Experimental quantum key distribution over highly noisy channels

Error filtration is a method for encoding the quantum state of a single particle into a higher dimensional Hilbert space in such a way that it becomes less sensitive to phase noise. We experimentally demonstrate this method by distributing a secret key over an optical fiber whose noise level otherwise precludes secure quantum key distribution. By filtering out the phase noise, a bit error rate of 15.3% +/- 0.1%, which is beyond the security limit, can be reduced to 10.6% +/- 0.1%, thereby guaranteeing the cryptographic security.Comment: 4 pages, 2 figure

Provably Secure Experimental Quantum Bit-String Generation

Coin tossing is a cryptographic task in which two parties who do not trust each other aim to generate a common random bit. Using classical communication this is impossible, but non trivial coin tossing is possible using quantum communication. Here we consider the case when the parties do not want to toss a single coin, but many. This is called bit string generation. We report the experimental generation of strings of coins which are provably more random than achievable using classical communication. The experiment is based on the ``plug and play'' scheme developed for quantum cryptography, and therefore well suited for long distance quantum communication.Comment: 4 pages, 3 figures. Submitted to Phys. Rev. Lett. A complete security analysis for the experiment is given in quant-ph/040812

Experimental asymmetric phase-covariant quantum cloning of polarization qubits

We report on two optical realizations of the $1 \to 2$ asymmetric phase-covariant cloning machines for polarization states of single photons. The experimental setups combine two-photon interference and tunable polarization filtering that enables us to control the asymmetry of the cloners. The first scheme involves a special unbalanced bulk beam splitter exhibiting different splitting ratios for vertical and horizontal polarizations, respectively. The second implemented scheme consists of a balanced fiber coupler where photon bunching occurs, followed by a free-space part with polarization filters. With this later approach we were able to demonstrate very high cloning fidelities which are above the universal cloning limit.Comment: 7 pages, 8 figure

Validity of the Adaptation to Age-related Vision Loss Scale in an Australian Cataract Population

Purpose: The Adaptation to Age-related Vision Loss (AVL) scale was developed to measure the adjustment of older adults who are adapting to late-life vision loss. The purpose of this study was to assess whether the AVL scale satisfies the Rasch model in a cataract population. Methods: The 24-item AVL scale (18 negatively and 6 positively coded) was mailed to 436 cataract patients for self-administration whilst they were on the waiting list for cataract surgery at the Flinders Eye Centre, Adelaide, South Australia. Rasch analysis was performed to determine whether the items were measuring a single construct (unidimensionality) as examined with fit statistics and principal components analysis (PCA) of the residuals. The ability of the scale to distinguish between the levels of adaptation of the participants (person separation) was investigated, with a value ≥ 2.0 established as the minimum acceptable. Results: The AVL scale was unable to differentiate sufficiently between participants’ levels of adaptation, indicating poor person separation. One item did not fit the construct, causing misfit. Furthermore, the five positively worded items did not appear either to measure the same construct as other items, resulting in lack of unidimensionality evidenced by PCA. Following the deletion of these items, the AVL scale was one-dimensional but a single item continued to misfit, so it had to be deleted, resulting in an 18-item AVL scale. Even so, the discriminating abilities of the scale continued to be poor. Conclusions: The AVL scale is not an appropriate measure of adaptation to vision loss in a cataract population

Economical quantum cloning in any dimension

The possibility of cloning a d-dimensional quantum system without an ancilla is explored, extending on the economical phase-covariant cloning machine found in [Phys. Rev. A {\bf 60}, 2764 (1999)] for qubits. We prove the impossibility of constructing an economical version of the optimal universal cloning machine in any dimension. We also show, using an ansatz on the generic form of cloning machines, that the d-dimensional phase-covariant cloner, which optimally clones all uniform superpositions, can be realized economically only in dimension d=2. The used ansatz is supported by numerical evidence up to d=7. An economical phase-covariant cloner can nevertheless be constructed for d>2, albeit with a lower fidelity than that of the optimal cloner requiring an ancilla. Finally, using again an ansatz on cloning machines, we show that an economical version of the Fourier-covariant cloner, which optimally clones the computational basis and its Fourier transform, is also possible only in dimension d=2.Comment: 8 pages RevTe

Validity of a visual impairment questionnaire in measuring cataract surgery outcomes [post print]

PURPOSE: To test the validity of the Impact of Visual Impairment (IVI) questionnaire in a cataract population. SETTING: Flinders Eye Centre, Flinders Medical Centre, Flinders University, Adelaide, South Australia. METHODS: Cataract patients recruited from a hospital waiting list completed the IVI questionnaire. The scale was assessed for fit to the Rasch model. Unidimensionality, item and person fit to the model, response category performance, differential item functioning (whether different subgroups responded differently), and targeting of item difficulty to patient ability were assessed. RESULTS: Overall, the IVI questionnaire performed well; there were ordered thresholds, person separation reliability was 0.97, and it was free from differential item functioning. One item (worry about eyesight getting worse) misfit the model and was removed. There was evidence of multidimensionality, indicating that the overall IVI score should be discarded; however, the 3 subscales (reading and accessing information, mobility and independence, and emotional well-being) functioned well. Several items calibrated differently in cataract patients compared with low-vision patients, indicating different issues are important to each population and that there is a need for population-specific conversion algorithms. Targeting of the IVI items was biased toward more impaired patients. CONCLUSIONS: The 3 subscales of the IVI questionnaire functioned well in a cataract population. However, additional items targeting the less impaired patients, especially second-eye cataract patients, would improve measurement

The Importance of Rating Scale Design in the Measurement of Patient-Reported Outcomes Using Questionnaires or Item Banks

This article is made available with the permission of the publisher, Association for Research in Vision and OphthalmologyPurpose.: To investigate the effect of rating scale designs (question formats and response categories) on item difficulty calibrations and assess the impact that rating scale differences have on overall vision-related activity limitation (VRAL) scores. Methods.: Sixteen existing patient-reported outcome instruments (PROs) suitable for cataract assessment, with different rating scales, were self-administered by patients on a cataract surgery waiting list. A total of 226 VRAL items from these PROs in their native rating scales were included in an item bank and calibrated using Rasch analysis. Fifteen item/content areas (e.g., reading newspapers) appearing in at least three different PROs were identified. Within each content area, item calibrations were compared and their range calculated. Similarly, five PROs having at least three items in common with the Visual Function (VF-14) were compared in terms of average item measures. Results.: A total of 614 patients (mean age ± SD, 74.1 ± 9.4 years) participated. Items with the same content varied in their calibration by as much as two logits; “reading the small print” had the largest range (1.99 logits) followed by “watching TV” (1.60). Compared with the VF-14 (0.00 logits), the rating scale of the Visual Disability Assessment (1.13 logits) produced the most difficult items and the Cataract Symptom Scale (0.24 logits) produced the least difficult items. The VRAL item bank was suboptimally targeted to the ability level of the participants (2.00 logits). Conclusions.: Rating scale designs have a significant effect on item calibrations. Therefore, constructing item banks from existing items in their native formats carries risks to face validity and transmission of problems inherent in existing instruments, such as poor targeting

Mice expressing HLA-DQ6α8β transgenes develop polychondritis spontaneously

Relapsing polychondritis (RP) is a human autoimmune disease of unknown etiology in which cartilaginous sites are destroyed by cyclic inflammatory episodes beginning, most commonly, during the fourth or fifth decade of life. We have previously described collagen-induced polychondritis that closely mirrors RP occurring in young (6–8 weeks old) HLA-DQ6αβ8αβ transgenic Aβ0 mice, following immunization with heterologous type II collagen (CII). We present evidence here that transgenic strains expressing the DQ6α8β transgene develop spontaneous polychondritis (SP) at the mouse equivalent of human middle age (4.5–6 months and 40–50 years old, respectively) and display polyarthritis, auricular chondritis and nasal chondritis – three of the most common sites affected in RP. Auricular chondritis in SP, like RP but unlike CII-induced polychondritis, exhibited a relapsing/remitting phenotype, requiring several inflammatory cycles before the cartilage is destroyed. Elevated serum levels of total IgG corresponded with the onset of disease in SP, as in RP and CII-induced polychondritis. No CII-specific immune response was detected in SP, however – more closely mirroring RP, in which as few as 30% of RP patients have been reported to have CII-specific IgG. CII-induced polychondritis displays a strong CII-specific immune response. SP also demonstrated a strong female preponderance, as some workers have reported in RP but has not observed in CII-induced polychondritis. These characteristics of SP allow for the examination of the immunopathogenesis of polychondritis in the absence of an overwhelming CII-specific immune response and the strong adjuvant-induced immunostimulatory influence in CII-induced polychondritis. This spontaneous model of polychondritis provides a new and unique tool to investigate both the initiatory events as well as the immunopathogenic mechanisms occurring at cartilaginous sites during the cyclic inflammatory assaults of polychondritis

Extremal quantum cloning machines

We investigate the problem of cloning a set of states that is invariant under the action of an irreducible group representation. We then characterize the cloners that are "extremal" in the convex set of group covariant cloning machines, among which one can restrict the search for optimal cloners. For a set of states that is invariant under the discrete Weyl-Heisenberg group, we show that all extremal cloners can be unitarily realized using the so-called "double-Bell states", whence providing a general proof of the popular ansatz used in the literature for finding optimal cloners in a variety of settings. Our result can also be generalized to continuous-variable optimal cloning in infinite dimensions, where the covariance group is the customary Weyl-Heisenberg group of displacements.Comment: revised version accepted for publicatio