PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 707 November 3, 2004
by Phillip F. Schewe, Ben Stein

ACCELERATOR FOR BECs.  Two research groups have banged quantum gases
together at record high velocities. Both groups begin by cooling clouds of
rubidium atoms to ultralow temperatures. Next, through magnetic
manipulation the clouds could be split into two separate clouds, each
containing a native population with a characteristic spin value. Physicists
in the Netherlands (FOM Institute for Atomic and Molecular Physics and the
University of Amsterdam) further cool the clouds to produce Bose-Einstein
condensates (BEC) before using the same magnetic control over the atoms to
urge the clouds back together again at an increasing speed. Earlier
experiments had managed to "collide" separate BEC samples at slow
speeds of mm/sec (slow in relation to the velocity of sound in the
BEC---several mm/sec) in order to observe characteristic interference
stripes, and affirm the intrinsic wavelike nature of BEC as a whole.  Now,
the Dutch experiment is able to achieve speeds of 20 cm/sec; in effect
their apparatus is a linear accelerator for BECs. The respective clouds are
about 10 microns in size; the relative size of the clouds and their initial
separation (up to record distances of 4 mm) is analogous to the separation
of two tennis balls on opposite sides of a tennis court. When the two
"tennis balls" collide, a spherical interference pattern shows up
(see animation at staff.science.uva.nl/~walraven/walraven/Highlights.htm).
Why is the higher speed important?  It's because below sound speed, the
superfluid BEC behaves like one giant matter wave, while above sound speed
the BEC behaves like a collection of individual atoms. So in this
experiment it is more accurate to think of 100,000 atoms (in the one cloud)
scattering with 100,000 atoms (in the other
cloud) rather then to think of two interacting clouds.  Furthermore,
because the speeds are still slow, the atom-atom collision can still be
thought of as being the collision of two waves (like separate ripples in a
pond passing through each other).  In other words, the experiment probes
the interaction between atoms rather than between BECs.  In the BEC
accelerator, matter waves of atom pairs are scattered out of the clouds at
an energy of 10^-7 eV. (Compare this to Fermilab's 10^12 eV energy scale.) 
These matter waves are a superposition of spherical-shaped "s"
and dumbbell-shaped "d" waves and hence show quantum mechanical
interference.  This interference is being directly imaged for the first
time (Buggle et al., Physical Review Letters, 22 October 2004; contact
Jeremie Leonard, jleonard{at}science.uva.nl), and yields accurate measurement
of the interaction properties between ultracold atoms. Comparable
observations are being reported by physicists from the University of Otago
in New Zealand, although in this experiment the atoms were at microkelvin
temperatures but did not constitute a BEC.  (Thomas et al., Physical Review
Letters, 22 October 2004; contact Niels Kaergaard, nk{at}physics.otago.ac.nz)

COOPER PAIRS UNPAIRED.  In a low-temperature superconductor electrons don't
travel singly but in weakly tethered pairs, Cooper pairs.  In a new
experiment at the Forschungszentrum Karlsruhe in Germany, physicists have
been able to send the two partners from Cooper pairs down separate wires
spaced more closely than the effective size of the Cooper pairs themselves
(see figure at www.aip.org/png).  The Cooper pairs (which have the property
that if one electron's spin is up, then the spin of its partner must be
down) start out in a piece of superconducting aluminum and proceed to a
frontier where they can travel down either of two normally-conducting and
magnetized iron wires.  (In general, when Cooper pairs move from a
superconducting into a normally-conducting material they can maintain their
pair status for a bit into the new material---a distance referred to as the
normal-metal coherence length---before breaking up.)  By magnetizing the
wires so as to filter out pairings of any electrons that don't have the
characteristic Cooper opposite-spin-orientation, and by varying the
distance between wires, and by measuring the resistance across the iron
wires, the experimenters can learn specific things about the Cooper pairing
mechanism (such as how large the pair is under various circumstances). 
This work is part of the larger study of spintronics---the exploitation of
electron spin for performing high-control electronics---and entangled
states---the quantum behavior in which two spatially separated objects have
a correlated behavior.  (Beckmann et al., Physical Review Letters, 5
November 2004; contact Detlef Beckmann, detlef.beckmann{at}int.fzk.de,
49-7247-82-6413

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