Number 723, March 15, 2005
by Phil Schewe and Ben Stein
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.
“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, email@example.com),
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)
A Puzzling Signal in RHIC Experiments
A puzzling signal in RHIC experiments has now been explained by two
researchers as evidence for a primordial state of nuclear matter
believed to have accompanied a quark-gluon plasma or similarly
exotic matter in the early universe. Colliding two beams of gold
nuclei at Brookhaven's Relativistic Heavy Ion Collider (RHIC) in New
York, physicists have been striving to make the quark-gluon plasma,
a primordial soup of matter in which quarks and gluons circulate
However, the collision fireball has been smaller and
shorter-lived than expected, according to two RHIC collaborations
(STAR and PHENIX) of pions (the lightest form of quark-antiquark
pairs) coming out of the fireball. The collaborations employ the
Hanbury-Brown-Twiss method, originally used in astronomy to measure
the size of stars. In the subatomic equivalent, spatially separated
detectors record pairs of pions emerging from the collision to
estimate the size of the fireball.
Now an experimentalist and a
theorist, both from the University of Washington, John G. Cramer
(206-543-9194, firstname.lastname@example.org) and Gerald A. Miller
(206-543-2995, email@example.com), have teamed up for the
first time to propose a solution to this puzzle. Reporting
independently of the RHIC collaborations, they take into account the
fact that the low-energy pions produced inside the fireball act more
like waves than classical, billiard-ball-like particles; the pions'
relatively long wavelengths tend to overlap with other particles in
the crowded fireball environment.
This new quantum-mechanical
analysis leads the researchers to conclude that a primordial
phenomenon has taken place inside the hot, dense RHIC fireballs.
According to Miller and Cramer, the strong force is so powerful that
the pions are overcome by the attractive forces exerted by
neighboring quarks and anti-quarks. As a result, the pions act as
nearly massless particles inside the medium.
Such a situation is
believed to have existed shortly after the big bang, when the
universe was extremely hot and dense. As the pions work against the
attraction to escape RHIC's primordial fireball, they must convert
some of their kinetic energy into mass, restoring their lost
weight. But the pions' experience in the hot, dense environment
leaves its mark: the strong attractive force (and the absorption of
some of the pions in the collision) would make the fireball appear
reduced in size to the detectors that record the pions. According
to Miller, looking at the fireball using pions is like looking
through a distorted lens: the pions see the radius as about 7 fermi
(fm), about the radius of an ordinary gold nucleus, while the
researchers deduce the true radius of the fireball to be about 11.5
fm (Cramer, Miller, Wu and Yoon, Phys Rev Lett, tent. 18 March 2005).