Atoms in a Trap Measure Gravity at the Micron Level

Nowadays many of the most sensitive measurements in science depend on some quantum phenomenon which very subtly can often be exploited to gain maximum precision. In an experiment conducted at the Università di Firenze (University of Florence) the quantum phenomenon in question is called Bloch oscillation. This weird effect occurs when particles subject to a periodic potential -- such as electrons feeling the regular gridlike electric force of a crystalline lattice of atoms -- are exposed to an additional static force, say, an electric force in a single direction; what happens is that the electrons do not, as you would expect, all move in the direction of the force, but instead oscillate back and forth in place.

In a new experiment conducted by Guglielmo Tino and his Florence colleagues, the particles are supercold strontium atoms held in a vertically oriented optical trap formed by criss-crossing laser beams, while the static force is merely the force of gravity pulling down on the atoms (see figure at Physics News Graphics).

What are the unique features of this experiment? First of all, although Bloch oscillations have been observed before, they have never been sustained for as long as 10 seconds, which is the case here. Experiments that mix gravity and quantum mechanics are rare.

Furthermore, even though the cloud of Sr atoms in use do not exist in the form of a Bose-Einstein condensate (BEC), the atoms do absorb the trapping laser light in a coherent way; that is, they absorb the light in a stimulated (not random) way. They quickly re-emit the light and then absorb still another photon. The number of photons per atom transferred in this way -- in the thousands rather than tens -- is the largest ever for a physics experiment.

Finally, close observation of the Bloch oscillations allows you to measure the strength of the static force, gravity, with high precision -- in this case to measure gravity with an uncertainty of a part in a million.

With planned improvements to the apparatus, the researchers will be able to bring the atoms to within a few microns of a test mass and will measure g with an uncertainty of 0.1 parts per million. With these conditions, one can probe theories which say that gravity should depart from the Newtonian norm, perhaps signifying the existence of unknown spatial dimensions.

According to Tino (guglielmo.tino@fi.infn.it, 39-055-457-2034) unlike gravity-measuring experiments which use torsional balances or cantilevers, the Florence approach measures gravity directly and over shorter distances. The atom-trap setup should also prove useful for future inertial guidance systems and optical clocks.

Ferrari et al., Physical Review Letters, 11 August 2006
Contact Guglielmo Tino, University of Florence
guglielmo.tino@fi.infn.it, 39-055-457-2034
Image at Physics News Graphics