PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 621 January 17, 2003   by Phillip F. Schewe, Ben Stein, and James
Riordon

SCATTERED CHATTER.  Tall buildings are often the bane of cell phone users
when making calls in big cities, but such scattering structures may someday
enhance communication. Researchers at the Laboratoire Ondes et Acoustique in
Paris recently demonstrated the effect with ultrasonic antennas in a
water-filled tank. When the space between a 23-element transmitter array and
an array of
5 receivers was devoid of scattering structures, the error rate in
transmission of a set of 5 messages sent simultaneously to the receivers was
about 28%. By placing a forest of randomly arranged steel rods between the
transmitter and receivers, the researchers found that they could transmit
the same messages without any error at all. The effect is due to the fact
that, without scatterers, there is a substantial amount of cross-talk
between the receivers - that is, each antenna detects some of the data
intended for its neighbors, and has no way to distinguish between the
message it is supposed to receive and the messages it should ignore. By
adding scatterers, the researchers ensured that signals intended for
different antennas were distinct. The situation can be understood by
imagining a single antenna sending a signal to multiple receivers. Without
scatterers, each receiver detects
very nearly the same thing. Adding scatterers distorts the signal, and each
antenna detects something slightly different. In fact, by reversing the
experiment and sending signals from each of the receivers back to the
transmitter (an arrangement known as a time-reversal antenna, Update 190) it
is possible to ensure that with the scatterers, a transmitter array can send
multiple unique signals that are only detectable by the intended receivers.
In essence, scatterers make it possible to establish multiple communication
channels, and the more scatterers between the transmitters and receivers,
the more channels that are available. For the time being, the communication
technique is limited to ultrasonic communication - the electronics necessary
for exploiting scatterers with wide-band time-reversal antennas at cell
phone frequencies simply don't yet exist. But when they
are developed, the buildings that currently hamper wireless communication
will become a cell phone user's boon. (A. Derode et al, Physical Review
Letters, 10 January 2003)

A STRONGLY INTERACTING DEGENERATE FERMI GAS, an ultracold lithium-6 gas
which expands in a strangely lopsided fashion, has been produced for the
first time by Duke University researchers (John Thomas, 919-660-2508,
jet{at}phy.duke.edu). Performed on a tabletop, these results can provide
universal insights into all strongly interacting fermions, including the
neutrons in neutron stars, the quarks in atomic nuclei, and the electrons in
superconductors.  In
addition to producing never-before-seen behavior in fermions, this
experiment may have provided the first evidence of a previously unseen
fermion-pairing phenomenon called "resonance superfluidity." The specially
prepared lithium-6 gas behaves in a markedly different fashion from ordinary
gases, those whose atoms essentially do not interact with one another.  When
a cloud of ordinary gas expands in a vacuum, it usually spreads out with
equal speed in all directions.  This means that a spherical cloud becomes a
larger sphere.   Even a cigar-shaped cloud smooths out the differences in
its dimensions and adopts a spherical shape.
But something very different occurs in the lithium-6 gas, whose atoms
interact with the maximum amount allowed by the laws of quantum mechanics.
Trapped in a laser beam and cooled to 800 nanokelvins by optical methods,
the gas cloud started off with the shape of a vertical cigar. But when
released from its laser trap, the cloud hardly expanded in the vertical
direction, but it  spread out rapidly in the horizontal direction.  The
cloud ended up as a wide, horizontal ellipsoid (a 3D ellipse; see figures at
http://www.aip.org/mgr/png/2002/175.htm).
What had happened? The researchers had created a special version of a
degenerate Fermi gas (see Updates 447,
http://www.aip.org/enews/physnews/1999/split/pnu447-1.htm, and
580, http://www.aip.org/enews/physnews/2002/split/580-1.html). "Degenerate"
means that the deBroglie wavelength of the fermion atoms is greater than the
average distance between them, causing the atoms to "overlap" with each
other just as bosons overlap with each other in a Bose-Einstein condensate.
But in previous degenerate Fermi gases, the atoms did not interact strongly
with one another.
In this experiment, the researchers used a magnetic field that caused the
lithium atoms to interact quite strongly with one another, to an extent
never before reached in a degenerate Fermi gas.  As a result, each atom
interacted with its kin over a region significantly larger than the average
distance between atoms. Because of the strong interactions among the atoms,
Thomas says, the gas completely changed its own shape while spreading out.
To explain fully this
"anisotropic expansion," the researchers suggest two possibilities, neither
of which they can distinguish at the present time: Either they were
observing a new kind of long-range collision between atoms, or they
witnessed resonance superfluidity, a relatively high-temperature form of
superfluidity that would be triggered by tuning the interactions between
fermions. (O'Hara et al., Science, 13 December 2002.)

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PHYSICS NEWS UPDATE is a digest of physics news items arising
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