A RUDIMENTARY ATOM LASER has been created at MIT, promising significant improvements in high precision measurements with atoms and offering the prospect of future nanotechnology applications, such as atom lithography, in which lines are drawn on integrated circuits (by directly depositing atoms) with greater precision than ever before. In an atom laser the output beam consists of a single coherent atom wave, just as in a regular laser the beam consists of a coherent light wave. The working substance for the atom laser is a Bose-Einstein condensate (BEC) of sodium atoms, cooled and contained within an atom trap by a shaped magnetic field. BEC itself was achieved for the first time only as recently as 1995 (see Update 233). It is a condition in which atoms are chilled to such low energies that, in a wavelike sense, the atoms begin to overlap and enter into a single quantum state.
Wolfgang Ketterle and his colleagues at MIT make their claim of producing the first atom laser on the basis of two experimental developments, as reported in two journals this week. In the first effort (M.-O.Mewes et al., Physical Review Letters, 27 January 1997) a portion of a sodium condensate was successfully extracted under controlled conditions. They achieve "output coupling" by applying radiofrequency radiation to the BEC; this "tips" the atoms' spins by an adjustable amount, putting the atoms in a superposition of quantum states. Thereafter some of the atoms feel the effect of the surrounding magnetic field in a different way and are able to leave the atom trap. It is these departing atoms, still enjoying the coherent properties of the BEC state, that constitute an atom laser beam. Pulled downward by gravity, the beam was observed over a distance of millimeters, although in principle it could travel further in an undisturbed vacuum environment.
The second development was to verify that the atom waves are indeed coherent (M.R. Andrews et al., Science, 31 January 1997). At the time of the original BEC discovery, many physicists expected the atoms in the condensate to fall into a single quantum state; some hypothesized that it could take a time equal to the age of the universe for true coherence to come about. The MIT group addressed this issue by creating two BEC clouds in a special trap. Turning off the trap allows the clouds to expand, overlap, and interfere, producing a pattern of light and dark fringes. The observed patterns (viewed with an electronic camera) could only exist if each BEC was an intense coherent wave. The MIT team determined that the atom wave associated with each BEC had a wavelength of 30 microns, a million times larger than the wavelengths associated with room-temperature atoms.
In addition to coherence, the atom laser waves are analogous to the light waves in an optical laser in another respect as well. Just as a laser beam is more intense than an equivalent stream of light from the Sun, the MIT atom beam is also more intense (for a given beam spotsize) than ordinary atom beams (whose atoms possess a variety of energies) since it delivers a powerful, directional stream of atoms in a single quantum state. In other ways, the atom lasers and light lasers are different. According to Ketterle, "Photons can be created but not atoms. The number of atoms in an atom laser is not amplified. What is amplified is the number of atoms in the lowest-energy quantum state, while the number of atoms in other states decreases."
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