How cold is cold enough? Eliminating entropy picokelvins from absolute zero

The team demonstrated their technique with two experiments, using a gas of rubidium-87 atoms in a square optical lattice. In the first, they started with a known number of atoms at each site (between one and four) all at the ground energy level. Then, by modulating the frequency, they gradually removed all the extra atoms, finishing with only one in each lattice site—a minimal entropy configuration.

In the second experiment, instead of starting with a known number of atoms all at the ground level, they loaded the lattice with a random number per site, with some excited and some at the ground level. As before, by sweeping the frequency, they removed all the extra atoms.