Asymmetric quantum channel for quantum teleportation

There are a few obstacles, which bring about imperfect quantum teleportation of a continuous variable state, such as unavailability of maximally entangled two-mode squeezed states, inefficient detection and imperfect unitary transformation at the receiving station. We show that all those obstacles can be understood by a combination of an {\it asymmetrically-decohered} quantum channel and perfect apparatuses for other operations. For the asymmetrically-decohered quantum channel, we find some counter-intuitive results; one is that teleportation does not necessarily get better as the channel is initially squeezed more and another is when one branch of the quantum channel is unavoidably subject to some imperfect operations, blindly making the other branch as clean as possible may not result in the best teleportation result. We find the optimum strategy to teleport an unknown field for a given environment or for a given initial squeezing of the channel.Comment: 4pages, 1figur

Experimentally realizable characterizations of continuous variable Gaussian states

Measures of entanglement, fidelity and purity are basic yardsticks in quantum information processing. We propose how to implement these measures using linear devices and homodyne detectors for continuous variable Gaussian states. In particular, the test of entanglement becomes simple with some prior knowledge which is relevant to current experiments.Comment: 4 pages, This paper supersedes quant-ph/020315

Demonstrating various quantum effects with two entangled laser beams

We report on the preparation of entangled two mode squeezed states of yet unseen quality. Based on a measurement of the covariance matrix we found a violation of the Reid and Drummond EPR-criterion at a value of only 0.36\pm0.03 compared to the threshold of 1. Furthermore, quantum state tomography was used to extract a single photon Fock state solely based on homodyne detection, demonstrating the strong quantum features of this pair of laser-beams. The probability for a single photon in this ensemble measurement exceeded 2/3

Surface states and their possible role in the superconductivity of MgB2

We report layer-Korringa-Kohn-Rostocker calculations for bulk and surface states as well as the corresponding angle resolved photoemission (ARPES) intensities of MgB2. Our theoretical results reproduce very well the recent ARPES data by Uchiyama et al., cond-mat/0111152. At least two surface states are assigned. Consequences of SFS on the anisotropy of the upper critical fields and other properties in the superconducting state of small grains in micropowder samples are briefly discussed.Comment: 4pages, 6figures, corrected typos, references adde

Bethe Ansatz for the Weakly Asymmetric Simple Exclusion Process and phase transition in the current distribution

The probability distribution of the current in the asymmetric simple exclusion process is expected to undergo a phase transition in the regime of weak asymmetry of the jumping rates. This transition was first predicted by Bodineau and Derrida using a linear stability analysis of the hydrodynamical limit of the process and further arguments have been given by Mallick and Prolhac. However it has been impossible so far to study what happens after the transition. The present paper presents an analysis of the large deviation function of the current on both sides of the transition from a Bethe ansatz approach of the weak asymmetry regime of the exclusion process.Comment: accepted to J.Stat.Phys, 1 figure, 1 reference, 2 paragraphs adde

Preoperative Posturing of Patients with Macula-On Retinal Detachment Reduces Progression Toward the Fovea

Purpose: Traditionally, preoperative posturing consisting of bed rest and positioning is prescribed to patients with macula-on retinal detachment (RD) to prevent RD progression and detachment of the fovea. Execution of such advice can be cumbersome and expensive. This study aimed to investigate if preoperative posturing affects the progression of RD. Design: Prospective cohort study. Participants: Ninety-eight patients with macula-on RD were included. Inclusion criteria were volume optical coherence tomography (OCT) scans could be obtained with sufficient quality; and the smallest distance from the fovea to the detachment border was 1.25 mm or more. Methods: Patients were admitted to the ward for bed rest in anticipation of surgery and were positioned on the side where the RD was mainly located. At baseline and before and after each interruption for meals or toilet visits, a 37°×45° OCT volume scan was performed using a wide-angle Spectralis OCT (Heidelberg Engineering, Heidelberg, Germany). The distance between the nearest point of the RD border and fovea was measured using a custom-built measuring tool. Main Outcome Measures: The RD border displacement and the average RD border displacement velocity moving toward (negative) or away (positive) from the fovea were determined for intervals of posturing and interruptions. Results: The median duration of intervals of posturing was 3.0 hours (interquartile range [IQR], 1.8-14.0 hours; n = 202) and of interruptions 0.37 hours (IQR, 0.26-0.50 hours; n = 197). The median RD border displacement was 2 μm (IQR, -65 to +251 μm) during posturing and -61 μm (IQR, -140 to 0 μm) during interruptions, a statistically significant difference (P < 0.001, Mann-Whitney U test). The median RD border displacement velocity was +1 μm/hour (IQR, -21 to +49 μm/hour) during posturing and -149 μm/hour (IQR, -406 to +1 μm/hour) during interruptions, a statistically significant difference (P < 0.001). Conclusions: By making use of usual interruptions of preoperative posturing we were able to show, in a prospective and ethically acceptable manner, that RD stabilizes during posturing and progresses during interruptions in patients with macula-on RD. Preoperative posturing is effective in reducing progression of RD

Entanglement and purity of two-mode Gaussian states in noisy channels

We study the evolution of purity, entanglement and total correlations of general two--mode Gaussian states of continuous variable systems in arbitrary uncorrelated Gaussian environments. The time evolution of purity, Von Neumann entropy, logarithmic negativity and mutual information is analyzed for a wide range of initial conditions. In general, we find that a local squeezing of the bath leads to a faster degradation of purity and entanglement, while it can help to preserve the mutual information between the modes.Comment: 10 pages, 8 figure

Generalised quantum weakest preconditions

Generalisation of the quantum weakest precondition result of D'Hondt and Panangaden is presented. In particular the most general notion of quantum predicate as positive operator valued measure (POVM) is introduced. The previously known quantum weakest precondition result has been extended to cover the case of POVM playing the role of a quantum predicate. Additionally, our result is valid in infinite dimension case and also holds for a quantum programs defined as a positive but not necessary completely positive transformations of a quantum states.Comment: 7 pages, no figures, added references, changed conten

Entangled light in transition through the generation threshold

We investigate continuous variable entangling resources on the base of two-mode squeezing for all operational regimes of a nondegenerate optical parametric oscillator with allowance for quantum noise of arbitrary level. The results for the quadrature variances of a pair of generated modes are obtained by using the exact steady-state solution of Fokker-Planck equation for the complex P-quasiprobability distribution function. We find a simple expression for the squeezed variances in the near-threshold range and conclude that the maximal two-mode squeezing reaches 50% relative to the level of vacuum fluctuations and is achieved at the pump field intensity close to the generation threshold. The distinction between the degree of two-mode squeezing for monostable and bistable operational regimes is cleared up.Comment: 7 pages, 4 figures; Content changed: more details added to the discussion. To be published in Phys. Rev.