by Pierre Le Hir
THE PHYSICS OF COLD ATOMS SHOWS INCREASINGLY PRECISE RESULTS
The interior courtyard of the Paris Observatory contains a number of box-spaces that used to serve as stables and, later, artists' studios. The runners that inhabit them currently are atoms, and the artists masters of light. We are here in the world of the infinitely small, the infinitely cold, the infinitely precise.
On Friday May 30th, the Francilian (Ile-de-France) Institute for Research on Cold Atoms Ifraf, created in 2005 by Claude Cohen-Tannoudji, Nobel Prize in Physics 1997, opened new laboratories on this site, which will permit work in this domaine of excellence within French physics. A very competitive area, where the thirty-two teams working for Ifraf will be up against those of the Massachusetts Institute of Technology (MIT) at Harvard in the United States, of Hanover, in Germany, or of Innsbruck, in Austria. This is because the physics of cold atoms is part of fundamental physics, yet it also has numerous applications in measurement science, in geophysics or in computer science. The basic principle underlying this science is that of slowing down - or what is essentially the same thing, cooling - clouds made up of a few million atoms, in order to precisely measure certain characteristics.
With the aid of laser beams, whose photons create a massive deceleration, it is possible to slow down atoms from a few hundred meters per second to a mere few centimeters per second. Thus caught up in an optical molasses, the atoms are in a super cold-state, a few billionth of a degree above absolute zero (-273,15°C).
One can then measure, for a time period of up to one second - an eternity for a physicist -, phenomena such as the transition between two energy levels in an atom of cesium 133. And thus improve on the accuracy of atomic clocks which calibrate their seconds by matching frequencies with this transition.
COLD MEASUREMENTS IN SPACE
Ever since the advent, in the 1950s, of the first atomic clocks, which have replaced the movements of Earth around the Sun as the standard of time, for example in Global positioning Systems, the precision of such clocks has improved by a factor of ten every ten years. With current systems - cold atom fountains - , these clocks are no more off than one second every 300 million years!
Ifraf scientists are also at work on the Pharao project (Projet d'horloge atomique par refroidissement d'atomes en orbite) which, in 2012 or 2013, should furnish the International Space Station with a new metronome. This would then become, given it's unequaled precision -microgravity permitting slowing down atoms even more -, the universal time referent.
Other projects are aiming for ultra sensitive gravity meters, permitting for example tracking the movement of an earthquake or finding an oil deposit. Or again cryptography or quantum memory.
But cold atoms are also 'tools of choice for fundamental research', points out M. CohenTannoudji. Theoretical physicists, who are having trouble reconciling the laws of general relativity, which hold for large bodies, with those of quantum mechanics, which govern elementary particles, know that their models are imperfect. Greater precision in the observations of atoms thanks to extreme cold could reveal that certain constants, which we have held to be unchanging since the Big Bang, are nothing of the sort. Blowing hot and cold on modern physics, as it were.
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