Physics News Update
Number 744, September 6, 2005
by Phil Schewe and Ben Stein

Atom-Molecule Dark States
Physicists at the University of Innsbruck have demonstrated that atom
pairing (molecule formation) in Bose-Einstein condensates (BECs) using
photoassociation is coherent. Coherent pairing of atoms (locking them into
a particular quantum relationship) has been observed before using a tuned
magnetic condition---a Feshbach resonance---between the atoms. But
molecules made that way are only feebly attached. By contrast the process
of photoassociation---i.e. using light to fuse two atoms into one
molecule---allows more deeply bound molecule states to be established. The
trouble is that the same laser light can also be absorbed to dissociate the
molecules rather than only perform its associative task. The counter
measure used by the Innsbruck researchers (contact Johannes Hecker
Denschlag, 43-512-507-6340, johannes.denschlag{at}uibk.ac.at) is to create a
"dark state" in which the light cannot be absorbed. A dark state
is a special quantum condition: it consists of three quantum energy levels,
two stable ground sta
tes and one excited level. If laser light at the two frequencies needed for
the transitions from both the ground states to the excited state are
present simultaneously, the two excitations (from the two lower energy
states) can destructively interfere with each other if there is phase
coherence between the ground states. (Homely example: offer one cookie to
two children and, if they fall into the right kind of arguing, the cookie
goes uneaten.) The consequence is that no light gets absorbed and the
molecules are stable. Such "electromagnetically induced
transparency" has been observed before for transitions within atoms
(PNU 343) but the Innsbruck scientists are the first to use it for a
transition between a BEC of atoms and molecules. In their experiments, the
same (two-color) laser light that creates the dark state is also the light
that photoassociates rubidium atoms into molecules. Johannes Hecker
Denschlag says that atom-molecule dark states are a convenient tool to
analyze the atom-molecule system a
nd to optimize the conversion of atomic into molecular BECs. BECs of
ultracold molecules represent, because of their many internal degree of
freedom (vibrational and rotational), a new field of research beyond atomic
BECs. (Winkler et al., Physical Review Letters, 5 August 2005; lab website,
www.dark.ultracold.at)

Measuring Nanotubes' Conductivity
How well nanotubes conduct electricity depends a lot on their environment.
Hongjie Dai and his colleagues at Stanford have made the first electrical
measurements of currents flowing under high voltage (high bias) through
single-walled carbon nanotubes suspended like miniature power lines. They
discovered that in suspended form a micron-scale-long nanotube could carry
about 5 micro-amps of current, whereas lying in the plane of a substrate
the same tube can carry about 25 micro-amps. The reason for the better
in-the-plane performance is that the substrate helps to dampen
"optical phonons," high-energy vibrations of the nanotube atomic
lattice. Dai (650-723-4518, hdai1{at}stanford.edu) believes that with careful
engineering of the interface between a nanotube and a substrate, maximum
currents could be raised to higher levels than previously possible, which
might make carbon nanotubes useful for applications in high-power
transistors and even nanoscale transmission lines. To make the kind of
transmission lines you see in the countryside out of nanotubes, you'd have
to develop a process for producing km-length carbon tubes, which is not
feasible for the foreseeable future. (Pop et al., Physical Review Letters,
upcoming article)

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