When ultracold is not cold enough

Abstract

A technique for cooling ultracold atoms in optical lattices has been demonstrated. This advance should allow the physics of strongly correlated systems, including that of quantum magnetism, to be explored. See Letter p.500 This work demonstrates an atom-number-sensitive blockade mechanism with potential applications in condensed matter physics and quantum information processing. Blockade can occur when strong interactions in a confined few-body system prevent a particle from occupying an otherwise accessible quantum state. The authors observe a new form of interaction blockade for ultracold atoms in optical lattices, which they term orbital excitation blockade. In this system, a single atom on a lattice site can be excited to a higher orbital by resonantly modulating the lattice depth. But when two atoms occupy the same site, interactions between them lead to orbital-dependent energy shifts. Therefore, modulation at an appropriate frequency to excite one atom to the higher orbital is off-resonant for exciting the second, which is 'blocked'. The effect is used here to demonstrate algorithmic cooling with ultracold atoms, potentially important for reaching the ultralow entropies required for quantum simulation. In addition, orbital excitation blockade could enable the implementation of quantum gates in optical lattices.