PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 788 August 10, 2006  by Phillip F. Schewe, Ben Stein,
and Davide Castelvecchi        www.aip.org/pnu

ATOMS IN A TRAP MEASURE GRAVITY at the micron level.  Nowadays many
of the most sensitive measurements in science depend on some quantum
phenomenon which very subtly can often be exploited to gain maximum
precision.  In an experiment conducted at the Universita di Firenze
(University of Florence) the quantum phenomenon in question is
called Bloch oscillation.  This weird effect occurs when particles
subject to a periodic potential (such as electrons feeling the
regular gridlike electric force of a crystalline lattice of atoms)
are exposed to an additional static force, say, an electric force in
a single direction; what happens is that the electrons do not, as
you would expect, all move in the direction of the force, but
instead oscillate back and forth in place.  In a new experiment
conducted by Guglielmo Tino and his Florence colleagues, the
particles are supercold strontium atoms held in a vertically
oriented optical trap formed by criss-crossing laser beams, while
the static force is merely the force of gravity pulling down on the
atoms (see figure at http://www.aip.org/png/2006/263.htm ).
What are the unique features of this experiment?  First of all,
although Bloch oscillations have been observed before, they have
never been sustained for as long as 10 seconds, which is the case
here.  Experiments that mix gravity and quantum mechanics are rare.
Furthermore, even though the cloud of Sr atoms in use do not exist
in the form of a Bose-Einstein condensate (BEC), the atoms do absorb
the trapping laser light in a coherent way; that is, they absorb the
light in a stimulated (not random) way.  They quickly re-emit the
light and then absorb still another photon.  The number of photons
per atom transferred in this way--in the thousands rather than
tens--is the largest ever for a physics experiment.  Finally, close
observation of the Bloch oscillations allows you to measure the
strength of the static force, gravity, with high precision--in this
case to measure gravity with an uncertainty of a part in a million.

With planned improvements to the apparatus, the researchers be able
to bring the atoms to within a few microns of a test mass and will
measure g with an uncertainty of 0.1 parts per million.  With these
conditions, one can probe theories which say that gravity should
depart from the Newtonian norm, perhaps signifying the existence of
unknown spatial dimensions.  According to Tino
(guglielmo.tino{at}fi.infn.it, 39-055-457-2034) unlike
gravity-measuring experiments which use torsional balances or
cantilevers, the Florence approach measures gravity directly and
over shorter  distances.  The atom-trap setup should also prove
useful for future inertial guidance systems and optical clocks.
(Ferrari et al., Physical Review Letters, 11 August 2006)

THE SHARPEST OBJECT YET MADE is a tungsten needle tapering down to
about the thickness of single atom.  The needle, made by postdoc
Moh'd Rezeq in the group of  Robert Wolkow at the University of
Alberta and the National Institute for Nanotechnology, starts out
much blunter.  Exposed to a pure nitrogen atmosphere, however, a
rapid slimming begins.  To start with the tungsten is chemically
very reactive and the nitrogen roughens the tungsten surface. But at
the tip, where the electric field created by applying a voltage to
the tungsten is at its maximum, N2 molecules are driven away.  This
process reaches an equilibrium condition in which the point is very
sharp.  (For a single picture, go to
http://www.aip.org/png/2006/264.htm; for a movie showing the
evaporation process all the way down to a single atom at the tip,
see the website at www.phys.ualberta.ca/~wolkow )  Furthermore, what
N2 is present near the tip helps to stabilize the tungsten against
further chemical degradation.  Indeed, the resultant needle is
stable up to temperatures of 900 C even after 24 hours of exposure
to air.
The probe tips used in scanning tunneling microscopes (STMs), even
though they produce atomic-resolution pictures of atoms sitting on
the top layer of a solid material, are not themselves atomically
thin.  Rather their radius of curvature at the bottom is typically
10 nm or more.  Wolkow (rwolkow{at}ualberta.ca) says that although a
narrower tip will be useful in the construction of STM arrays (you
can pack more tips into a small area; and a wide array might even
permit movies of atomic motions) the spatial resolution won't
improve thereby.  The real benefit of the sharp tungsten tips, he
believes, will be as superb electron emitters.  Being so slender,
they would emit electrons in a bright, narrow, stable stream.
(Rezeq, Pitters, Wolkow, Journal of Chemical Physics, 28 May 2006)

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