Cool a gas of rubidium atoms to one-hundred-millionth of a degree above absolute zero or less and something strange happens. The atoms lose their individuality and merge into a single quantum state, forming what is known as a Bose-Einstein condensate (BEC). In this condensate atoms flow without friction, endowing the ultracold gas with the property of superfluidity. Scientists have known that much since 1995, when Eric A. Cornell of the National Institute of Standards and Technology and his colleagues created the first BEC in the laboratory¿an achievement that earned them the 2001 Nobel Prize in Physics. Now new research has taken that work one step further, revealing a surprising BEC behavior. It appears that under certain conditions, the condensate undergoes a reversible quantum phase transition, switching from a superfluid to a patterned fluid¿a new type of matter. The finding, announced today in the journal Nature, could aid efforts to build quantum computers.
To coax their rubidium BEC into switching states, Markus Greiner of the Ludwig-Maximilians University in Munich, Germany, and colleagues placed the quantum gas in an optical lattice¿a three-dimensional light interference pattern generated by laser beams. In the superfluid phase, the rubidium atoms moved freely in this landscape of high-energy peaks and low-energy valleys, with varying numbers of atoms settling into each valley. But when the investigators increased the intensity of the laser beams making up the optical lattice¿thereby heightening the energy landscape's peaks¿the atoms lost their freedom and each became trapped in a single valley, forcing the superfluid into an insulating phase. Decreasing the intensity of the laser fields lowered the peaks and freed the atoms, returning the gas to its superfluid state.
In a commentary accompanying the Nature report physicist Henk T. Stoof of Utrecht University in the Netherlands notes that the ideal array of single atoms created in the insulating phase lends itself to quantum computing. "Every rubidium atom has a magnetic moment and so has two internal states that may serve as the 0 and 1 of a quantum bit," he writes. Given the large number of rubidium atoms in the optical lattice, he says, they could provide the memory for a quantum computer. Moreover, "if there are two such memories that can be moved relative to each other, we can even make use of the interactions between atoms to perform a quantum computation," Stoof muses. "The first step towards this exciting goal has now been taken."