Quantum communication and the creation of maximally entangled pairs of atoms over a noisy channel

We show how to create maximally entangled EPR pairs between spatially distant atoms, each of them inside a high-Q optical cavity, by sending photons through a general, noisy channel, such as a standard optical fiber. An error correction scheme that uses few auxiliary atoms in each cavity effectively eliminates photoabsorption and other transmission errors. This realizes the `absorption free channel.' A concatenation protocol using the absorption free channel allows for quantum communication with single qubits over distances much larger than the coherence length of the channel.Comment: 12 pages, latex, rspublic.sty, 4 figures, uses epsf macro. For the Royal Society meeting on quantum computatio

Entanglement purification for Quantum Computation

We show that thresholds for fault-tolerant quantum computation are solely determined by the quality of single-system operations if one allows for d-dimensional systems with $8 \leq d \leq 32$. Each system serves to store one logical qubit and additional auxiliary dimensions are used to create and purify entanglement between systems. Physical, possibly probabilistic two-system operations with error rates up to 2/3 are still tolerable to realize deterministic high quality two-qubit gates on the logical qubits. The achievable error rate is of the same order of magnitude as of the single-system operations. We investigate possible implementations of our scheme for several physical set-ups.Comment: 4 pages, 1 figure; V2: references adde

Quantum random walks in optical lattices

We propose an experimental realization of discrete quantum random walks using neutral atoms trapped in optical lattices. The random walk is taking place in position space and experimental implementation with present day technology --even using existing set-ups-- seems feasible. We analyze the influence of possible imperfections in the experiment and investigate the transition from a quantum random walk to the classical random walk for increasing errors and decoherence.Comment: 8 pages, 4 figure

Pengawetan Kayu Karet (Havea brasiliensis) dan Kayu Kopi (Coffea sp.) dengan Brotowali (Tinospora crispa)

ABSTRAKPohon karet (Hevea brasiliensis) dan kopi (Coffea sp.) merupakan tanaman perkebunan yang dibudidayakan di Indonesia. Kayu karet lemah terhadap serangan kutu dan jamur blue stain. Pohon karet dan kopi mempunyai karakteristik kelas kuat II-III, dan kelas awet IV-V. Menurut SNI 03-5010.1- 1999, kayu dengan kelas kuat III- IV-V dan kelas awet III-IV-V harus dilakukan pengawetan untuk menambah umur kayu. Kegiatan ini bertujuan untuk mengetahui kinerja brotowali (Tinospora cordifolia) sebagai pengawet alami pada kayu karet dan kopi sekaligus memanfaatkan kayu karet sebagai bahan baku mebel. Kayu karet dan kopi diberikan perlakuan dengan larutan brotowali 2,5%, 5% dan 7,5% dengan durasi lama perendaman kayu 1, 2 dan 3 minggu. Perbedaan perlakuan memberikan hasil yang berbeda, retensi pada kayu berbanding lurus dengan lama perendaman dan konsentrasi. Pengujian ketahanan rayap pada hasil proses pengawetan, memberikan hasil yang positif, dengan nilai mortalitas mencapai 100%. Brotowali dapat memberikan hasil pengawetan yang baik dengan komposisi larutan terbaik 7,5% selama 3 minggu.ABSTRACTRubber trees (Hevea brasiliensis) and coffee (Coffea sp.) are plants that have been cultivated in Indonesia. Rubber and coffee trees have wood strength at category III-IV and it has durability at category IV-V. Rubberwood is weak against termites and blue stain fungus. According to SNI 03- 5010.1-1999, wood with category III-IV-V of strength and category III-IV-V of durability have to be preserved to prolong the age of the wood. The aim of the study was to evaluate the perfomance of Brotowali as a natural preservative in rubberwood and coffee wood while also using it as raw material for furniture. Rubberwood and coffee treated with 2.5%, 5%, and 7.5% solutions with the duration of soaking wood are 1, 2, and 3 weeks. The treatment gives different results; retention in wood is directly proportional to the soaking time and concentration. Termite endurance testing on the results of the preservation process, gave positive results, with a mortality value reaching 100%. Brotowali can provide good preservation results with the best composition of a 7.5% solution

Lower Bounds for Attainable Fidelity in Entanglement Purification

We derive lower bounds for the attainable fidelity of standard entanglement purification protocols when local operations and measurements are subjected to errors. We introduce an error parameter which measures the distance between the ideal completely positive map describing a purification step and the one in the presence of errors. We derive non--linear maps for a lower bound of the fidelity at each purification step in terms of this parameter.Comment: 11 page

Universal architecture of bacterial chemoreceptor arrays

Chemoreceptors are key components of the high-performance signal transduction system that controls bacterial chemotaxis. Chemoreceptors are typically localized in a cluster at the cell pole, where interactions among the receptors in the cluster are thought to contribute to the high sensitivity, wide dynamic range, and precise adaptation of the signaling system. Previous structural and genomic studies have produced conflicting models, however, for the arrangement of the chemoreceptors in the clusters. Using whole-cell electron cryo-tomography, here we show that chemoreceptors of different classes and in many different species representing several major bacterial phyla are all arranged into a highly conserved, 12-nm hexagonal array consistent with the proposed “trimer of dimers” organization. The various observed lengths of the receptors confirm current models for the methylation, flexible bundle, signaling, and linker sub-domains in vivo. Our results suggest that the basic mechanism and function of receptor clustering is universal among bacterial species and was thus conserved during evolution

Modematching an optical quantum memory

We analyse the off-resonant Raman interaction of a single broadband photon, copropagating with a classical `control' pulse, with an atomic ensemble. It is shown that the classical electrodynamical structure of the interaction guarantees canonical evolution of the quantum mechanical field operators. This allows the interaction to be decomposed as a beamsplitter transformation between optical and material excitations on a mode-by-mode basis. A single, dominant modefunction describes the dynamics for arbitrary control pulse shapes. Complete transfer of the quantum state of the incident photon to a collective dark state within the ensemble can be achieved by shaping the control pulse so as to match the dominant mode to the temporal mode of the photon. Readout of the material excitation, back to the optical field, is considered in the context of the symmetry connecting the input and output modes. Finally, we show that the transverse spatial structure of the interaction is characterised by the same mode decomposition.Comment: 17 pages, 4 figures. Brief section added treating the transverse spatial structure of the memory interaction. Some references added. A few typos fixe

Entanglement on mixed stabilizer states: normal forms and reduction procedures

Published versio