Condensed Matter Theory

Abstract:

We model a reaction-diffusion-convection system which comprises a liquid drop containing solutes that undergo an Oregonator reaction producing chemical waves. The reactants are taken to have surfactant properties so that the variation in their concentrations induces Marangoni flows at the drop interface which lead to a displacement of the drop. We discuss the mechanism by which the chemical-mechanical coupling leads to drop motion and the way in which the net displacement of the drop depends on the strength of the surfactant action. The equations of motion are solved using a lattice Boltzmann approach.

Abstract:

We describe the consequences of time-reversal invariance of the Stokes equations for the hydrodynamic scattering of two low-Reynolds-number swimmers. For swimmers that are related to each other by a time-reversal transformation, this leads to the striking result that the angle between the two swimmers is preserved by the scattering. The result is illustrated for the particular case of a linked-sphere model swimmer. For more general pairs of swimmers, not related to each other by time reversal, we find that hydrodynamic scattering can alter the angle between their trajectories by several tens of degrees. For two identical contractile swimmers, this can lead to the formation of a bound state.

Abstract:

Topological quantum computation has emerged as one of the most exciting approaches to constructing a fault-tolerant quantum computer. The proposal relies on the existence of topological states of matter whose quasiparticle excitations are neither bosons nor fermions, but are particles known as non-Abelian anyons, meaning that they obey non-Abelian braiding statistics. Quantum information is stored in states with multiple quasiparticles, which have a topological degeneracy. The unitary gate operations that are necessary for quantum computation are carried out by braiding quasiparticles and then measuring the multiquasiparticle states. The fault tolerance of a topological quantum computer arises from the nonlocal encoding of the quasiparticle states, which makes them immune to errors caused by local perturbations. To date, the only such topological states thought to have been found in nature are fractional quantum Hall states, most prominently the ν=5/2 state, although several other prospective candidates have been proposed in systems as disparate as ultracold atoms in optical lattices and thin-film superconductors. In this review article, current research in this field is described, focusing on the general theoretical concepts of non-Abelian statistics as it relates to topological quantum computation, on understanding non-Abelian quantum Hall states, on proposed experiments to detect non-Abelian anyons, and on proposed architectures for a topological quantum computer. Both the mathematical underpinnings of topological quantum computation and the physics of the subject are addressed, using the ν=5/2 fractional quantum Hall state as the archetype of a non-Abelian topological state enabling fault-tolerant quantum computation. © 2008 The American Physical Society.

Abstract:

The thermophoretic motion of a charged spherical colloidal particle and its accompanying cloud of counterions and coions in a temperature gradient is studied theoretically. Using the Debye-Hückel approximation, the Soret drift velocity of a weakly charged colloid is calculated analytically. For highly charged colloids, the nonlinear system of electrokinetic equations is solved numerically, and the effects of high surface potential, dielectrophoresis, and convection are examined. Our results are in good agreement with some of the recent experiments on highly charged colloids without using adjustable parameters.