Bragg Reflection of Bose Condensates by Optical Lattices
In recent years, the interface between atomic and condensed matter physics has been explored using ultra-cold atoms in "optical lattices" formed by using laser beams to set up an electromagnetic standing wave. This forms a "crystal of light" in which the atoms are held together by the laser beams. When the atoms are cooled to ultra-low temperatures (below 1 micro Kelvin) they move through energy bands, just like electrons in a real crystal structure. But, in contrast to real crystals, the laser beams that bind the crystal of light can be switched off at any time. This makes optical lattices ideals for studying how quantum objects move through energy bands when they are subjected to forces. We are currently investigating the quantum dynamics of individual cold atoms and of Bose-Einstein condensates, comprising thousands of atoms, moving through crystals of light. Our recent prediction that lasers can cause condensates to undergo dramatic explosions stimulated new experiments at the University of Pisa, which have just observed this effect. This catastrophic destruction of the atom cloud is of particularly broad interest because it also has implications for understanding galactic structure, which is at the extreme opposite end of size scale to atomic condensates, but involves very similar equations.