After years of trying, scientists have coldly gone where no one has gone before. Two groups of physicists have created a strange quantum ensemble of atoms called Bose-Einstein condensate (BEC) out of a fundamentally different kind of particle, called a fermion. Up to now, BECs have been made only from particles called bosons. The achievement opens up a whole new area of research that might soon help scientists understand the mystery of high-temperature superconductivity.
The excitement stems from the difference between bosons and fermions, based on a quantum-mechanical property known as "spin." Spin comes in packets that physicists reckon in multiples of 1/2. Fermions are particles (such as the electron) that have a half-integer spin (1/2, 3/2, 5/2, and so on), whereas bosons (such as the photon) have an integer spin (0, 1, 2, and so on).
The distinction gives bosons and fermions very different properties. No two fermions can have the same quantum state: They can't have the same properties and be in the same place at the same time. Bosons can. So when cooled to a few billionths of a degree above absolute zero, bosons sink into the same quantum state, becoming, to all intents, a single, giant, ensemble atom: a BEC. Fermions such as potassium-40 or lithium-6, on the other hand, can't do this because each particle must have slightly different properties, even at the very coldest temperature. But in theory, fermion pairs act as bosons, and a gas of paired fermions should be able to condense into a BEC as well.
Now, for the first time, two groups have made this happen. The first, based at the University of Innsbruck in Austria, and the second, at the Joint Institute for Laboratory Astrophysics (JILA) in Boulder, Colorado, cooled and trapped lithium-6 and potassium-40 atoms respectively. Although the techniques differed slightly, they both used magnetic fields to induce the atoms to pair up and form a BEC. "Somewhere in the final stages of cooling" the fermions pair up, says the Innsbruck team leader Rudolf Grimm. "I don't know where." The Austrian work is described in a paper published online by Science this week, while the American paper has been submitted to Nature and is available in the arXiv preprint server.
"It's a big step," says Randall Hulet, a physicist at Rice University in Houston, Texas. "It's really very different and a major new direction." The work might lead to a way of studying high-temperature superconductivity, in which a process similar to Bose-Einstein condensation takes place.