412 research outputs found
Teleportation is necessary for faithful quantum state transfer through noisy channels of maximal rank
Quantum teleportation enables deterministic and faithful transmission of
quantum states, provided a maximally entangled state is pre-shared between
sender and receiver, and a one-way classical channel is available. Here, we
prove that these resources are not only sufficient, but also necessary, for
deterministically and faithfully sending quantum states through any fixed noisy
channel of maximal rank, when a single use of the cannel is admitted. In other
words, for this family of channels, there are no other protocols, based on
different (and possibly cheaper) sets of resources, capable of replacing
quantum teleportation.Comment: 4 pages, comments are welcom
Object-oriented development: knowledge base support for design
The research is conducted in the area of Software Engineering, with emphasis on the design phase of the Software Development Life Cycle (SDLC). The object-oriented paradigm is the point of departure. The investigation deals with the problem of creating support for the design phase of object-oriented system development. This support must be able to guide the system designer through the design process, according to a sound design method, highlight opportunities for prototyping and point out where to re-iterate a design step, for example. A solution is proposed in the form of a knowledge-based support system; In the prototype this support guides a designer partially through the first step of the
System Design task for object-oriented design. The intention is that the knowledge-based system should capture the know-how of an expert system designer and assist an inexperienced system designer to create good designs
Building Gaussian Cluster States by Linear Optics
The linear optical creation of Gaussian cluster states, a potential resource
for universal quantum computation, is investigated. We show that for any
Gaussian cluster state, the canonical generation scheme in terms of QND-type
interactions, can be entirely replaced by off-line squeezers and beam
splitters. Moreover, we find that, in terms of squeezing resources, the
canonical states are rather wasteful and we propose a systematic way to create
cheaper states. As an application, we consider Gaussian cluster computation in
multiple-rail encoding. This encoding may reduce errors due to finite
squeezing, even when the extra rails are achieved through off-line squeezing
and linear optics.Comment: 5 Pages, 3 figure
Rate analysis for a hybrid quantum repeater
We present a detailed rate analysis for a hybrid quantum repeater assuming
perfect memories and using optimal probabilistic entanglement generation and
deterministic swapping routines. The hybrid quantum repeater protocol is based
on atomic qubit-entanglement distribution through optical coherent-state
communication. An exact, analytical formula for the rates of entanglement
generation in quantum repeaters is derived, including a study on the impacts of
entanglement purification and multiplexing strategies. More specifically, we
consider scenarios with as little purification as possible and we show that for
sufficiently low local losses, such purifications are still more powerful than
multiplexing. In a possible experimental scenario, our hybrid system can create
near-maximally entangled (F = 0.98) pairs over a distance of 1280 km at rates
of the order of 100 Hz
Detecting genuine multipartite continuous-variable entanglement
We derive necessary conditions in terms of the variances of position and
momentum linear combinations for all kinds of separability of a multi-party
multi-mode continuous-variable state. Their violations can be sufficient for
genuine multipartite entanglement, provided the combinations contain both
conjugate variables of all modes. Hence a complete state determination, for
example by detecting the entire correlation matrix of a Gaussian state, is not
needed.Comment: 13 pages, 3 figure
Experiment towards continuous-variable entanglement swapping: Highly correlated four-partite quantum state
We present a protocol for performing entanglement swapping with intense
pulsed beams. In a first step, the generation of amplitude correlations between
two systems that have never interacted directly is demonstrated. This is
verified in direct detection with electronic modulation of the detected
photocurrents. The measured correlations are better than expected from a
classical reconstruction scheme. In the entanglement swapping process, a
four--partite entangled state is generated. We prove experimentally that the
amplitudes of the four optical modes are quantum correlated 3 dB below shot
noise, which is due to the potential four--party entanglement.Comment: 9 pages, 10 figures, update of references 9 and 10; minor
inconsistency in notation removed; format for units in the figures change
Entanglement properties of optical coherent states under amplitude damping
Through concurrence, we characterize the entanglement properties of optical
coherent-state qubits subject to an amplitude damping channel. We investigate
the distillation capabilities of known error correcting codes and obtain upper
bounds on the entanglement depending on the non-orthogonality of the coherent
states and the channel damping parameter. This work provides a first, full
quantitative analysis of these photon-loss codes which are naturally
reminiscent of the standard qubit codes against Pauli errors.Comment: 7 pages, 6 figures. Revised version with small corrections; main
results remain unaltere
Greenberger-Horne-Zeilinger paradox for continuous variables
We show how to construct states for which a Greenberger-Horne-Zeilinger type
paradox occurs if each party measures either the position or momentum of his
particle. The paradox can be ascribed to the anticommutation of certain
translation operators in phase space. We then rephrase the paradox in terms of
modular and binary variables. The origin of the paradox is then due to the fact
that the associativity of addition of modular variables is true only for
c-numbers but does not hold for operators.Comment: 4 pages, no figure
Entanglement, Purity, and Information Entropies in Continuous Variable Systems
Quantum entanglement of pure states of a bipartite system is defined as the
amount of local or marginal ({\em i.e.}referring to the subsystems) entropy.
For mixed states this identification vanishes, since the global loss of
information about the state makes it impossible to distinguish between quantum
and classical correlations. Here we show how the joint knowledge of the global
and marginal degrees of information of a quantum state, quantified by the
purities or in general by information entropies, provides an accurate
characterization of its entanglement. In particular, for Gaussian states of
continuous variable systems, we classify the entanglement of two--mode states
according to their degree of total and partial mixedness, comparing the
different roles played by the purity and the generalized entropies in
quantifying the mixedness and bounding the entanglement. We prove the existence
of strict upper and lower bounds on the entanglement and the existence of
extremally (maximally and minimally) entangled states at fixed global and
marginal degrees of information. This results allow for a powerful, operative
method to measure mixed-state entanglement without the full tomographic
reconstruction of the state. Finally, we briefly discuss the ongoing extension
of our analysis to the quantification of multipartite entanglement in highly
symmetric Gaussian states of arbitrary -mode partitions.Comment: 16 pages, 5 low-res figures, OSID style. Presented at the
International Conference ``Entanglement, Information and Noise'', Krzyzowa,
Poland, June 14--20, 200
Graphical calculus for Gaussian pure states
We provide a unified graphical calculus for all Gaussian pure states,
including graph transformation rules for all local and semi-local Gaussian
unitary operations, as well as local quadrature measurements. We then use this
graphical calculus to analyze continuous-variable (CV) cluster states, the
essential resource for one-way quantum computing with CV systems. Current
graphical approaches to CV cluster states are only valid in the unphysical
limit of infinite squeezing, and the associated graph transformation rules only
apply when the initial and final states are of this form. Our formalism applies
to all Gaussian pure states and subsumes these rules in a natural way. In
addition, the term "CV graph state" currently has several inequivalent
definitions in use. Using this formalism we provide a single unifying
definition that encompasses all of them. We provide many examples of how the
formalism may be used in the context of CV cluster states: defining the
"closest" CV cluster state to a given Gaussian pure state and quantifying the
error in the approximation due to finite squeezing; analyzing the optimality of
certain methods of generating CV cluster states; drawing connections between
this new graphical formalism and bosonic Hamiltonians with Gaussian ground
states, including those useful for CV one-way quantum computing; and deriving a
graphical measure of bipartite entanglement for certain classes of CV cluster
states. We mention other possible applications of this formalism and conclude
with a brief note on fault tolerance in CV one-way quantum computing.Comment: (v3) shortened title, very minor corrections (v2) minor corrections,
reference added, new figures for CZ gate and beamsplitter graph rules; (v1)
25 pages, 11 figures (made with TikZ
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