Unconditionally secure quantum key distribution over 50km of standard telecom fibre

We demonstrate a weak pulse quantum key distribution system using the BB84 protocol which is secure against all individual attacks, including photon number splitting. By carefully controlling the weak pulse intensity we demonstrate the maximum secure bit rate as a function of the fibre length. Unconditionally secure keys can be formed for standard telecom fibres exceeding 50 km in length.Comment: 9 pages 2 figure

Tungsten Behavior at High Temperature and High Stress

Re­cent­ly re­port­ed re­sults on the tung­sten life­time/fa­tigue tests under con­di­tions ex­pect­ed in the Neu­tri­no Fac­to­ry tar­get have strength­ened the case of solid tar­get op­tion for a Neu­tri­no Fac­to­ry. This paper gives de­scrip­tion of the de­tailed mea­sure­ments of the tung­sten prop­er­ties at high tem­per­a­ture and high stress. We have per­formed ex­ten­sive set of mea­sure­ments of the sur­face dis­place­ment and ve­loc­i­ty of the tung­sten wires that were stressed by pass­ing a fast, high cur­rent pulse through a thin sam­ple. Ra­di­al and lon­gi­tu­di­nal os­cil­la­tions of the wire were mea­sured by a Laser Doppler Vi­brom­e­ter. The wire was op­er­at­ed at tem­per­a­tures of 300-2500 K by ad­just­ing the pulse rep­e­ti­tion rate. In doing so we have tried to sim­u­late the con­di­tions (high stress and tem­per­a­ture) ex­pect­ed at the Neu­tri­no Fac­to­ry. Most im­por­tant re­sult of this study is an ex­per­i­men­tal con­fir­ma­tion that strength of tung­sten re­mains high at high tem­per­a­ture and high stress. The ex­per­i­men­tal re­sults have been found to agree very well with LS-DY­NA mod­elling re­sults

Stabilizer notation for Spekkens' toy theory

Spekkens has introduced a toy theory [Phys. Rev. A, 75, 032110 (2007)] in order to argue for an epistemic view of quantum states. I describe a notation for the theory (excluding certain joint measurements) which makes its similarities and differences with the quantum mechanics of stabilizer states clear. Given an application of the qubit stabilizer formalism, it is often entirely straightforward to construct an analogous application of the notation to the toy theory. This assists calculations within the toy theory, for example of the number of possible states and transformations, and enables superpositions to be defined for composite systems.Comment: 7+4 pages, 5 tables. v2: Clarifications added and typos fixed in response to referee comment

Enhanced indistinguishability of in-plane single photons by resonance fluorescence on an integrated quantum dot

Integrated quantum light sources in photonic circuits are envisaged as the building blocks of future on-chip architectures for quantum logic operations. While semiconductor quantum dots have been proven to be the highly efficient emitters of quantum light, their interaction with the host material induces spectral decoherence, which decreases the indistinguishability of the emitted photons and limits their functionality. Here, we show that the indistinguishability of in-plane photons can be greatly enhanced by performing resonance fluorescence on a quantum dot coupled to a photonic crystal waveguide. We find that the resonant optical excitation of an exciton state induces an increase in the emitted single-photon coherence by a factor of 15. Two-photon interference experiments reveal a visibility of 0.80 ± 0.03, which is in good agreement with our theoretical model. Combined with the high in-plane light-injection efficiency of photonic crystal waveguides, our results pave the way for the use of this system for the on-chip generation and transmission of highly indistinguishable photons

Genetic differentiation in Scottish populations of the pine beauty moth Panolis flammea (Lepidoptera: Noctuidae)

Pine beauty moth, Panolis flammea (Denis & Schiffermüller), is a recent but persistent pest of lodgepole pine plantations in Scotland, but exists naturally at low levels within remnants and plantations of Scots pine. To test whether separate host races occur in lodgepole and Scots pine stands and to examine colonization dynamics, allozyme, randomly amplified polymorphic DNA (RAPD) and mitochondrial variation were screened within a range of Scottish samples. RAPD analysis indicated limited long distance dispersal (FST = 0.099), and significant isolation by distance (P < 0.05); but that colonization between more proximate populations was often variable, from extensive to limited exchange. When compared with material from Germany, Scottish samples were found to be more diverse and significantly differentiated for all markers. For mtDNA, two highly divergent groups of haplotypes were evident, one group contained both German and Scottish samples and the other was predominantly Scottish. No genetic differentiation was evident between P. flammea populations sampled from different hosts, and no diversity bottleneck was observed in the lodgepole group. Indeed, lodgepole stands appear to have been colonized on multiple occasions from Scots pine sources and neighbouring populations on different hosts are close to panmixia.A.J. Lowe, B.J. Hicks, K. Worley, R.A. Ennos, J.D. Morman, G. Stone and A.D. Wat

Generation of entangled states and error protection from adiabatic avoided level crossings

We consider the environment-affected dynamics of $N$ self-interacting particles living in one-dimensional double wells. Two topics are dealt with. First, we consider the production of entangled states of two-level systems. We show that by adiabatically varying the well biases we may dynamically generate maximally entangled states, starting from initially unentangled product states. Entanglement degradation due to a common type of environmental influence is then computed by solving a master equation. However, we also demonstrate that entanglement production is unaffected if the system-environment coupling is of the type that induces ``motional narrowing''. As our second but related topic, we construct a different master equation that seamlessly merges error protection/detection dynamics for quantum information with the environmental couplings responsible for producing the errors in the first place. Adiabatic avoided crossing schemes are used in both topics.Comment: 14 pages, 6 figures. Minor changes. To appear in Phys. Rev.

Controlled order rearrangement encryption for quantum key distribution

A novel technique is devised to perform orthogonal state quantum key distribution. In this scheme, entangled parts of a quantum information carrier are sent from Alice to Bob through two quantum channels. However before the transmission, the orders of the quantum information carrier in one channel is reordered so that Eve can not steal useful information. At the receiver's end, the order of the quantum information carrier is restored. The order rearrangement operation in both parties is controlled by a prior shared control key which is used repeatedly in a quantum key distribution session.Comment: 5 pages and 2 figure

Phylogenetic relationships within Orobanche and Phelipanche (Orobanchaceae) from Central Europe, focused on problematic aggregates, taxonomy, and host ranges

Holoparasitic genera within the family Orobanchaceae are characterized by greatly reduced vegetative organs; therefore, molecular analysis has proved to be a useful tool in solving taxonomic problems in this family. For this purpose, we studied all species of the genera Orobanche and Phelipanche occurring in Central Europe, specifically in Poland, the Czech Republic, Slovakia, and Austria, supplemented by samples mainly from Spain, France, Germany, and Ukraine. They were investigated using nuclear sequences (ITS region) and a plastid trnL- trnF region. The aim of this study was to examine phylogenetic relationships within Orobanche and Phelipanche from Central Europe; we focused on problematic species and aggregates, recent taxonomic changes in these (rank and secondary ranks), and host ranges. The most interesting results concern the exlusion of O. mayeri from O. alsatica aggr. Additionally, following the rules of traditional taxonomy, the correct names and types of some secondary ranks are given and, as a result of this, a new combination below the Phelipanche genus is made ( P . sect. Trionychon ). The host ranges of the investigated species in Central Europe include 102 species from 12 families, most often from Asteraceae. For this purpose, ca. 400 localities were examined in the field. Moreover, data acquired from the literature and European and Asian herbaria were use

Entanglement between a qubit and the environment in the spin-boson model

The quantitative description of the quantum entanglement between a qubit and its environment is considered. Specifically, for the ground state of the spin-boson model, the entropy of entanglement of the spin is calculated as a function of $\alpha$, the strength of the ohmic coupling to the environment, and $\epsilon$, the level asymmetry. This is done by a numerical renormalization group treatment of the related anisotropic Kondo model. For $\epsilon=0$, the entanglement increases monotonically with $\alpha$, until it becomes maximal for $\alpha \lim 1^-$. For fixed $\epsilon>0$, the entanglement is a maximum as a function of $\alpha$ for a value, $\alpha = \alpha_M < 1$.Comment: 4 pages, 3 figures. Shortened version restricted to groundstate entanglemen

Robustness of Decoherence-Free Subspaces for Quantum Computation

It was shown recently [D.A. Lidar et al., Phys. Rev. Lett. 81, 2594 (1998)] that within the framework of the semigroup Markovian master equation, decoherence-free (DF) subspaces exist which are stable to first order in time to a perturbation. Here this result is extended to the non-Markovian regime and generalized. In particular, it is shown that within both the semigroup and the non-Markovian operator sum representation, DF subspaces are stable to all orders in time to a symmetry-breaking perturbation. DF subspaces are thus ideal for quantum memory applications. For quantum computation, however, the stability result does not extend beyond the first order. Thus, to perform robust quantum computation in DF subspaces, they must be supplemented with quantum error correcting codes.Comment: 16 pages, no figures. Several changes, including a clarification of the derivation of the Lindblad equation from the operator sum representation. To appear in Phys. Rev