In the wide weird world of quantum mechanics, the atom laser is out there on the fringe. Instead of photons, it shoots out ultracooled atoms, atoms so cold that they no longer move or interact like particles and instead behave just like waves. Marching coherently, their peaks and troughs all in step, like regular laser light, these wavy atoms form a beam that, unlike laser light, moves so slowly a person can outwalk it.
The beam dribbles out along an arc--like water from a squirt gun. As the beam falls, it accelerates under gravity, and the waves dramatically shorten until they are one-70 000th as long as a light wave.
The first atom laser was built in 1997 by Wolfgang Ketterle and co-workers at the Massachusetts Institute of Technology, in Cambridge. Since then, technophiles have wondered: what can we do with this incredible thing?
How about shrinking lithographic circuit patterns to nearly an atomic scale? Or measuring acceleration so precisely that you could steer a submarine blind from San Francisco to Yokohama? Or detecting gravitational variations so subtle that they show the movement of magma deep inside the earth? These applications won't arrive tomorrow morning, but they're on their way, and they could be big [see sidebar, "What's a Superatom Good For?"].
Thanks to its freakishly short waves, an atom laser can resolve much smaller features than an ordinary laser. To get an idea of just how small, consider today's most exquisitely sensitive measuring stick: the laser interferometer. To make one, you split a laser's beam into two parts that proceed along paths of different length, reflect off mirrors, and finally overlap at a detector.
Because one beam takes a little longer than the other to arrive at the detector, the waves are slightly out of phase. Not all the peaks meet up to reinforce each other in a bright line; some coalesce with troughs and cancel, forming a dark band. The resulting interference pattern turns out to be exquisitely sensitive to phase differences. Keep one mirror steady and accelerate the other, and the pattern will change; measure the change, and you can calculate the acceleration. What you've got is an accelerometer.
An atom laser interferometer would be able to register gravitational fields with great precision. That's because the atoms--unlike the photons in an optical laser interferometer--feel the pull of gravity. Such a detector could pick up even the slight variations in gravitational field strength coming from small hollows in rock formations, like underground oil deposits.
To build an atom laser, you chill atoms of a single kind to near absolute zero and herd them together. This maneuver makes them condense into a quantum mechanical blob in which they all have the same energy and position. The blob is called a Bose-Einstein condensate, because it was predicted in 1924 by Albert Einstein, who extrapolated from the work of Satyendra Nath Bose on the nature of photons.
There's a lot of counterintuitive stuff going on here, but the strangest thing must surely be the way that many atoms occupy a single position. Credit the famous Heisenberg Uncertainty Principle, which states that you can precisely know a particle's position or its energy, but not both. Because we've cooled the atoms to nearly zero, we know their energy very precisely indeed: it's practically zero.
That means we no longer know their position well at all. Instead of sitting in a sphere a few hundredths of a nanometer in diameter, the region in which each atom can exist has ballooned to micrometers in diameter. In that space, millions of them overlap, fall into the same quantum mechanical state, and become one giant superatom.
It took 70 years to create Einstein's blob, because it was necessary to chill the atoms to less than a millionth of a Kelvin--that is, less than a millionth of a degree above absolute zero, the point at which all particle motion stops. The atoms floating in interstellar space are nearly three degrees hotter. In the 1980s, when refrigeration technology finally got good enough to make the goal seem achievable, it sparked a scientific race in the classic style that ended in 1995. That was the year Eric Cornell and Carl Wieman at JILA, a physics laboratory in Boulder, Colo., jointly run by the University of Colorado and the National Institute of Standards and Technology, created the world's--perhaps the universe's--first Bose-Einstein condensate. Four months later, MIT's Ketterle created another. For their efforts, the three men shared the Nobel Prize in Physics in 2001.
Not every atom can become a superatom. Researchers have to pick an element that will resonate at a wavelength that can be produced by an optical laser. One such element is rubidium. According to one method perfected at JILA, scientists place a few grams of rubidium inside a vacuum chamber pierced by six intersecting laser beams [see illustration, ]. Rubidium atoms rise off the sample in a vapor, and the light-pressure of the beams slows the atoms, reducing their effective temperature to about one-10 000th of a Kelvin. The force of the lasers holds the atoms in place at the beams' intersection and away from the room-temperature walls of the chamber.
Next, the scientists turn off the lasers and use a magnetic field to confine the atoms. The field makes the atoms slosh back and forth so that they bang together, transferring momentum at random. Some atoms get more than the average amount of momentum and achieve escape velocity; they leave the trap, carrying energy out of the system. This cools the gas further, much as escaping steam cools your coffee.
As the atoms cool, their velocity approaches zero, their positional uncertainty grows, and they behave less like particles and more like waves. At around a millionth of a Kelvin, the atoms' wave packets are large enough to overlap. Suddenly, the material reorganizes its structure and undergoes a phase transition. The transition is somewhat comparable to that of water when its temperature reaches the point of freezing, boiling, or condensing, except that rather than trying to minimize their energy state, the atoms in the magnetic trap are all falling into the lowest quantum state. And so a Bose-Einstein condensate is formed.