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.
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
(firstname.lastname@example.org, 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
Image at Physics News Graphics