Piyush Jain

Efficient, low-threshold collinear and noncollinear beta-barium borate optical parametric oscillators.

The performance of collinear and noncollinear pulsed barium borate optical parametric oscillators with third-harmonic Nd:YAG pumping is analyzed. The effects of tangential phase matching and pump Poynting vector walk-off compensation are shown to enhance noncollinear operation. However, we show that these effects are mutually exclusive for a typeI beta-barium borate optical parametric oscillator and furthermore that tangential phase matching is dominant. The selection of a collinear or a noncollinear configuration is determined by pump divergence and spot size as well as by crystal aperture. With a pump divergence of 4mrad, noncollinear operation is optimal. The highest slope efficiency, 37%, and the lowest threshold, 5mJ, are obtained with nearly perfect tangential phase matching. For a pump divergence of less than 2mrad and a similar spot size, collinear operation gives the lowest threshold, 3.2mJ, and the highest slope efficiency, 33%.

Quantum de Laval nozzle: Stability and quantum dynamics of sonic horizons in a toroidally trapped Bose gas containing a superflow

We study an experimentally realizable system containing stable black hole–white hole acoustic horizons in toroidally trapped Bose-Einstein condensates—the quantum de Laval nozzle. We numerically obtain stationary flow configurations and assess their stability using Bogoliubov theory, finding both in hydrodynamic and nonhydrodynamic regimes there exist dynamically unstable regions associated with the creation of positive and negative energy quasiparticle pairs in analogy with the gravitational Hawking effect. The dynamical instability takes the form of a two mode squeezing interaction between resonant pairs of Bogoliubov modes. We study the evolution of dynamically unstable flows using the truncated Wigner method, which confirms the two mode squeezed state picture of the analogue Hawking effect for low winding number.

On the observation of nonclassical excitations in Bose–Einstein condensates

In the recent experimental and theoretical literature well-established nonclassicality criteria from the field of quantum optics have been directly applied to the case of excitations in matter-waves. Among these are violations of Cauchy-Schwarz inequalities, Glauber-Sudarshan P-nonclassicality, sub-Poissonian number-difference squeezing (also known as the two-mode variance) and the criterion of nonseparability. We review the strong connection of these criteria and their meaning in quantum optics, and point out differences in the interpretation between light and matter waves. We then calculate observables for a homogenous Bose-Einstein condensate undergoing an arbitrary modulation in the interaction parameter at finite initial temperature, within both the quantum theory as well as a classical reference. We conclude that to date in experiments relevant for analogue gravity, nonclassical effects have not conclusively been observed and conjecture that additional, noncommuting, observables have to be measured to this end.

Structure, Bose-Einstein condensation, and superfluidity of two-dimensional confined dipolar assemblies

Low temperature properties of harmonically confined two-dimensional assemblies of dipolar bosons are systematically investigated by Monte Carlo simulations. Calculations carried out for different numbers of particles and strengths of the confining potential yield evidence of a quantum phase transition from a superfluid to a crystal-like phase, consistently with what is observed in the ho- mogeneous system. It is found that the crystal phase nucleates in the center of the trap, as the density increases. Bose-Einstein condensation vanishes at T = 0 upon entering the crystalline phase, concurrently with the disappearance of the superfluid response.

Trend analysis of fire season length and extreme fire weather in North America between 1979 and 2015

We have constructed a fire weather climatology over North America from 1979 to 2015 using the North American Regional Reanalysis dataset and the Canadian Fire Weather Index (FWI) System. We tested for the presence of trends in potential fire season length, based on a meteorological definition, and extreme fire weather using the non-parametric Theil–Sen slope estimator and Mann–Kendall test. Applying field significance testing (i.e. joint significance of multiple tests) allowed the identification of the locations of significant trends, taking into account spatial correlations. Fire season length was found to be increasing over large areas of North America, especially in eastern Canada and the south-western US, which is consistent with a later fire season end and an earlier fire season start. Both positive and negative trends in potential fire spread days and the 99th percentile of FWI occurred in Canada and the contiguous United States, although the trends of largest magnitude and statistical significance were mostly positive. In contrast, the proportion of trends with significant decreases in these variables were much lower, indicating an overall increase in extreme fire weather. The smaller proportion of significant positive trends found over Canada reflects the truncation of the time series, necessary because assimilation of precipitation observations over Canada ceased in the reanalysis post-2002.

Evaluation of the Canadian Precipitation Analysis (CaPA) to improve forest fire danger rating

The Fire Weather Index (FWI) System is the basic method of fire danger rating for fire management agencies in Canada. The spatial interpolation of this weather-based system can be difficult in areas where weather stations are sparse. Previous studies have shown that interpolation of precipitation is especially problematic owing to the isolated nature of convective summer storms. Environment and Climate Change Canada (ECCC) has recently developed the Canadian Precipitation Analysis (CaPA), a gridded precipitation product that integrates forecast data, station observations and Doppler radar, to improve precipitation estimates. The objective of the present study was to evaluate whether CaPA is indeed superior to the currently used interpolation method (thin-plate spline), and to assess the impact of the two precipitation estimates on the FWI System for the province of Ontario. Overall, the results showed the CaPA had increased skill in areas of radar coverage. In non-radar areas, where the ECCC station network was much sparser, there were less significant differences between the methods. Precipitation estimates from the CaPA also significantly improved the estimation of the Fine Fuel Moisture Code, Initial Spread Index and FWI in areas of radar coverage, improving estimates of potential fire danger.

Quantum demixing in binary mixtures of dipolar bosons

Quantum Monte Carlo simulations of a two-component Bose mixture of trapped dipolar atoms of identical masses and dipole moments, provide numerical evidence of demixing at low finite temperatures. Demixing occurs as a consequence of quantum statistics, which results in an effective attraction between like bosons. Spatial separation of two components takes place at low temperature with the onset of long exchanges of identical particles, underlying Bose-Einstein condensation of both components. Conversely, at higher temperature the system is miscible due to the entropy of mixing. Exchanges are also found to enhance demixing in the case of mixtures of nonidentical and distinguishable species.