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.
“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
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, firstname.lastname@example.org),
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.
al., Physical Review Letters, March 4, 2005; lab website, www.quantumoptics.ethz.ch)