389 research outputs found
Beating the One-half Limit of Ancilla-free Linear Optics Bell Measurements
We show that optically encoded two-qubit Bell states can be unambiguously
discriminated with a success probability of more than 50% in both single-rail
and dual-rail encodings by using active linear-optical resources that include
Gaussian squeezing operations. These results are in contrast to the well-known
upper bound of 50% for unambiguous discrimination of dual-rail Bell states
using passive, static linear optics and arbitrarily many vacuum modes. We
present experimentally feasible schemes that improve the success probability to
64.3% in dual-rail and to 62.5% in single-rail for a uniform random
distribution of Bell states. Conceptually, this demonstrates that neither
interactions that induce nonlinear mode transformations (such as Kerr
interactions) nor auxiliary entangled photons are required to go beyond the
one-half limit. We discuss the optimality of our single-rail scheme, and talk
about an application of our dual-rail scheme in quantum communication.Comment: 4+5 pages, 4 figure
Local Gaussian operations can enhance continuous-variable entanglement distillation
Entanglement distillation is a fundamental building block in long-distance
quantum communication. Though known to be useless on their own for distilling
Gaussian entangled states, local Gaussian operations may still help to improve
non-Gaussian entanglement distillation schemes. Here we show that by applying
local squeezing operations, both the performance and the efficiency of existing
distillation protocols can be enhanced. We derive the optimal enhancement
through local Gaussian unitaries, which can be obtained even in the most
natural scenario when Gaussian mixed entangled states are shared after their
distribution through a lossy-fiber communication channel.Comment: 4 figure
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
Quantum versus classical domains for teleportation with continuous variables
By considering the utilization of a classical channel without quantum entanglement, fidelity Fclassical=1/2 has been established as setting the boundary between classical and quantum domains in the teleportation of coherent states of the electromagnetic field [S. L. Braunstein, C. A. Fuchs, and H. J. Kimble, J. Mod. Opt. 47, 267 (2000)]. We further examine the quantum-classical boundary by investigating questions of entanglement and Bell-inequality violations for the Einstein-Podolsky-Rosen states relevant to continuous variable teleportation. The threshold fidelity for employing entanglement as a quantum resource in teleportation of coherent states is again found to be Fclassical=1/2. Likewise, violations of local realism onset at this same threshold, with the added requirement of overall efficiency η>2/3 in the unconditional case. By contrast, recently proposed criteria adapted from the literature on quantum-nondemolition detection are shown to be largely unrelated to the questions of entanglement and Bell-inequality violations
Distillation of mixed-state continuous-variable entanglement by photon subtraction
We present a detailed theoretical analysis for the distillation of one copy
of a mixed two-mode continuous-variable entangled state using beamsplitters and
coherent photon-detection techniques, including conventional on-off detectors
and photon number resolving detectors. The initial Gaussian mixed-entangled
states are generated by transmitting a two-mode squeezed state through a lossy
bosonic channel, corresponding to the primary source of errors in current
approaches to optical quantum communication. We provide explicit formulas to
calculate the entanglement in terms of logarithmic negativity before and after
distillation, including losses in the channel and the photon detection, and
show that one-copy distillation is still possible even for losses near the
typical fiber channel attenuation length. A lower bound for the transmission
coefficient of the photon-subtraction beamsplitter is derived, representing the
minimal value that still allows to enhance the entanglement.Comment: 13 pages, 8 figure
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
Die verband tussen die blootstelling aan en gebruik van elektroniese media en akademiese taalvaardigheid
The focus of this article is the possible contribution of exposure to and use of modern electronic media to the problem of students’ perceived weak command of formal academic language. Concepts such as academic literacy, the relationship between language and thought, and the effect of modern media on thought patterns will be discussed. The frequent exposure to images on the one hand and the absence of formal reading material on the other, are seen as important factors contributing to language problems experienced by university students. A questionnaire was used to examine the frequency at which grade 8 learners in four Bloemfontein schools, as well as a group of second year students at the University of the Free State, use computers and cell phones, watch television and films, etc. User patterns were established. The results of this research are discussed here.Keywords: Academic language proficiency, literacy, Web generation, social context, technology and media, interactive learning environment, language and thought, patterns of thought, image thinkingSleutelwoorde: akademiese taalvaardigheid, geletterdheid, Net-generasie, sosiale konteks, tegnologie en media, interaktiewe leeromgewing, taal en denke, denkpatrone, beelddenke.The article is in Afrikaans
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
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