Performance of Photon-Pair Quantum Key Distribution Systems

We analyze the quantitative improvement in performance provided by a novel quantum key distribution (QKD) system that employs a correlated photon source (CPS) and a photon-number resolving detector (PNR). Our calculations suggest that given current technology, the CPR implementation offers an improvement of several orders of magnitude in secure bit rate over previously described implementations

Green construction in India: gaining a deeper understanding

A workshop on green construction was organized in New Delhi in July 2008 by Jamia Millia University, India and University of Salford, U.K., aiming to increase 'green' practices that are environmentally friendly and energy efficient. The workshop included participants from regulatory bodies, public and private construction companies and researchers. The object of the workshop was to provide an opportunity to capture issues, challenges and research issues in green practices throughout the supply chain in the construction sector in India. The methodology constituted a survey that was provided to all participants, where eight questions were composed by the facilitators. The participants were asked about their views on voluntary and compulsory ways of assessment and audit of green implementation in India. The findings of the workshop featured major challenges, drivers, initiatives, and the ways of effective implementation as well as enforcement on the discussed topic

Spectral Changes in the Hyperluminous Pulsar in NGC 5907 as a Function of Super-Orbital Phase

We present broad-band, multi-epoch X-ray spectroscopy of the pulsating ultra-luminous X-ray source (ULX) in NGC 5907. Simultaneous XMM-Newton and NuSTAR data from 2014 are best described by a multi-color black-body model with a temperature gradient as a function of accretion disk radius significantly flatter than expected for a standard thin accretion disk (T(r) ~ r^{-p}, with p=0.608^{+0.014}_{-0.012}). Additionally, we detect a hard power-law tail at energies above 10 keV, which we interpret as being due to Comptonization. We compare this observation to archival XMM-Newton, Chandra, and NuSTAR data from 2003, 2012, and 2013, and investigate possible spectral changes as a function of phase over the 78d super-orbital period of this source. We find that observations taken around phases 0.3-0.4 show very similar temperature profiles, even though the observed flux varies significantly, while one observation taken around phase 0 has a significantly steeper profile. We discuss these findings in light of the recent discovery that the compact object is a neutron star and show that precession of the accretion disk or the neutron star can self-consistently explain most observed phenomena.Comment: 7 pages, 5 figures, submitted to ApJ; comments welcom

Transport coefficients for an inelastic gas around uniform shear flow: Linear stability analysis

The inelastic Boltzmann equation for a granular gas is applied to spatially inhomogeneous states close to the uniform shear flow. A normal solution is obtained via a Chapman-Enskog-like expansion around a local shear flow distribution. The heat and momentum fluxes are determined to first order in the deviations of the hydrodynamic field gradients from their values in the reference state. The corresponding transport coefficients are determined from a set of coupled linear integral equations which are approximately solved by using a kinetic model of the Boltzmann equation. The main new ingredient in this expansion is that the reference state $f^{(0)}$ (zeroth-order approximation) retains all the hydrodynamic orders in the shear rate. In addition, since the collisional cooling cannot be compensated locally for viscous heating, the distribution $f^{(0)}$ depends on time through its dependence on temperature. This means that in general, for a given degree of inelasticity, the complete nonlinear dependence of the transport coefficients on the shear rate requires the analysis of the {\em unsteady} hydrodynamic behavior. To simplify the analysis, the steady state conditions have been considered here in order to perform a linear stability analysis of the hydrodynamic equations with respect to the uniform shear flow state. Conditions for instabilities at long wavelengths are identified and discussed.Comment: 7 figures; previous stability analysis modifie

Quantum Mechanics Another Way

Deformation quantization (sometimes called phase-space quantization) is a formulation of quantum mechanics that is not usually taught to undergraduates. It is formally quite similar to classical mechanics: ordinary functions on phase space take the place of operators, but the functions are multiplied in an exotic way, using the star product. Here we attempt a brief, pedagogical discussion of deformation quantization, that is suitable for inclusion in an undergraduate course.Comment: 14 pages, 3 figures, to be published in Eur. J. Phy

The hard X-ray perspective on the soft X-ray excess

The X-ray spectra of many active galactic nuclei (AGN) exhibit a `soft excess' below 1keV, whose physical origin remains unclear. Diverse models have been suggested to account for it, including ionised reflection of X-rays from the inner part of the accretion disc, ionised winds/absorbers, and Comptonisation. The ionised reflection model suggests a natural link between the prominence of the soft excess and the Compton reflection hump strength above 10keV, but it has not been clear what hard X-ray signatures, if any, are expected from the other soft X-ray candidate models. Additionally, it has not been possible up until recently to obtain high-quality simultaneous measurements of both soft and hard X-ray emission necessary to distinguish these models, but upcoming joint XMM-NuSTAR programmes provide precisely this opportunity. In this paper, we present an extensive analysis of simulations of XMM+NuSTAR observations, using two candidate soft excess models as inputs, to determine whether such campaigns can disambiguate between them by using hard and soft X-ray observations in tandem. The simulated spectra are fit with the simplest "observer's model" of a black body and neutral reflection to characterise the strength of the soft and hard excesses. A plot of the strength of the hard excess against the soft excess strength provides a diagnostic plot which allows the soft excess production mechanism to be determined in individual sources and samples using current state-of-the-art and next generation hard X-ray enabled observatories. This approach can be straightforwardly extended to other candidate models for the soft excess.Comment: 12 pages, 11 figures, accepted for publication in ApJ. Added reference

Lie group weight multiplicities from conformal field theory

Dominant weight multiplicities of simple Lie groups are expressed in terms of the modular matrices of Wess-Zumino-Witten conformal field theories, and related objects. Symmetries of the modular matrices give rise to new relations among multiplicities. At least for some Lie groups, these new relations are strong enough to completely fix all multiplicities.Comment: 12 pages, Plain TeX, no figure