Going beyond optical molasses, Cohen- Tannoudji, Alain Aspect, Ennio Arimondo, Robin Kaiser and Nathalie Vansteenkiste, then all at the Ecole Normale, invented an ingenious scheme capable of cooling helium atoms below the recoil velocity of a single scattered photon. Helium atoms have been cooled to two microkelvins along one dimension, and work is under way to extend this technique to two and three dimensions. This cooling method captures an atom in a well-defined velocity state in much the same way atoms were trapped in space in our first optical trap. As the atom scatters photons, its velocity randomly changes. The French experiment establishes conditions that allow an atom to recoil and land in a particular quantum state, which is a combination of two states with two distinct velocities close to zero. Once in this state, the chance of scattering more photons is greatly reduced, meaning that additional photons cannot scatter and increase the velocity. If the atom does not happen to land in this quantum state, it continues to scatter photons and has more opportunities to seek out the desired low-velocity state. Thus, the atoms are cooled by letting them randomly walk into a "velocity trapped" quantum state.
Besides the cooling and trapping of atoms, investigators have demonstrated various atomic lenses, mirrors and diffraction gratings for manipulating atoms. They have also fashioned devices that have no counterpart in light optics. Researchers at Stanford and the University of Bonn have made "atomic funnels" that transform a collection of hot atoms into a well-controlled stream of cold atoms. The Stanford group has also made an "atomic trampoline" in which atoms bounce off a sheet of light extending out from a glass surface. With a curved glass surface, an atom trap based on gravity and light can be made.
Clearly, we have learned to push atoms around with amazing facility, but what do all these tricks enable us to do? With very cold atoms in vapor form, physicists are in a position to study how the atoms interact with one another at extremely low temperatures. According to quantum theory, an atom behaves like a wave whose length is equal to Planck's constant divided by the particle's momentum. As the atom is cooled, its momentum decreases, thereby increasing its wavelength. At sl\fficiently low temperatures, the average wavelength becomes comparable to the average distance between the atoms. At these low temperatures and high densities, quantum theory says that a significant fraction of all the atoms will condense into a single quantum ground state. This unusual form of matter, called a Bose-Einstein condensation, has been predicted but never observed in a vapor of atoms. Thomas]. Greytak and Daniel Kleppner of M.LT. and look T. M. Walraven of the University of Amsterdam are trying to achieve such a condensation with a collection of hydrogen atoms in a magnetic trap. Meanwhile other groups are attempting the same feat in a laser-cooled sample of alkali atoms such as cesium or lithium.
Atom-manipulation techniques are also offering new opportunities in highresolution spectroscopy. By combining several such techniques, the Stanford group has created a device that will allow the spectral features of atoms to be measured with exquisite accuracy. We have devised an atomic fountain that launches ultra-cold atoms upward gently enough to have gravity turn them around. Atoms for the fountain are collected by a magneto-optic trap for 0. 5 second. After that amount of time, about 10 million atoms are launched upward at a velocity of roughly two meters per second. At the top of the trajectory, an atom is probed with two pulses of microwave radiation separated in time. If the frequency of the radiation is properly tuned, the two pulses cause the atom to change from one quantum state to another. (Norman Ramsey shared the Nobel Prize in Physics in 1989 for inventing and applying this technique.) In our first experiment we measured the energy difference between two states of an atom with a resolution of two parts in 100 billion.