Quantum Horse Races and Crystals of Light

Quantum Horse Races and Crystals of Light: Bloch’s team and others bring them to heel by cooling them to a temperature of nanokelvins and pouring them into an optical lattice, which, depending on your poetic frame of mind, you might call an optical egg crate or a crystal of light... The atoms are spaced perhaps 400 nanometers apart, so they reach a density of about 100 trillion atoms per cubic centimeter—which is a lot of atoms per cubic centimeter, but still only about a hundred-thousandth the density of hydrogen gas at room temperature and pressure. So these systems let physicists explore a domain they seldom otherwise enter, a frigid, sparse realm where quantum is king...

There are all sorts of other fun experiments you can do. Last year, Bloch’s team tracked the insulator-superfluid transition and showed that the system goes through a “hidden” phase of matter—a subtly patterned arrangement that conventional theory doesn’t capture...

Yet another experiment touches on the fundamental question of what determines the speed of events in the world... They began with an insulator, dialed up the interaction energy, and watched the atoms start to self-organize. A wave of activity spread though the system at twice the speed of sound. What governed the velocity was that atoms did not passively roll on the wave, but actively contributed to it. Some quantum gravity theorists have speculated that the speed of light represents the Lieb-Robinson bound of some underlying quantum system out of which space and time emerge.