DEGENERATE GAS STUCK IN OPTICAL LATTICE. The forces that govern the
motions of macroscopic objects like planets and tennis balls are
complicated enough. Forces among atoms at ultracold temperatures
are even more complicated. In this regime atoms (pictured as being
waves) spread out so much that they overlap with neighboring atoms.
If the atoms are bosons (that is, if the total spin of each atom is
an integer) then they all fall into a single quantum state, namely a
Bose Einstein condensate (BEC). If, however, the atoms are fermions
(the total spin is half-integral-valued), then quantum reality, in
the form of the Pauli exclusion principle, also decrees a special
status: not a single ensemble BEC state (all atoms having the same
energy), but a state in which none of the atoms has the same
energy. In this "Fermi degenerate" state the atoms fill up all
possible quantum energy levels, one by one (or two by two, providing
that the two atoms sharing a level have opposite spins), until the
last atom is accounted for. (For the first demonstration of a Fermi
degenerate state in atoms, see
www.aip.org/pnu/1999/split/pnu447-1.htm.) Now, physicists at the
ETH lab in Zurich have, for the first time, not only made a quantum
degenerate Fermi gas but have been able to load the atoms into the
criss-cross interstices of an optical lattice, an artificial 3D
crystal in which atoms are held in place by the electric fields of
well-aimed laser beams. Then, by adjusting an external magnetic
field, the pairs of atoms lodged in their specified sites can be
made to interact (courtesy of the "Feshbach resonance") with a
varying strength.   According to Tilman Esslinger (41-1-633-2340,
esslinger@phys.ethz.ch), it is this ability to put atoms where you
want them in a crystal-like scaffolding, and then to make them
interact with a strength that you can control, that makes this setup
so useful. It might be possible to test various condensed matter
theories, such as those that strive to explain high-temperature
superconductivity, on a real physical system. (Kohl et al.,
Physical Review Letters, March 4; lab site,
www.quantumoptics.ethz.ch
)