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@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 states 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 and 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. (Winkleret 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@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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