An extended Falicov-Kimball model on a triangular lattice

The combined effect of frustration and correlation in electrons is a matter of considerable interest of late. In this context a Falicov-Kimball model on a triangular lattice with two localized states, relevant for certain correlated systems, is considered. Making use of the local symmetries of the model, our numerical study reveals a number of orbital ordered ground states, tuned by the small changes in parameters while quantum fluctuations between the localized and extended states produce homogeneous mixed valence. The inversion symmetry of the Hamiltonian is broken by most of these ordered states leading to orbitally driven ferroelectricity. We demonstrate that there is no spontaneous symmetry breaking when the ground state is inhomogeneous. The study could be relevant for frustrated systems like $GdI_2$, $NaTiO_2$ (in its low temperature C2/m phase) where two Mott localized states couple to a conduction band.Comment: 6 pages, 8 figure

Topological Quantum Computing with Only One Mobile Quasiparticle

In a topological quantum computer, universal quantum computation is performed by dragging quasiparticle excitations of certain two dimensional systems around each other to form braids of their world lines in 2+1 dimensional space-time. In this paper we show that any such quantum computation that can be done by braiding $n$ identical quasiparticles can also be done by moving a single quasiparticle around n-1 other identical quasiparticles whose positions remain fixed.Comment: 4 pages, 5 figure

Braid Topologies for Quantum Computation

In topological quantum computation, quantum information is stored in states which are intrinsically protected from decoherence, and quantum gates are carried out by dragging particle-like excitations (quasiparticles) around one another in two space dimensions. The resulting quasiparticle trajectories define world-lines in three dimensional space-time, and the corresponding quantum gates depend only on the topology of the braids formed by these world-lines. We show how to find braids that yield a universal set of quantum gates for qubits encoded using a specific kind of quasiparticle which is particularly promising for experimental realization.Comment: 4 pages, 4 figures, minor revision

Randomisation of Pulse Phases for Unambiguous and Robust Quantum Sensing

We develop theoretically and demonstrate experimentally a universal dynamical decoupling method for robust quantum sensing with unambiguous signal identification. Our method uses randomisation of control pulses to suppress simultaneously two types of errors in the measured spectra that would otherwise lead to false signal identification. These are spurious responses due to finite-width $\pi$ pulses, as well as signal distortion caused by $\pi$ pulse imperfections. For the cases of nanoscale nuclear spin sensing and AC magnetometry, we benchmark the performance of the protocol with a single nitrogen vacancy centre in diamond against widely used non-randomised pulse sequences. Our method is general and can be combined with existing multipulse quantum sensing sequences to enhance their performance

Evaluations of NAM forecasts on midtropospheric perturbation-induced convective storms over the U.S. northern plains

In the U.S. northern plains, summer progressive convective storms that occur in weakly forced environ- ments are often coupled with short-wave perturbations that are embedded in the midlevel northwesterly flow. These midtropospheric perturbations (MPs) are capable of inducing propagating convection that contributes to a majority of the rainfall over the northern plains during July and August. There is a possibility that the difficulties of numerical weather prediction models in forecasting summer convective rainfall over the northern plains are partly attributed to their deficiency in forecasting MPs. The present study tests this possibility through examining operational forecasts by the North American Mesoscale (NAM) model during the summers of 2005 and 2006. Forecasted MPs exhibit slower propagation speeds and weaker relative vorticity than the observations leading to systematic position errors. Underpredicted vorticity magnitudes weaken horizontal vorticity ad- vection that influences the vorticity tendency throughout the MP life cycle and, in turn, slows the propagation speed of MPs. Moreover, biases of weak ambient flow speed and vortex stretching contribute to the mag- nitude and propagation speed errors of MPs. Skill scores of precipitation forecasts associated with MPs are low, but can be considerably improved after removing the MP position error that displaces the rainfall pat- tern. The NAM also tends to underpredict precipitation amounts. A modified water vapor budget analysis reveals that the NAM insufficiently generates atmospheric humidity over the central United States. The shortage of moisture in the forecast reduces the water vapor flux convergence that is part of the precipitation process. The precipitation bias may feed back to affect the MP growththrough the bias in heating, thus further slowing the perturbation

Assessing a novel contact heater as a new method of recovering explosives traces from porous surfaces

It can be very challenging to recover explosives traces from porous surfaces, such as clothing and car seats, compared to non-porous surfaces. The contact heater has been developed as a novel instrument designed to recover explosives traces from porous surfaces. Samples are taken by heating and drawing air across a surface, with the air flowing through a sampling cartridge containing adsorbent polymer beads, which act to trap any recovered explosive material. Any collected explosive can then be eluted from this cartridge using a solvent, prior to analysis. This paper outlines work performed to evaluate the usefulness of the contact heater with regards to the recovery of explosives traces from porous materials. Ethylene glycol dinitrate (EGDN) and triacetone triperoxide (TATP) were chosen as two representative explosives for this study. Quantification was performed using GC–MS for EGDN and LC–MS/MS for TATP. Different sampling temperatures, sampling times and elution solvents were investigated. Recovery was trialled from leather, carpet and denim. Recoveries of up to 71% were obtained following optimisation. It was also possible to recover TATP from fabrics exposed to TATP vapour in a vapour-laden jar up to two hours after exposure. The contact heater therefore appears to be a very useful tool for the recovery of explosives traces from porous materials

Modulated Entanglement Evolution Via Correlated Noises

We study entanglement dynamics in the presence of correlated environmental noises. Specifically, we investigate the quantum entanglement dynamics of two spins in the presence of correlated classical white noises, deriving Markov master equation and obtaining explicit solutions for several interesting classes of initial states including Bell states and X form density matrices. We show how entanglement can be enhanced or reduced by the correlation between the two participating noises.Comment: 9 pages, 4 figures. To be published in Quantum Information Processing, special issue on Quantum Decoherence and Entanglemen