She is able to manipulate light this way because the density and the temperature of the BEC slows pulses of light down. (She recently took the experiments a step further, stopping a pulse in one BEC, converting it into electrical energy, transferring it to another BEC, then releasing it and sending it on its way again.) Hau uses BECs to discover more about the nature of light and how to use "slow light"—that is, light trapped in BECs—to improve the processing speed of computers and provide new ways to store information.
Not all ultracold research is performed using BECs. In Finland, for instance, physicist Juha Tuoriniemi magnetically manipulates the cores of rhodium atoms to reach temperatures of 180 trillionths of a degree F above absolute zero. (The Guinness record notwithstanding, many experts credit Tuoriniemi with achieving even lower temperatures than Ketterle, but that depends on whether you're measuring a group of atoms, such as a BEC, or only parts of atoms, such as the nuclei.)
It might seem that absolute zero is worth trying to attain, but Ketterle says he knows better. "We're not trying," he says. "Where we are is cold enough for our experiments." It's simply not worth the trouble—not to mention, according to physicists' understanding of heat and the laws of thermodynamics, impossible. "To suck out all the energy, every last bit of it, and achieve zero energy and absolute zero—that would take the age of the universe to accomplish."
Tom Shachtman is the author of Absolute Zero and the Conquest of Cold, the basis for a future PBS "Nova" documentary.