61 research outputs found
Iterative Entanglement Distillation: Approaching full Elimination of Decoherence
The distribution and processing of quantum entanglement form the basis of
quantum communication and quantum computing. The realization of the two is
difficult because quantum information inherently has a high susceptibility to
decoherence, i.e. to uncontrollable information loss to the environment. For
entanglement distribution, a proposed solution to this problem is capable of
fully eliminating decoherence; namely iterative entanglement distillation. This
approach builds on a large number of distillation steps each of which extracts
a number of weakly decohered entangled states from a larger number of strongly
decohered states. Here, for the first time, we experimentally demonstrate
iterative distillation of entanglement. Already distilled entangled states were
further improved in a second distillation step and also made available for
subsequent steps.Our experiment displays the realization of the building blocks
required for an entanglement distillation scheme that can fully eliminate
decoherence
Multiple copy distillation and purification of phase diffused squeezed states
We provide a detailed theoretical analysis of multiple copy purification and
distillation protocols for phase diffused squeezed states of light. The
standard iterative distillation protocol is generalized to a collective
purification of an arbitrary number of N copies. We also derive a
semi-analytical expression for the asymptotic limit of the iterative
distillation and purification protocol and discuss its properties.Comment: 11 pages, 13 figure
Comparing human and automatic thesaurus mapping approaches in the agricultural domain
Knowledge organization systems (KOS), like thesauri and other controlled
vocabularies, are used to provide subject access to information systems across
the web. Due to the heterogeneity of these systems, mapping between
vocabularies becomes crucial for retrieving relevant information. However,
mapping thesauri is a laborious task, and thus big efforts are being made to
automate the mapping process. This paper examines two mapping approaches
involving the agricultural thesaurus AGROVOC, one machine-created and one human
created. We are addressing the basic question "What are the pros and cons of
human and automatic mapping and how can they complement each other?" By
pointing out the difficulties in specific cases or groups of cases and grouping
the sample into simple and difficult types of mappings, we show the limitations
of current automatic methods and come up with some basic recommendations on
what approach to use when.Comment: 10 pages, Int'l Conf. on Dublin Core and Metadata Applications 200
Towards Einstein-Podolsky-Rosen quantum channel multiplexing
A single broadband squeezed field constitutes a quantum communication
resource that is sufficient for the realization of a large number N of quantum
channels based on distributed Einstein-Podolsky-Rosen (EPR) entangled states.
Each channel can serve as a resource for, e.g. independent quantum key
distribution or teleportation protocols. N-fold channel multiplexing can be
realized by accessing 2N squeezed modes at different Fourier frequencies. We
report on the experimental implementation of the N=1 case through the
interference of two squeezed states, extracted from a single broadband squeezed
field, and demonstrate all techniques required for multiplexing (N>1). Quantum
channel frequency multiplexing can be used to optimize the exploitation of a
broadband squeezed field in a quantum information task. For instance, it is
useful if the bandwidth of the squeezed field is larger than the bandwidth of
the homodyne detectors. This is currently a typical situation in many
experiments with squeezed and two-mode squeezed entangled light.Comment: 4 pages, 4 figures. In the new version we cite recent experimental
work bei Mehmet et al., arxiv0909.5386, in order to clarify the motivation of
our work and its possible applicatio
Programmable Multimode Quantum Networks
Entanglement between large numbers of quantum modes is the quintessential
resource for future technologies such as the quantum internet. Conventionally
the generation of multimode entanglement in optics requires complex layouts of
beam-splitters and phase shifters in order to transform the input modes in to
entangled modes. These networks need substantial modification for every new set
of entangled modes to be generated. Here we report on the highly versatile and
efficient generation of various multimode entangled states with the ability to
switch between different linear optics networks in real time. By defining our
modes to be combinations of different spatial regions of one beam, we may use
just one pair of multi-pixel detectors each with M photodiodes in order to
measure N entangled modes, with a maximum number of N=M modes. We program
virtual networks that are fully equivalent to the physical linear optics
networks they are emulating. We present results for N=2 up to N=8 entangled
modes here, including N=2,3,4 cluster states. Our approach introduces
flexibility and scalability to multimode entanglement, two important attributes
that are highly sought after in state of the art devices.Comment: 10 pages, 5 figures, 2 tables, comments welcome
Experimental characterization of Gaussian quantum communication channels
We present a full experimental characterization of continuous variable
quantum communication channels established by shared entanglement together with
local operations and classical communication. The resulting teleportation
channel was fully characterized by measuring all elements of the covariance
matrix of the shared two-mode squeezed Gaussian state. From the experimental
data we determined the lower bound to the quantum channel capacity, the
teleportation fidelity of coherent states and the logarithmic negativity and
the purity of the shared state. Additionally, a positive secret key rate was
obtained for two of the established channels.Comment: 9 pages, 4 figures, submitted to Physical Review
Observation of squeezed light with 10dB quantum noise reduction
Squeezing of light's quantum noise requires temporal rearranging of photons.
This again corresponds to creation of quantum correlations between individual
photons. Squeezed light is a non-classical manifestation of light with great
potential in high-precision quantum measurements, for example in the detection
of gravitational waves. Equally promising applications have been proposed in
quantum communication. However, after 20 years of intensive research doubts
arose whether strong squeezing can ever be realized as required for eminent
applications. Here we show experimentally that strong squeezing of light's
quantum noise is possible. We reached a benchmark squeezing factor of 10 in
power (10dB). Thorough analysis reveals that even higher squeezing factors will
be feasible in our setup.Comment: 10 pages, 4 figure
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