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Unfriendly Atoms Coaxed Into Quantum Harmony
10 September 1999 7:30 pm
In 1995, physicists captured the world's imagination by creating a new kind of matter--the Bose-Einstein condensate (BEC), in which supercold atoms called bosons lock into a kind of superatom. Now, reported in today's Science, another team has gone further by coaxing atoms of another, less cooperative ilk, known as fermions, into a similar low-temperature quantum state.
The creation of a cold fermion condensate could be the first step toward more accurate atomic clocks and strange new substances that behave like superconductors and superfluids. "The experiment is ingenious, and the challenge of cooling a gas of fermions is considerable," says Dan Kleppner of the Massachusetts Institute of Technology.
Part of the challenge was that the original BEC technique won't cool fermions. Bosons, which include many atoms, are particles whose "spin"--a property that makes them act like tiny bar magnets--is zero or an integer value. They are happy to be neighbors even when their energies are identical. But two fermions--all other atoms and particles, which have half-integer spin--can't stand each other's company if they are in the same state. That unfriendliness means that identical fermions can't be made to collide, and collisions were crucial to removing energy from the atoms to make the original condensate.
To get around this problem, physicist Deborah Jin and grad student Brian DeMarco of JILA, a government/university lab in Boulder, Colorado, placed a gas of fermions--in this case, potassium atoms--in a magnetic field. The field created many more energy levels, each a different quantum state. Fermions in various quantum states are not forbidden from colliding--and thus removing energy from their neighbors. "It's just a beautiful scheme," says Kleppner.
When Jin and DeMarco had succeeded in cooling the gas down to about 300 nanokelvin (0.3 millionth of a degree above absolute zero), they saw the critical signs of the atoms locking into a low-energy quantum state. One sign was that the energy of the potassium vapor was ever-so-slightly higher than expected for a classical gas at the same temperature, because the exclusion principle, as the quantum unfriendliness of fermions is called, prevents fermionic atoms from all dropping to the lowest energy level. Physicists hope that if a fermionic atomic vapor can be cooled to still lower temperatures, the atoms will pair up to form a kind of atomic superconductor.