Text 227, 72 rader
Skriven 2004-11-05 08:25:22 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 707
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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@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@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@int.fzk.de, 49-7247-82-6413

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 * Origin: Big Bang (1:106/2000.7)