COOL things happen when atoms get close to absolute zero. But getting them
cold enough can be tricky. Now researchers have come a step closer to making one
type of atoms so cold that they behave like the electrons in a
superconductor—a quantum phenomenon never before seen in atoms.
Atoms come in two varieties: bosons have an even sum total of electrons,
neutrons and protons, while fermions have an odd total. That distinction makes
all the difference when a gas of atoms is cooled to near absolute zero.
Bosons like to clump together in the same quantum state, called a
Bose-Einstein condensate—a feat first achieved five years ago (New
Scientist, 22 July 1995, p 16). Fermions can’t form such a condensate, because
quantum mechanics forbids more than one fermion from occupying each quantum
state. Fermions effectively avoid each other, and if atoms don’t collide it’s
difficult to draw heat energy out of them. “If you don’t have collisions, you
don’t have thermal equilibrium and temperature is not well defined,” says
Deborah Jin of the National Institute of Standards and Technology in Boulder,
Colorado.
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However, the collisions don’t cease if bosons are mixed in with the fermions,
Randall Hulet and his colleagues at Rice University in Houston, Texas, will
report in a forthcoming issue of Science. The researchers cooled a mixture of
lithium-6 and lithium-7 atoms to within a fraction of a degree of absolute zero.
The lithium-7 bosons fell into the condensed state, and collisions between the
two types of atoms cooled the lithium-6 fermions enough to make them fill up the
lowest energy states of the container one by one, as if occupying the rungs of a
ladder. Previously, only Jin had observed this phenomenon, called “degeneracy”,
which is thought to keep neutron stars from collapsing under gravity.
The new technique could be used to reach a long-sought goal: getting fermions
to team up into “Cooper pairs”, just as electrons do in a superconductor. At
temperatures very near absolute zero, the fermions near the top of the energy
ladder should form Cooper pairs. But making the unsociable fermions cooperate
won’t be easy. “Cooper pairing isn’t going to happen this week or next,” Hulet
says. “But it could happen this year.”
