The Sciences

Interfering Atoms

By David H FreedmanJan 1, 1996 6:00 AM


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Stopping atoms in their tracks is not the only way to get them to show their wavelike nature. Another way is to throw them at a grating with slits so small and tightly spaced that each atom wave passes through two slits at once and is thus split in two. The split waves can then be recombined to produce an interference pattern--alternating bands of intensity in which the matter waves either cancel each other or reinforce each other, just as interfering light waves do. MIT physicist David Pritchard first measured such atomic interference in 1988.

Last February Pritchard and his colleagues reported another first: using the silicon nitride grating shown here, whose slits are just a few hundred-millionths of an inch apart, they managed to separate the split atom waves enough to do separate experiments on them. (The closer the spacing of the slits, the more the waves diverge after they pass through the grating.) The researchers passed one of the waves through a gas or an electric field while leaving the other alone. By observing the effect on the interference pattern--which is extremely sensitive to any tampering with one of the component waves--Pritchard and his team made fundamental measurements that were not possible before. They measured the susceptibility of sodium atoms to electric fields and the degree to which sodium atom waves are refracted--bent and attenuated--as they pass through another gas and the atoms in that gas attract them.

Physicists armed with optical interferometers have been able to make similar measurements on light waves for the last century or so--but light waves are 10,000 times longer than atom waves, which means they can be diffracted with much coarser gratings than the one in Pritchard’s atom interferometer. Pritchard has managed to send entire sodium molecules through his device, and in principle, even something as large as a living bacterium could hurtle through it in wave form. But quantum mechanical trade-offs mean that such a large chunk of matter would take thousands of years to pass through the grating. For the moment at least, physicists must be content with finally being able to exploit the wave nature of atoms.

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