From: scerir (scerir@libero.it)
Date: Mon Mar 25 2002 - 01:22:08 MST
[FWD]
A MOLECULAR BEC has been achieved by Carl Wieman and his
colleagues at the University of Colorado. To be precise, what
Wieman reported at this week's APS March Meeting in
Indianapolis was the observation of a quantum superposition of
diatomic molecules and disassociated atoms in a trap. Having long
used Rb-87 in his BEC experiments, Wieman has as of late been
studying Rb-85 which, although it is harder to condense, possesses
just the right fine-grained set of quantum energy levels (hyperfine
levels) so that the application of a magnetic field can alter the
interaction force among the atoms in the trap, even as they reside
in the single quantum state which is the hallmark of Bose Einstein
condensates. By adjusting the magnetic field to be very close to the
point where the interatomic force goes from attractive to repulsive,
a "Feshbach resonance" occurs and some of the atoms form
molecules. The atoms and molecules are thought to be coherent
(share a single quantum state) at least locally, and maybe over
longer distances too. In this process the condensate appears first to
implode and then rebound somewhat like a supernova, even to the
extent of sending out jets of particles and leaving behind a
remnant. The physics behind this "Bosenova" behavior is still a
mystery.
Wolfgang Ketterle of MIT, like Wieman a winner of the 2001
Nobel Prize in physics for BEC discoveries, spoke at the same
APS session and reported findings in three areas. (1) He has used
a sodium-23 BEC to help cool a gas of lithium-6. Li-6 is a
fermionic atom (one with a half-integral net spin). The Pauli-
exclusion principle forbids such atoms from falling into the single
state available to bosonic atoms such as Na-23, but the Li-6 atoms
can, if cooled low enough, occupy all the lowest energy quantum
states possible. This has now been done in the MIT experiment,
the first time such a "degenerate Fermi sea" has co-existed with a
large BEC. One wants to see how such a fermi gas behaves at nK
temperatures and whether the atoms can be coaxed (by
manipulating the interaction between them) into forming Cooper
pairs, becoming thereby a superfluid.
(2) Ketterle reported the propagation of a condensate in a
magnetic waveguide. First, his group made a large (2 million
atoms) BEC in the usual way (in a magnet trap), then loaded it into
a magnetic trap by 40 cm, and finally loaded it into a microtrap on
a printed circuit board. The micro-journey around the chip was
partly smooth and partly bumpy, especially when the cigar-shaped
BEC came toward a Y divide. (Such a beam splitter would be a
useful step toward making an interferometer for atom waves.) At
the divide the condensate wiggled itself into a snake shape. Close
to the chip surface, the condensate broke up into several detached
segments. Future atom chips will need better control of surface
roughness.
(3) Another goal is the generation of pair correlated atoms.
Ironically, the atoms in a condensate all share a single quantum
state but are not otherwise entangled. The MIT researchers have
created two BEC blobs (let us call them 1 and 2) together with
another small "seed" condensate (blob 3). The elastic collision of
these blobs produced a fourth blob in a process called four-wave
mixing (for an earlier version of this experiment, at NIST.
In effect the atoms in blobs 1 and 2 help to amplify
blob 3 (a gain of 20, in this case). For each atom added to blob 3
one atom is put into blob 4. This created two pair-correlated
atomic beams. In some future experiment this pair correlation
might be verified directly if one could detect single atoms in the
two condensates, which are moving off in opposite directions.
Right now it is difficult to spot single neutral atoms in a BEC.
Single-atom detection is likely in helium BECs since the atoms,
deliberately put into an excited state in order to confine and cool
them in the first place, are easily ionized, making it far easier to detect
them. Chris Westbrook, a member of Alain Aspect's team at Orsay,
summarized recent helium work and described a scheme for
producing helium molecules within a BEC. This, he said, might
allow an atom-wave equivalent to the current process of down-
conversion, by which UV photons can be converted, in a special
crystal, into a pair of lower-energy but entangled photons (if one
photon has a horizontal polarization, the other must have a vertical
polarization. A beam of related atoms could,
analogously, be sundered into beams of pair-correlated atoms.
Finally, Jakob Reichel (Max Planck/Univ Munich), a member
of one of Ted Hansch's groups, said at the APS meeting that he and
his colleagues were aiming to achieve single-atom detection in
rubidium condensates. Furthermore, he hoped that the single
atoms, maneuvered into resonant cavities, might be able to carry
out quantum computing chores.
http://www.aip.org/enews/physnews/2002/split/581-1.html
http://www.nist.gov/public_affairs/bosenova.htm
http://www.nature.com/nsu/010322/010322-3.html
http://www.aip.org/physnews/update/530-1.html
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