Beams of Stuff

To get closer to the true, quantum nature of matter, physicist David Pritchard has been splitting atoms down the middle, fiddling with the halves, and then putting them back together. In principle, he says, he could do the same to a bacterium. Or even a baby grand.

By Robert Pool|Monday, December 1, 1997
In room 240 of building 26 on the mit campus, physicist David Pritchard has built an attitude-adjustment machine. That’s not what he calls it, of course. Officially it is an atom interferometer, built to measure various properties of atoms with astonishing accuracy, and this it does quite well. The machine has allowed Pritchard to pin down some atomic quantities that were once unmeasurable and to improve the accuracy of other measurements ten times over--the particle-physics equivalent of adding a couple of inches to the world high-jump record or shaving several minutes from the previous best marathon time. But as good as it is at making measurements, the interferometer is even better at opening eyes and shaking beliefs. Once you understand what it does, you can never think of ordinary matter in the same way again.

From the outside, the machine appears to be nothing more than a cylindrical stainless-steel shell, 11 or 12 feet long and a foot in diameter, with a dozen airtight ports that provide access for vacuum pumps, control cables, and various instruments. It is the sort of container physicists typically use when they want to perform experiments in a near vacuum. Inside, however, is where its secrets lie. The first section of the interferometer creates a very narrow atomic beam--a collection of sodium atoms, all moving in the same direction and at the same speed--and shoots it down the axis of the machine. After a few inches, the beam gets split into two components, one angling left, the other right. Two feet farther on, the two beams encounter a second device that angles them back toward the middle, so that a few feet later they meet.

To understand what happens next, you need a passing acquaintance with wave-particle duality--the notion that matter at the quantum mechanical scale, the scale of individual subatomic particles, acts sometimes like a wave, sometimes like a particle, depending on the circumstances. In this case, when the beams meet they act like two sets of ripples colliding on a pond--they create an interference pattern of peaks and valleys. Wherever the crests of two ripples meet, or two troughs, they add together to create a large peak or valley; when crest meets trough, they cancel each other out. At the far end of the interferometer, a detector records the interference pattern created.

One’s first impression on hearing this description is that the atoms have done nothing unusual. After all, you could set up a similar system with water waves and get the same sort of interference pattern. But then Pritchard adds one detail that changes everything. He has made the beam so weak that only one atom passes through the interferometer at a time. Thus, when the atom beam splits in two, it’s not that some atoms are going right and others left. The entire beam is a single atom, and when the beam is split, the atom is split. Suddenly you find yourself scrambling to visualize what has happened--and failing miserably.

It is difficult to picture what the atom is doing in there, Pritchard admits. Nearly all similar beam-splitting experiments use fundamental particles, like photons or electrons, that have no internal structure. Because a photon is a featureless, dimensionless speck of light, it’s easy to squint mentally and fancy that you see it as a fuzzy, spread- out wave. Stop squinting, and it’s a point particle again. You fool yourself into believing that you really understand wave-particle duality.

But atoms are different. They have a definite, complex structure. Each sodium atom passing through the interferometer is composed of 34 individual particles--11 protons, 12 neutrons, and 11 electrons--arrayed in the familiar atomic pattern, with the protons and neutrons in a central nucleus orbited by the electrons. It is not easy to squint and pretend that these well-defined compound objects are fuzzy waves. To make imagining even more difficult, Pritchard has sent sodium molecules--pairs of sodium atoms, 68 particles in all--through the interferometer and shown that these doubly complex entities also traverse the machine as waves.

The wave nature of an atom is necessarily hidden because, according to theory, the moment a quantum mechanical object is observed, it no longer behaves like a wave. It assumes the guise of an ordinary classical particle--its behavior is not significantly different from a billiard ball’s. For this reason, ever since Niels Bohr and other physicists constructed quantum theory back in the 1920s, the details of the transformation from wave to particle have been shrouded in mystery. In the days of Bohr, we just accepted it as a fact of life, Pritchard says. Now we want to understand it and hopefully be able to work with it. And Pritchard believes he may finally have built an instrument that can do that.

The beam begins with a bit of sodium metal heated to 1300 degrees Fahrenheit. Sodium atoms come bubbling off into an atmosphere of neon, argon, or other noble gas and form a thick sodium vapor. Then, whoosh, the gas mixture jets through a tiny nozzle at several times the speed of sound and enters a vacuum chamber, in which it expands and cools, leaving the sodium beam monochromatic--that is, with all its atoms moving at about the same speed. This torrent strikes an inverted funnel with an opening the size of a pinhole, which collimates the beam, allowing only those atoms nearest the centerline to advance to the next chamber. Farther on, the beam is collimated again, this time by passing through a narrow slit, and then again, so that only those few atoms pointed straight down the axis of the interferometer remain in the beam.

Of the sodium atoms that began the journey, just a minute fraction are now left to carry on, but the wastefulness is necessary, Pritchard says. You need an atom beam that is sufficiently monochromatic and well-collimated that it doesn’t wash out the interference pattern. If the atoms are moving at different speeds or in different directions, the interference pattern will be as difficult to make out as the picture on a scrambled cable tv channel.

In normal atmosphere, the atoms in the beam would collide with surrounding atoms before they had traveled a millionth of an inch, and the beam would dissipate almost immediately. Pritchard’s chambers, however, are nearly a perfect vacuum. An atom in this vacuum could travel a hundred meters without hitting anything, he says. And since the atoms in the beam, all moving in the same direction and at the same speed, are also unlikely to run into each other, each sodium atom sails as though through empty space.

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.

This ratherawkward beam splitter is necessary, Pritchard says, because there is no better, easier way to separate an atom wave into two components. In a light interferometer, the beam splitter is a half-silvered mirror set at an angle to the light beam. Half the light goes through, and the other half is reflected off at an angle. But there is no material that is transparent to atoms, nor is there an easy way to reflect atoms off a surface.

As the two beams move away from the diffraction grating, they diverge ever so slightly. By the time they reach a second grating two feet farther on, the distance between the beams is the width of a very fine hair--about 50,000 nanometers, or two-thousandths of an inch. Like the first beam splitter, this grating divides each of the two beams into a fan of spreading waves, and this time Pritchard uses only one finger from each. From the beam on the left, he chooses the first component on the right, and from the beam to its right, he takes the first component on the left. Two feet farther on, these two sub-beams intersect to create an interference pattern.

To detect the interference pattern, Pritchard uses a third diffraction grating paired with a hot wire detector. Whenever an atom comes through this third grating, it strikes a thin rhenium wire heated to 1500 degrees. At this temperature the rhenium can suck an electron off the atom, giving the atom a positive electric charge and making it perceptible to a charge detector behind the wire. Pritchard counts the atoms hitting the wire for a few thousandths of a second, moves the grating a little and counts again, gradually building up a picture of the interference pattern. When this third diffraction grating is lined up with the high-intensity parts of the pattern, many atoms will come through. When the grating is lined up with the low-intensity sections, only a few will.

The interference pattern, Pritchard notes, is a statistical phenomenon. It cannot be seen in the behavior of an individual atom, because the act of striking the detector causes the atom to shed its quantum mechanical nature. The pattern can be inferred, however, by watching thousands of atoms and calculating the probability that a given atom will be found in this place or that. The effect is much like viewing a pointillist painting--a single atom contributes a single dot, and together they create a clear image.

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 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.

We calculated that somewhere between a bacterium and an amoeba, we’d run out of time, Pritchard jokes. At this size, he says, an object would need two years to pass through the interferometer instead of the two milliseconds typical for an atom, and if it took longer than that he couldn’t attract any graduate students to help him run the experiments.

Of course, Pritchard notes, a number of practical obstacles would prevent him from going this far, even if he could figure out how to make a bacterium beam. The interferometer is exceptionally sensitive to vibrations, changes in temperature, and other disturbances, and to obtain an interference pattern with atoms demanded such measures as hanging the machine from the ceiling to reduce vibrations. Working with something as large as bacteria would make the interferometer so sensitive that the gravitational pull from a truck pulling up at the loading dock would screw things up.

Talk of creating interference patterns with bacteria or large molecules--or even single atoms--leads to the question, What is really going on in the interferometer? When pressed on how he visualizes the activity in his machine, Pritchard professes to be an agnostic. I think of the interferometer as a black box. We put things in one end and get a pattern out the other end. But what does the atom really do? It’s a mistake even to ask, he says, for this assumes that there is a reality in that black box, and that assumption will inevitably get you tangled in the contradictions of quantum mechanics. The most that one can say is that when observed, the atom appears as a particle, and when no one is looking, it seems to take on the form of a wave, which can be described by a wave function--a purely mathematical construct. Although Pritchard and other physicists fall back on metaphors, such as water waves splitting up and recombining, Pritchard cautions against attaching physical significance to the language they use when they try to put the mathematics into words. Quantum mechanics says there is no reality when you don’t make a measurement, he says. From this point of view, the atom waves passing through the interferometer aren’t real; they are merely convenient fictions invented by physicists to predict where the atoms will appear when they are observed.

These convenient fictions can have real payoffs, however, when put to work in an interferometer. A hundred years ago, some of the most important measurements in physics were being performed with light interferometers--such as the Michelson-Morley experiment, which found the speed of light to be the same in all directions and paved the way for Einstein’s special theory of relativity. The atom interferometer is a useful measuring device for precisely the same reason that the light interferometer is: its interference pattern is extremely sensitive to conditions along the paths that the two beams take. If something causes one beam to, say, slow down slightly, the interference pattern will shift noticeably and the shift will indicate how big the slowdown was.

This sensitivity has allowed Pritchard to measure the electric polarizability of the sodium atom--the extent to which a sodium atom’s electrons are pulled off center by an electric field--with an accuracy ten times greater than ever before. He did it by placing an electric field across the path of only one of the beams in his interferometer and observing the resulting shift in the interference pattern. In another experiment, Pritchard measured the force of attraction between sodium and atoms of different gases. This he accomplished by placing a gas in the path of one arm of the sodium beam. Each sodium atom sped up as it approached an atom of gas and slowed down as it left the atom behind, and in the process the interference pattern shifted almost imperceptibly. By measuring this subtle shift, Pritchard obtained details about these interactions between atoms that no one else had.

Pritchard even tried rotating the interferometer at about the same speed as the hour hand on a clock. Since the atoms in the beam continued to fly straight, the resulting interference pattern shifted slightly. By measuring this shift, he was able to reverse calculate the speed at which the interferometer was rotating. Eventually, Pritchard suggests, this capability might make atom interferometers useful in ultrasensitive inertial navigation systems or other applications that require the detection of tiny movements.

Although using the interferometer as an exquisitely sensitive yardstick is interesting and potentially very useful, Pritchard’s favorite experiment was far more academic. It told him nothing that quantum physicists haven’t known since 1925. Rather it had value as a demonstration of theory--a particularly vivid and exactingly controlled demonstration, and one that allowed him to linger at the boundary at which a particle begins to act like a wave.

The experiment started merely by observing the sodium atom as it passed through the interferometer. To do this, Pritchard placed a laser slightly before the second diffraction grating and shone it across the paths of both arms of the sodium beam. When a photon from the laser struck a passing atom, it bounced off, providing a way to see the atom. Actually, Pritchard didn’t even bother to place a photo-detector in the path of the deflected photons. What mattered was that he could have if he’d wanted to. As far as the atoms in the interferometer were concerned, they had been caught in the act. They were forced to take on the guise of particles, as quantum mechanics says they should. As a result, the interference pattern vanished.

Next, Pritchard moved the laser much closer to the first diffraction grating, so that it passed across the two beams before they had diverged so much. As if by magic, the interference pattern reappeared. Why? In the act of observing, the details you can make out are limited by the wavelength of the light--or X-rays, or whatever type of electromagnetic radiation you happen to be using. Thus it is impossible to build a light microscope powerful enough to see an atom, no matter how large the lenses, because the wavelength of visible light is a thousand times larger than an atom. Likewise, when Pritchard moved the laser to its new position, the separation between the beams was less than the wavelength of the laser light, which made the two beams indistinguishable. In effect, even though the atoms passing through the interferometer had been struck by photons, their positions had not been observed because the picture, so to speak, was too fuzzy. That restored the uncertainty about which arm they were in, and the two components of the atom beam once again began to interfere with each other.

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.

Such splitting of atomic hairs may one day actually turn out to have some practical significance. It’s been suggested that quantum computers would carry out calculations with particles that are in two or more quantum states at once. This would allow them, in theory, to perform calculations that are unthinkable for present-day computers, whose electrons exist in only one place at a time. Whatever form quantum computers ultimately take, the need to control the transition from particle to wave and back again will be crucial to their feasibility. We’ll need to reverse it, protect it, distill it, error-correct it, and so on, says Pritchard. His experiment is an ambitious first step in that direction. And even in the strange, counterintuitive world of quantum mechanics, there needs to be a first step.
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