Text 797, 89 rader
Skriven 2006-07-17 16:32:05 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 785
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PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 785  July 17, 2006  by Phillip F. Schewe, Ben Stein,
and Davide Castelvecchi        www.aip.org/pnu
                                                
A NEW BEC MAGNETOMETER represents the first application for Bose
Einstein condensates (BECs) outside the realm of atomic physics.
Physicists at the University of Heidelberg have used a
one-dimensional BEC as a sensitive probe of the magnetic fields
emanating from a nearby sample.  The field sensitivity achieved
thereby is at the level of magnetic fields of nanotesla strength
(equivalent to an energy scale
of about 10^-14 eV) with a spatial resolution of only 3 microns.
Some methods (such as scanning hall probe microscopes) can attain
finer spatial resolution and some methods (such as superconducting
quantum interference devices---SQUIDs) can attain higher magnetic
sensitivity, but for its range, the Heidelberg device has a region
of the sensitivity-vs-resolution space all to itself.  Joerg Schmiedmayer
and his colleagues are pioneers in advancing the young science of
integrated atom optics (see www.aip.org/pnu/2000/split/516-1.html ),
which seeks to guide atoms around microchips and exploit them for
future practical applications much as electronics manipulates
electrons in integrated circuits and photonics uses photons in
optoelectronic structures.
To see how the BEC measures the electromagnetic potential above a
surface consider that potential to be a landscape covered with peaks
and valleys.  If now you flood the whole landscape with water you
would create an equi-potential flat surface at the top. To plumb the
submerged topography you could measure the total amount of the water
beneath the surface at any point.  This is what the Heidelberg
researchers do.  Across the sample, where the potential is deep
(that is, where the fields are particularly strong) more atoms in
the BEC pile up.  Thus the density of atoms in the BEC (which can be
measured by seeing how much light from a probe laser is absorbed at
points along the length of the BEC---see figure at
http://www.aip.org/png/2006/261.htm ) can be converted into a map of
the fields at the sample surface.  According to Schmiedmayer
(schmiedmayer@atomchip.org), the sensitivity of this process is
already so great that the measurement is limited to some extent by
"atomic shot noise," the atom equivalent of shot noise, the noise
encountered in measuring faint currents because of fluctuations in
the number of electrons arriving at a point a circuit or in
measuring light levels in a fiber because of fluctuations in the
number of arriving photons.  In the BEC case, the field measurements
will be more robust against such atom shot noise if more atoms can
be loaded into the BEC, which resides in a tiny atom trap mere
microns from the surface under study, while simultaneously keeping
the chemical potential constant.  The sensor's nT field sensitivity
and micron spatial resolution should make it useful for discovering
new solid state and surface physics phenomena.  (Wildermuth et al.,
Applied Physics Letters, published online 27 June 2006; lab website
at www.atomchip.org )

DUNE TUNES.  For centuries, world travelers have known of sand dunes
that issue loud sounds, sometimes of great tonal quality.  In the
12th century Marco Polo heard singing sand in China and Charles
Darwin described the clear sounds coming from a sand deposit up
against a mountain in Chile.  Now, a team of scientists has
disproved the long held belief that the sound comes from vibrations
of the dune as a whole and proven, through field studies and through
controlled experiments in a lab, that the sounds come from the
synchronized motions of the grains in avalanches of a certain size.
Small avalanches don't produce any detectable sound, while large
avalanches produce sound at lots of frequencies (leading to
cacophonous noise).  But sand slides of just the right size and
velocity result in sounds of a pure frequency, with just enough
overtones to give the sound "color," as if the dunes were musical
instruments.  In this case, however, the tuning isn't produced by
any outside influence but by critically self-organizing tendencies
of the dune itself.  The researchers thus rule out various "musical"
explanations.  For example, the dune sound does not come from the
stick-slip motion of blocks of sand across the body of the dune
(much as violin sounds are made by the somewhat-periodic stick-slip
motion of a bow across a string attached to the body of the
violin).  Nor does the dune song arise from a resonance effect (much
as resonating air inside a flute produces a pure tone) since it is
observed that the dune sound level can be recorded at many locations
around the dune.  Instead, the sand sound comes from the
synchronized, free sliding motion of dry larger-grained sand
producing lower frequency sound. The scientists---from the
University of Paris (France), Harvard (US), the CNRS lab in Paris,
and the Universite Ibn Zohr (Morocco)---have set up a website
(http://www.lps.ens.fr/~douady/SongofDunesIndex.html ) where one can
listen to sounds from different dunes in China, Oman, Morocco, and
Chile. (Douady et al., Physical Review Letters, upcoming article;
contact Stephane Douady at douady@lps.ens.fr)

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