Jason R. Williams

A cloud of 41K87Rb molecules held in an optical trap expands like fog at dawn. Each molecule is a weakly bound pair of potassium and rubidium atoms, teetering on the brink of dissociation. Cooling such a cloud to near absolute zero without shattering the molecules has long been a challenge.

An ordinary door swings both ways, but the physicists rigged a one-way door for atoms — enter at will, but leaving is off-limits. The atoms were cooled nearly to absolute zero — in such deep cold, they merge into a single cloud. This cloud was placed in a neat lattice of laser beams, adding a tiny irregularity, like a step at the threshold.