Text 155, 73 rader
Skriven 2004-08-28 19:45:33 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 698
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PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 698 August 26, 2004
by Phillip F. Schewe, Ben Stein
        
THE WORLD'S SMALLEST ATOMIC CLOCK, about the size of a rice grain, is built
around a microcell about 1 cubic mm in volume filled with cesium atoms.  It
draws only about 30 mA of current from a 2.5 V battery.  Atomic clocks are the
best timekeepers because they are able to convert the high-precision
information contained in the light emitted by alkali atoms (the light emerging
from an atomic transition from one energy level to another can be measured to
an uncertainty of better than a part in a billion) into a usable standard for
defining the second.  The new miniature clock has a precision of 3.5 x 10^-10. 
What this means is that events can be timed with an uncertainty of about one
part in 3 billion. Scientists at NIST in Boulder, Colorado make atomic clocks
that are far more precise---the F-1 clock is good to about one part in 10
trillion---but this requires a huge table-top's worth of equipment. The mini
version being reported now should eventually reach a stability of about 10^-11,
some 10,000 times better than any quartz oscillator clock of equivalent size
and power.  How will this new cheap, tiny, low-power, high-precision MEMS clock
be used?  In satellites, GPS receivers, networked computer CPU's, possibly in
cell phones.  (Knappe et al., Applied Physics Letters, 30 August 2004; contact
John Kitching, kitching@boulder.nist.gov, 303-497-3328; for an explanation of
precision and accuracy, see
www.boulder.nist.gov/timefreq/general/about.html)

OPTICAL FUNNEL FOR FOCUSING COLD ATOMS. A new experiment at the Tokyo Institute
of Technology uses evanescent light to focus cold atoms and output as a beam.
Evanescent light is the faint optical field (a sort of aura of light stuck on a
material) that is found on the material surface when a laser beam reflects away
from the material via "total internal reflection." In this case, the focusing
effect occurs when a hollow laser beam moving upwards splays outward around a
funnel-shaped piece of glass. The light, shone downward and covering the inner
edge of this funnel, helps to repel and cool a blob of atoms held and chilled
in a magneto-optical trap (MOT) and falling slightly under the force of
gravity.  Evanescent light has been used before to guide atoms through a hollow
optical fiber (see http://www.aip.org/pnu/1996/split/pnu272-2.htm), but in the
Tokyo work there are new features: high flux intensity, low temperature, and
small beam diameter.  The funnel focuses an atom swarm about 2 mm wide is
forced to collimate down to the size of the funnel's exit hole, which in the
experiment was 200 microns, for a net focusing factor of 100 (see figure at
www.aip.org/png). Furthermore, a micron-sized hole is now being tested, which
should result in a focusing factor of a million, and a beam flux intensity of
some 10^15 atoms/cm^2-s. Akifumi Takamiazwa
(Akifumi.Takamizawa@physik.uni-muenchen.de) says that he and his colleagues
hope to make a nanometer-sized funnel as small as atomic de Broglie wavelength
and use it eventually for single-atom manipulation, perhaps for processes in
which one atom can transfer one bit of information. (Takamizawa et al., Applied
Physics Letters, 6 September 2004; see
http://uuu.ae.titech.ac.jp/research-e.html and
http://www.coe21-pni.titech.ac.jp/eng/task/index.htm)

SUPERPROTONIC TRANSITIONS.  Electrons are the charge carriers in most
electronic transactions.  Sometimes, in semiconductors, holes, the moving voids
recently vacated by an electron, constitute a usable current flow.  But
positive ions can also act as an important current.  Lead-acid batteries in
cars are a prominent application of this principle.  A particularly interesting
phenomenon in this regard is the "superprotonic" transition, an effect
discovered in the 1980s by Russian scientists, in which the proton conductivity
jumps by several orders of magnitude at a certain temperature, when a
structural rearrangement of some of the molecular oxyamion groups (such as SO4)
occurs.  Sossina M. Haile and her colleagues at Caltech (smhaile@caltech.edu,
626-395-2958) have performed new experiments which have expanded the roster of
superprotonic materials, or cleared up past mysteries.  For example, they have
cleared up any doubt that the solid-form acid CsH2PO4, whose chemistry and
conducting properties are especially promising as a candidate for the
electrolyte in fuel cells, can undergo the superprotonic transition.  The new
results were reported at last month's meeting of the American Crystallographic
Association in Chicago
(http://www.hwi.buffalo.edu/ACA/; see also http://addis.caltech.edu/)

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 * Origin: Big Bang (1:106/2000.7)