Ultra-bright source of polarization-entangled photons

Using the process of spontaneous parametric down conversion in a novel two-crystal geometry, one can generate a source of polarization-entangled photon pairs which is orders of magnitude brighter than previous sources. We have measured a high level of entanglement between photons emitted over a relatively large collection angle, and over a 10-nm bandwidth. As a demonstration of the source intensity, we obtained a 242-$\sigma$ violation of Bell's inequalities in less than three minutes.Comment: 4 pages, 5 encapsulated Postscript figures. To appear in Physical Review A (Rapid Communication

A toolkit for measurement error correction, with a focus on nutritional epidemiology.

Exposure measurement error is a problem in many epidemiological studies, including those using biomarkers and measures of dietary intake. Measurement error typically results in biased estimates of exposure-disease associations, the severity and nature of the bias depending on the form of the error. To correct for the effects of measurement error, information additional to the main study data is required. Ideally, this is a validation sample in which the true exposure is observed. However, in many situations, it is not feasible to observe the true exposure, but there may be available one or more repeated exposure measurements, for example, blood pressure or dietary intake recorded at two time points. The aim of this paper is to provide a toolkit for measurement error correction using repeated measurements. We bring together methods covering classical measurement error and several departures from classical error: systematic, heteroscedastic and differential error. The correction methods considered are regression calibration, which is already widely used in the classical error setting, and moment reconstruction and multiple imputation, which are newer approaches with the ability to handle differential error. We emphasize practical application of the methods in nutritional epidemiology and other fields. We primarily consider continuous exposures in the exposure-outcome model, but we also outline methods for use when continuous exposures are categorized. The methods are illustrated using the data from a study of the association between fibre intake and colorectal cancer, where fibre intake is measured using a diet diary and repeated measures are available for a subset

Using full-cohort data in nested case-control and case-cohort studies by multiple imputation.

In many large prospective cohorts, expensive exposure measurements cannot be obtained for all individuals. Exposure-disease association studies are therefore often based on nested case-control or case-cohort studies in which complete information is obtained only for sampled individuals. However, in the full cohort, there may be a large amount of information on cheaply available covariates and possibly a surrogate of the main exposure(s), which typically goes unused. We view the nested case-control or case-cohort study plus the remainder of the cohort as a full-cohort study with missing data. Hence, we propose using multiple imputation (MI) to utilise information in the full cohort when data from the sub-studies are analysed. We use the fully observed data to fit the imputation models. We consider using approximate imputation models and also using rejection sampling to draw imputed values from the true distribution of the missing values given the observed data. Simulation studies show that using MI to utilise full-cohort information in the analysis of nested case-control and case-cohort studies can result in important gains in efficiency, particularly when a surrogate of the main exposure is available in the full cohort. In simulations, this method outperforms counter-matching in nested case-control studies and a weighted analysis for case-cohort studies, both of which use some full-cohort information. Approximate imputation models perform well except when there are interactions or non-linear terms in the outcome model, where imputation using rejection sampling works well

Pulse generation with ultra-superluminal pulse propagation in semiconductor heterostructures by superradiant-phase transition enhanced by transient coherent population gratings.

This paper reports the observation of ultra-superluminal pulse propagation in multiple-contact semiconductor heterostructures in a superradiant emission regime, and shows definitively that it is a different class of emission from conventional spontaneous or stimulated emission. Coherent population gratings induced in the semiconductor medium under strong electrical pumping have been shown to cause a major decrease of the group refractive index, in the range of 5-40%. This decrease is much greater than that caused by conventional carrier depletion or chirp mechanisms. The decrease in refractive index in turn causes faster-than-c propagation of femtosecond pulses. The measurement also proves the existence of coherent amplification of electromagnetic pulses in semiconductors at room temperature, the coherence being strongly enhanced by interactions of the light with coherent transient gratings locked to carrier gratings. This pulse-generation technique is anticipated to have great potential in applications where highly coherent femtosecond optical pulses must be generated on demand.We acknowledge support of the UK Engineering and Physical Sciences Research CouncilThis is the final version of the article. It first appeared from Nature Publishing Group via http://dx.doi.org/10.1038/lsa.2016.8

Effects of Classical Exposure Measurement Error on the Shape of Exposure-Disease Associations

In epidemiology many exposures of interest are measured with error. Random, or 'classical', error in exposure measurements attenuates linear exposure-disease associations. However, its precise effects on different nonlinear associations are not well known. We use simulation studies to assess how classical measurement error affects observed association shapes and power to detect nonlinearity. We focus on a proportional hazards model for the exposure-disease association and consider six true association shapes of relevance in epidemiology: linear, threshold, U-shaped, J- shaped, increasing quadratic, asymptotic. The association shapes are modeled using three popular methods: grouped exposure analyses, fractional polynomials, P-splines. Under each true association shape and each method we illustrate the effects of classical exposure measurement error, considering varying degrees of random error. We also assess what we refer to as MacMahon's method for correcting for classical exposure measurement error under grouped exposure analyses, which uses replicate measurements to estimate usual exposure within observed exposure groups. The validity of this method for nonlinear associations has not previously been investigated. Under nonlinear exposure-disease associations, classical measurement error results in increasingly linear shapes and not always an attenuated association at a given exposure level. Fractional polynomials and P-splines give similar results and offer advantages over grouped exposure analyses by providing realistic models. P-splines offer greatest power to detect nonlinearity, however random exposure measurement error results in a potential considerable loss of power to detect nonlinearity under all methods. MacMahon's method performs well for quadratic associations, but does not in general recover nonlinear shapes

Nonlinear optical effects during femtosecond superradiant emission generation in semiconductor laser structures.

This paper presents theoretical and experimental studies of ultrabright internal second harmonic during femtosecond superradiant emission generation in multiple sections GaAs/AlGaAs laser structures at room temperature. Experimentally measured conversion efficiencies are by 1-2 orders of magnitude greater than expected. To explain this fact, a model based on one-dimensional nonlinear Maxwell curl equations without taking into consideration the slowly-varying envelope approximation has been developed. It has been demonstrated that strong transient periodic modulation of e-h density and refraction index dramatically affects the process of superradiance in semiconductor media and can explain the ultrastrong internal second harmonic generation

Bandwidth Studies on a 1.4 m Long Multimode Polymer Spiral Waveguide

This version is the author accepted manuscript. The final version is available at http://publications.lib.chalmers.se/publication/201575-bandwidth-studies-of-a-14-m-long-multimode-polymer-spiral-waveguide.EPSR

Spectral Function for the S=1 Heisenberg Antiferromagetic Chain

We study the spectral function, $S(k,\omega)$ for the spin-1, one dimensional antiferromagnetic chain using a time-dependent density matrix renormalizaton group (DMRG) numerical method. We develop methods for extrapolating the time dependent correlation functions to larger times in order to enhance the frequency resolution. The resulting spectral functions are impressively precise and accurate. Our results confirm many qualitative expectations from non-linear $\sigma$ model methods and test them quantitatively. The critical wave-vector at which the single particle excitation emerges from the 2-particle continuum is estimated to be $0.23\pi-0.24\pi$.Comment: 12 pages, 19 fig