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

Purification and distillation of continuous variable entanglement

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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

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

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