Quantum Reflection of Bose condensates from solid surfaces
Ball rolling down a hill does not stop until it reaches the bottom. But in quantum mechanics, there is a chance that the ball will suddenly stop half-way down the hill and then roll back up to the top – a bizarre effect known as “quantum reflection”. Our recent theoretical studies of how Bose-Einstein condensates undergo quantum reflection from a silicon surface, helped to explain groundbreaking experiments (Pasquini et al, Physical Review Letters 93, 223201 (2004)] performed by Professor Wolfgang Ketterle's group at MIT. The experimental results were puzzling: at low incident velocities the reflection probability for the BEC decreased, contrary to single-particle predictions, and the atom cloud began to fragment. In collaboration with the MIT group, we showed that many-body effects play a crucial role in the quantum reflection process. In particular, by solving the time-dependent Gross-Pitaevskii equation, we demonstrated that the anomalously low reflection probabilities observed in experiment originate from the formation of vortex rings at low incident velocities as a result of interference between the incident and reflected parts of the BEC wavefunction. By contrast, at higher velocities, no vortices form and the BEC reflects cleanly. Our simulations of the condensate dynamics and plots of reflection probability, R, versus incident velocity , vx, accurately reproduced the experimental data, gave a clear physical explanation for the anomalies observed at low-velocity, and have general implications for atom optics and interferometry. We are currently investigating ways to make the reflected process stronger by using concepts from our work on advanced semiconductor devices to design new types of electronic “atom chips” that can manipulate very cold atoms.