTe

### The influence of large-scale motion on turbulent transport for confined coaxial jets

The existence of large-scale coherent structures in turbulent shear flows has been well documented in the literature. The importance of these structures in flow entrainment, momentum transport and mass transport in the shear layer has been suggested by several researchers. Comparisons between existing models and experimental data for shear flow in confined coaxial jets reinforce the necessity of further investigation of the large scale structures. These comparisons show the greatest discrepancy between prediction and actual results in the developing flow region where the large scales exist. It was also observed that the momentum transport rate comparisons were very bad. Finally, Schetz has reviewed mixing flows and concluded that large-scale structures were essential aspects of future modeling efforts

### Entanglement Swapping Chains for General Pure States

We consider entanglement swapping schemes with general (rather than
maximally) entangled bipartite states of arbitary dimension shared pairwise
between three or more parties in a chain. The intermediate parties perform
generalised Bell measurements with the result that the two end parties end up
sharing a entangled state which can be converted into maximally entangled
states. We obtain an expression for the average amount of maximal entanglement
concentrated in such a scheme and show that in a certain reasonably broad class
of cases this scheme is provably optimal and that, in these cases, the amount
of entanglement concentrated between the two ends is equal to that which could
be concentrated from the weakest link in the chain.Comment: 18 pages, 5 figure

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