Physics news Update 785

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Number 785, July 17, 2006 by Phil Schewe and Ben Stein

A New BEC Magnetometer

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 electronvolt) 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.
Jörg Schmiedmayer and his colleagues are pioneers in advancing the young science of integrated atom optics (see PNU 516), 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 Physics News Graphics) 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 nanotesla 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
Contact Jörg Schmiedmayer, University of Heidelberg, schmiedmayer@atomchip.org
Jörg Schmiedmayer's lab

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, Harvard University in Cambridge, Mass., the CNRS lab in Paris, and Ibn Zohr University, in Morocco -- have set up a Web site 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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