Number 297, November 27, 1996 by Phillip F. Schewe and Ben Stein
FERMILAB HAS MADE ANTIHYDROGEN ATOMS . Comparable to CERN's experiment
of a year ago (Update 253), the Fermilab effort involves sending a beam
of antiprotons through a gas jet. In collisions with atoms in the jet electron-
positron pairs are sometimes made, and in rare cases the positron will
bond with the antiproton to form an antihydrogen atom. Unfortunately, it
is not possible in this kind of arrangement to capture and study the anti-atoms.
(Fermilab press release, 21 November.)
A REFRACTIVE LENS FOR X RAYS , until now impractical because lenses absorb
too much or refract too little at x-ray wavelengths, has been developed
by scientists at the European Synchrotron Radiation Facility (ESRF) in
Grenoble. The compound lens consists simply of a series of closely-spaced
0.6- mm holes drilled in a piece of aluminum. With this lens, a 14- keV
beam of x rays was focused to a an 8-micron spot size. (A. Snigirev et
al., Nature, 7 November 1996.)
QUANTUM DECOHERENCE , the collapse of a quantum superposition (a uniquely
quantum-mechanical phenomenon in which an atom, photon, or other quantum
system acts as if it simultaneously exists in more than one state) into
a single definite state, has been quantitatively measured for the first
time. Serge Haroche and his co-workers at the Ecole Normale Superieure
in Paris (haroche@physique.ens.fr) send individual rubidium atoms-- each
of which is in a superposition of two states--through a cavity containing
a microwave field. Each of the two quantum states shifts the phase of the
microwave field by a different amount--so the field also falls into a superposition
of two states. However, as the cavity field exchanges energy with its surroundings,
the superposition can collapse into a single definite state. The researchers
measured this decoherence by measuring correlations between the energy
levels of pairs of atoms sent through the cavity with various time delays
between the atoms. The ENS team discovered that decoherence (as measured
by the disappearance of evidence of quantum-mechanical interference between
the two cavity states) proceeds at a faster rate with time, and also when
the differences between the two phase shifts are increased and therefore
made more distinguishable from one another. (M. Brune et al., Physical
Review Letters, 9 December 1996.)
MAGNETIC LEVITATION OF LIQUID HELIUM-4 has been achieved by physicists
at Brown University. Drops as large as 2 cm across, in both the normal
and superfluid state, were levitated. The scientists were surprised by
the fact that two such drops often came into contact but failed to coalesce,
apparently because of the vapor pressure between them. (M.A. Weilert et
al., Physical Review Letters, 2 December 1996.)
A NANOTUBE HAS BEEN USED AS A MICROSCOPE PROBE. Richard Smalley at Rice
University has managed to glue single carbon nanotubes (several microns
long but only 5-20 nm wide) to the tips of scanning tunneling microscopes
and scanning force microscopes. The resulting probes are conductive (good
for STM), reproducible, and sturdy (good for SFM) since they flex instead
of snapping when they make contact with atoms in the sample. (Hongjie Dai
et al., Nature, 14 November 1996.)
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