This Looks Like a Job for...Superatoms

Indeed, inertial guidance beats GPS not only by telling you which way you are pointing but by actually functioning when you're indoors, underground, underwater, in the canyons of Manhattan, in dense jungle, or in deep space. Those are places where GPS won't work, and they are just where people, and their robots, are the likeliest to get lost.

Another important application for atom lasers is holographic printing. Like optical holography, atom holography gives a true, three-dimensional image of an object, but because of its short wavelength, it can resolve details 70 000 times as fine. And just as atom particles can be made to undergo a phase change to a wavelike state, so can an atom wave revert to particles. That means that the holographic image can deposit itself, like frost forming on a windowpane.

This research is in the fundamental stages right now, but it has exciting possibilities for circuit fabrication. In the future, engineers might be able to deposit a complex integrated circuit pattern, just a few nanometers in size, on a semiconductor surface. Building a circuit like this, an atom at a time, would allow circuit components that are perfect down to an atom.

Conventional holograms are made by splitting a laser beam and sending one half, the reference beam, straight to a piece of film and the other half, the object beam, to the object to be imaged. Because the light bouncing off the object arrives later than the reference beam, it is out of phase with it and hence able to interfere with it. The interference pattern is the hologram, because it can now function as a diffraction grating: shine a laser on it, and it will reconstitute the 3-D image.

So a hologram creates an image by diffracting light the right way. Does that mean that any diffraction grating--a plate or "mask" drilled with thousands of holes--will also act like a hologram? Yes, it will, if it has the right pattern of holes. This is useful in optical holography when you don't want to start with an existing object but instead want to create an image of an object that is in your imagination, like a spaceship. You could use a diffraction grating with the right pattern of holes as a hologram that gives you your spaceship image.

When an atom-laser gyroscope can finally fit inside a cellphone, children will ask, "Daddy, how did you ever find your way around when all you had was GPS?"

This technique is critical to atom holography, because atom waves cannot illuminate an object; the minute they hit something, they revert to particles and stick. (For the same reason, an atom laser beam cannot work in the open air, only in a vacuum.) Instead, you must find a way to perforate a diffraction grating so that it constitutes a hologram.

How do you get the right pattern of holes for your diffraction grating, or holographic mask? You use a mathematical trick known as a Fourier transform, originally developed to describe complex shapes in mathematical terms. Any complex shape may be regarded as the sum of a number of simpler ones, notably sine waves. Such waves are described by a straightforward formula involving frequency and amplitude. A hologram, or any interference pattern created from an object, stores precisely this kind of information about an object, and therefore a hologram may be considered a Fourier transform of the object. Similarly, the 3-D image is the "transform" of the hologram--that is, the solution to its equation.

So if you know the image you want, you can work backward to get the hologram that specifies it. This information can be translated into a pattern for a holographic mask. You can then use electron-beam lithography to punch holes in a grid, making the mask.

Suppose the image you want is the pattern of an integrated circuit. You work backward to get the hologram, then perforate the pattern into a grid to make a mask. Then you shoot an atom laser through the mask. When the Bose-Einstein condensate in the laser beam passes through the mask, it breaks up into wavelets. The wavelets then interfere with each other to create the image of the integrated circuit on the substrate, "frosting" it in the features of the circuit [see illustration, ].

The first images made with atom holography were created by Fujio Shimizu at the University of Electro-Communications, in Tokyo, and his colleagues at the NEC Fundamental Research Laboratories, in Tsukuba, Japan, in 1996. They successfully deposited millimeter-wide letters and even tiny art images.

Since then, Shimizu's researchers have taken holographic masks a step further and developed a neat trick to produce layers in different patterns. Their holographic masks have strips of charged wires around the holes. A potential difference between two strips causes a phase change in the atomic wave, altering the image formed. Switching the charge on different wires thus makes different images without having to drill a new mask each time. The scientists have already built masks with 150-nm precision and are working on designs with lines as fine as 1 nm.

With atom holography, if you were willing to make a circuit out of superatom-suitable elements like rubidium and sodium, you could simply deposit the entire image directly onto the wafer in a single operation, in the solid analog of photolithography. If you needed a material that doesn't easily form a Bose-Einstein condensate--such as gold or aluminum--you could use rubidium atoms to expose a resist. Then you could use the resist to etch gold lines into the wafer with the usual techniques.

In regular patterns like gratings and grids, the lines could be as tiny as one atom wide and positioned to within less than an atom's diameter. This capability would make atom holography especially useful in the production of photonic devices, such as nanoscale Fresnel lenses and diffraction gratings used in optical sensing, telecommunications, and imaging. On a somewhat larger scale--feature sizes of around 5 to 10 nm--these lines can combine to form any desired image, including a complete integrated circuit.

No commercial systems exist yet for making nanoscale circuits with atom holography, but researchers are hopeful that someday they will. And if IBM Corp. was able to show off its atomic-force microscope by spelling out its name in xenon atoms, why shouldn't JILA and MIT go it one better with atom holography? They could imprint a transistor with their logos--or, better yet, a portrait of Einstein and Bose.