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Quotes from Discover Magazine's
Beams of Stuff
Article
About David E. Pritchard's Diffraction of Sodium Atoms

"Until, that is, it encounters the first beam splitter. To divide the atom beam into components,
Pritchard uses a diffraction grating—a series of closely spaced microscopic slits cut into a
silicon nitride membrane. The atom approaches the grating like a bb pellet fired at a picket
fence. Each slit in the grating is 100 nanometers wide—about four- millionths of an inch—as
is each of the silicon nitride slats. An individual sodium atom, on the other hand, appears
only a few tenths of a nanometer across when observed—hundreds of times smaller than the
slits in the grating. If the atom were indeed a bb-like particle, it would either slam into one of
the slats of the fence or else sail through one of the gaps between the slats."

"That doesn’t happen. About half the atoms in the beam do hit and stick to the slats of the
grating, but the half that get to the other side do not pass through just one slit. Each atom
passes through approximately a hundred slits simultaneously, like an ocean wave washing
through a line of pilings protecting a harbor, and exits the grating as a hundred individual
waves. Once past the grating, each of these component waves begins to spread out and
collide with the others, canceling out in some places and combining in others. The resulting
diffraction pattern consists of a series of waves that fan out behind the grating like the tines
on a leaf rake. The strongest wave is the one headed straight down the axis of the
interferometer, the next strongest are the two on either side of it, and so on. For the
interferometer, Pritchard needs just two of these, and he sets up the apparatus in such a way
that only the straight-ahead wave and the one to its right play a role."

. . .

"So far Pritchard has put only sodium atoms and sodium molecules through the
interferometer, but there is no reason he couldn’t create interference patterns with larger
objects. “There’s no real theoretical limit,” he says. “The practical limit is time.” To produce
a detectable interference pattern, the wavelength of an object moving through the
interferometer needs to stay above a certain minimum, which for Pritchard’s machine is
about a hundredth of a nanometer. A so-called matter wave, of course, is not to be
confused with an electromagnetic or any other kind of wave. To talk about the wavelength
of a bacterium, say, or a baseball, is to defy common sense, and yet, using the equations of
quantum mechanics, you can calculate a wavelength for each of these objects. According to
these equations, wavelength decreases [flux increases] as both mass and velocity increase—
the bigger an object is, the slower it has to move for the wavelength to stay the same.
Therefore, if you wanted to send a molecule with a hundred sodium atoms through Pritchard’s
machine, the molecule would need to travel through the interferometer at one-hundredth the
sodium atoms’ speed, if it were to have the same wavelength. Larger objects would have to go
even more slowly. “And at some point,” Pritchard points out, “you’d have to get a larger
diffraction grating so that things would fit through the slits.” That would demand slowing
things down even more because the visibility of the interference pattern depends partly on
how much the two beams diverge, which in turn depends on the closeness of the splits in the
grating."

. . .

"Pritchard then gradually moved the laser back toward the second grating, so that the beams
were farther and farther apart when the laser light hit them, making it possible to tell with
more and more certainty which beam an atom was in. As he did, the interference pattern
gradually faded. At a point where the wavelength of the photons was exactly twice the
separation between the beams—the minimum wavelength needed to tell with certainty which
beam the atom was in—the interference pattern disappeared completely. The atom in
principle could have been located, and it once again dropped any pretense of acting like a
wave."

Use Discover's Archives to search for Pool's entire article. Search on 'pritchard,' then choose
Beams of Stuff.

From Discover Magazine
Topic area: Astronomy & Physics, December 1997
Beams of Stuff by Robert Pool

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Rev. 12Feb2015  PDR — Created 8May1999  PDR
(25Aug2002 rev - Add 'consensus' link to common sense above.)
(27Mar2007 rev - Reformat page.)
(7Jun2009 rev - Add link to Pritchard'sweb site at MIT.)
(12Feb2015 rev - Update color. Reset legacy markups. Make page current.)