Bright matter-wave solitons: formation, dynamics and quantum reflection

The ripples that travel outwards from a pebble dropped in a pond are a familiar sight. Closer inspection reveals that each ripple or wave spreads out as it travels and in so doing decreases in height or amplitude until it has all but vanished by the time it reaches the edge of the pond. Remarkably, however, there exists a form of wave that does not spread out or disperse and which can therefore travel great distances without any change in amplitude. Such waves are known as solitons and were first observed as bow-waves produced by narrow boats on a canal in Scotland in 1834. Today solitons are seen in many different physical systems ranging from waves in plasmas to optical pulse propagation in nonlinear media. The latter example now finds important applications in long distance optical fibre communication systems. Common to all these examples is the existence of a nonlinear wave equation governing wave propagation in the system.Dilute gases of alkali atoms are now routinely cooled to within a millionth of a degree of absolute zero using laser light, permitting them to be confined in traps formed due to the interaction of an applied magnetic field with the minute magnetic moment of each atom. Further cooling by evaporation leads to the creation of a new state of matter, known as a Bose-Einstein condensate, in which the quantum mechanical nature of the particles dominates over their classical behaviour. The state of this system is also governed by a nonlinear wave equation in which the nonlinearity results from the atom-atom interactions. Moreover, if the atomic interactions in the system are attractive then the condensate can form a bright matter-wave soliton; a pulse or wave-packet of atoms which, just as for the bow-wave on the canal, does not spread out as it propagates. The objective of this proposal is to investigate the formation and dynamics of such solitons in condensates of rubidium-85 atoms. Collisions between rubidium-85 atoms exhibit a scattering resonance, known as a Feshbach resonance, which permits the precise control of the atomic interactions essential for a systematic investigation of soliton formation. Moreover, the use of optical dipole traps permits the real-time modification of the confinement potential and enables the manipulation of the position and velocity of the solitons for precise collision studies.Just as solitons have found applications in everyday life, the creation of bright matter-wave solitons offers potential future applications in atom interferometry and atom optics. This proposal will assess the feasibility of using matter-wave solitons to investigate the interaction between an atom and a solid surface, as part of the longer-term research goal to construct a tunable matter-wave surface probe . The attractive atom-surface interaction is a fundamental problem in QED and has a long and important theoretical history. It is only relatively recently, however, that this interaction has been measured experimentally. More recently, the high degree of control with which ultracold atoms can be manipulated has lead to several new experimental approaches to probe this interaction. Intimately connected to the measurement of atom-surface interactions is the phenomenon of quantum reflection, whereby a particle is reflected from a potential without reaching a classical turning point as a result of the wave nature of the particle. This proposal aims to demonstrate the quantum reflection of solitons from a solid surface, as a first step towards measuring the atom-surface interaction. The use of well-localized solitons, coupled to the precise control of their velocity, has the potential to take the study of atom-surface interactions to a new level. Such studies are motivated by the possibility that precision measurements of atom-surface interactions may, in the future, set new limits on short range corrections to gravity due to exotic forces beyond the Standard model.