How does the fountain make such precise measurements possible? First, the atoms fall freely and are easy to shield from any perturbation that might alter their energy levels. Second, such measurements are limited in precision by the Heisenberg uncertainty prinCiple. This principle states that the resolution of an energy measurement will be limited to Planck's constant divided by the time of the "measurement." In our case, this time corresponds to the time between the two microwave pulses. With an atomic fountain the measurement time for unperturbed atoms can be as long as one second, a period impossible with atoms at room temperature.
Because the atomic fountain allows extremely precise measurements of the energy levels of atoms, it may be possible to adapt the device to make an improved atomic clock. At present, the world time standard is defined by the energy difference between two particular energy levels in ground states of the cesium atom. Two years after the first atomic fountain, the group at the Ecole Normale used a fountain to measure the "clock transition" in the cesium atom with high precision. These two experiments suggested that a properly engineered instrument might be able to measure the absolute frequency of this transition to one part in 10^16, 1,000 times better than the accuracy of our best clocks. Lured by this potential, more than eight groups around the world are now trying to improve the cesium time standard with an atomic fountain.
Another application being intensively studied is atom interferometry. The first atom interferometers were built in 1991 by investigators at the University of Konstanz, M.LT., the Physikalisch-Technische Bundesanstalt and Stanford.
An atom interferometer splits an atom into two waves separated in space. The two parts of the atom are then recombined and allowed to interfere with each other. The simplest example of such a splitting occurs when the atom is made to go through two separated mechanical slits. If the atom is recombined after passing through the slits, wavelike interference fringes can be observed. The interference effects from atoms dramatically demonstrate the fact that their behavior needs both a wave and a particle description.
More important, atom interferometers offer the possibility of measuring physical phenomena with high sensitivity. In the first demonstration of the potential sensitivity, Mark Kasevich and I have created an interferometer that uses slow atoms. The atoms were split apart and recombined in a fountain. With this instrument we have already shown that the acceleration of gravity can be measured with a resolution of at least three parts in 100 million, and we expect another 100-fold improvement shortly. Previously, the effects of gravity on an atom have been measured at a level of roughly one part in 100.
In recent years the work on atom trapping has stimulated renewed interest in manipulating other neutral particles. The basic principles of atom trapping can be applied to micron-size particles, such as polystyrene spheres. The intense electric field at the center of a focused laser beam polarizes the particle, just as it would polarize an atom. The particle, like an atom, will also absorb light of certain frequencies. Glass, for example, strongly absorbs ultraviolet radiation. But as long as the light is tuned below absorption frequency, the particle will be drawn into the region of highest laser intensity.
In 1986 Ashkin, Bjorkholm, ]. B. Dziedzic and I showed that particles that range in size between 0.02 and 10 microns can be trapped in a single focused laser beam. In 1970 Ashkin trapped micron-size latex spheres suspended in water in between two fo- cused, counterpropagating beams of light [see "The Pressure of Laser Light," by Arthur Ashkin; SCIENTIFIC AMERICAN, February 1972]. But only much later was it realized that if a single beam is focused tightly enough, the dipole force would suffice to overcome the scattering force that pushes the particle in the direction that the laser beam is traveling.