BECs Undergo Bragg Explosion

Bose Einstein condensates (BEC) provide a versatile testbed for looking at quantum phenomena. And maybe cosmology too. In their calculations, physicists at the University of Nottingham first load an alkali BEC into an optical lattice, a honeycomb of laser light which holds atoms in a 3D gridwork. (For another recent BEC-in-a-lattice story, see Physics News Update #626) Then they jar the cloud of atoms, setting the BEC into motion, and have it scatter from the same "crystal" of light beams. Instead of x rays undergoing Bragg scattering from crystallized protein, the BEC waves scatter from a crystal of light. But as it threads through the optical lattice, the pattern of Bragg reflections can create traveling zones (essentially self-perpetuating solitons and local whirlpools, or vortices) where atoms in the condensate are actually excluded (see figure). These solitons can in turn destabilize the BEC, causing it to explode outward. The Nottingham researchers have been trying to model this explosion using a nonlinear Schrodinger equation, a modified version of the equation that governs electron waves inside atoms. According to Mark Fromhold (, 44-0115-9515192), similar equations are being used in the statistical study of galaxy distribution. (See for example, Scott et al., Physical Review Letters, 21 Mar 2003)