Physics, Instrumentation and Sensing
Cold atom devices
Physics
In 1997, the Frenchman Claude Cohen-Tanoudji and Americans Steven Chu and W. Philips won a Nobel prize for creating, describing, and laying the theoretical groundwork for a new field of atomic physics: cold atoms. It was five years earlier that their teams managed to cool atoms down to temperatures of the order of a few tens of microkelvins.
Once the atoms are cooled down, they allow for high precision acceleration or rotation measurements. This new generation of inertial sensors was first proposed and modeled by Christian Bordé in 1989 (site web). The first experimental confirmation was obtained two years laters. After 15 years of improvement, cold atoms inertial sensors are currently reaching better performances than other technologies.
But what is atomic cooling?
To explain this, it should be remembered that temperature is nothing other than a measurement of the kinetic energy of the atoms, i.e. their mean velocity. Cooling atoms removes their kinetic energy and thus slows them down. The atoms surrounding us move at a speed close to 300 m/s. It is now easy to slow them down to less than a few centimeters per second using laser cooling techniques.
How are atoms cooled?
We will not describe here the details of the techniques used to cool atoms. The interested reader may refer to the excellent American site JILA Colorado[EN] or to the site of the Kastler Brossel ENS laboratory. It should simply be known that atoms absorb light, and do so only at very precise wavelengths. When an atom absorbs a grain of light, a photon, it keeps the energy and adds its impulse to its own. If the photon beam is propagated in the opposite direction to atom motion, after a certain number of absorptions-emissions, it is stopped. If we use three pairs of concurrent laser beams in space, the atoms are stopped at their point of encounter, and thus give rise to a cloud of cold atoms.
Ball of a billion atoms
The temperatures reached are of the order of 1 to 100 microKelvins, depending on the atom.
Why cool atoms?
This research exhibits first-order experimental prowess and provides a spectacular verification of quantum mechanics. Beyond the fundamental aspect, the interest of the experimentation is in the use of atoms as universal measurement probes that are invariable in time. That is, atoms, because of their quantum nature, are absolute energy and thus frequency references by the simple relation of proportionality (E = hν, where h is Planck’s constant). When they are slowed down, they can be observed longer and thus the sensitivity of the measurement is increased decisively. In the same way as when measuring distance with a ruler, it is more accurate to concentrate a long time on the graduation rather than just taking a quick glance. Moreover, the statistical law followed by the atoms (Maxwell-Boltzman law) says that when the average velocity of the atoms decreases, their dispersion around this value decreases in the same proportion. Thus, cold atoms interact very similarly. The measurement approaches the perfect measurement on a single atom (reduced averaging effect).
