Small Wonders

Courtesy of IBM Research Division Atomic corral with probability waves

The scanning tunneling microscope has another trick, too. The tip of its probe can stick to individual atoms lying on a surface and move them into, say, circles. These are the quantum corrals. The microscope can then take pictures of how electrons confined inside a corral of atoms behave. (More precisely, the pictures are graphic representations of the probability waves created by the ephemeral electrons. The probability waves, which look like ripples in a pond, reflect measurements made by the microscope, and are graphical presentations of the probability of electrons being present in a certain space. Where there are crests in the waves, there are probably more electrons.)

The ability to move individual atoms is key to building nanomachines, not just nanostructures. The first atom-sized machines are likely to be switches, inside which atomic structures function as conductors and insulators, like today's microchips, but many times smaller and more efficient.

Crommie says his group's work is "pretty far ahead of today's industrial applications." But he expects the not-too-distant future to feature devices in which individual atoms function like toggles in a household light switch. The problem with Crommie's nanotechnology right now is that it all takes place at temperatures a few degrees above absolute zero, where the nearly perpetual movement of atoms is stilled. Whereas Tom Kenny's cantilevers, and Hongjie Dai's nanotubes, work at room temperature, Crommie's even smaller structures jitter themselves into smithereens at normal temperatures. Like many of his colleagues, though, Crommie looks to the living body for inspiration. DNA, proteins, and cells of all sorts already function as self-assembling nanoscale machines in animals and plants, and they function at normal temperatures.

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Charles Marcus, nanotechnologist and professor of physics at Stanford, shows off an artificial atom -- a quantum dot. Peering through the lenses of an optical microscope, it is possible to see little gold wires trailing off into nothingness. "Somewhere down there," muses Marcus, "is our little device."

Marcus is a nanotech enthusiast; as such, he believes that scientists should be dreamers. But it is important not to confuse scientific dreaming with the real thing, he opines. Like all nanoscientists, Marcus is aware that the media's perception of nanotech is largely shaped by the Foresight Institute. Marcus says he has nothing against the Foresight Institute's predictions. But ...

"Eric Drexler's book contains some useful engineering formulas. It's just not useful to my research. And I think it's fair to say that the future of nanostuff will be even wilder than Drexler has imagined," Marcus remarks.

Marcus' quantum dots usually live in the bottom of super-cooled refrigerators where electricity and magnetic fields are applied experimentally to test the dots' properties. A quantum dot can be as big as 500 nanometers, but its "walls" are only a few atoms thick. One of the most amazing things about a quantum dot is its ability to "element-shift." By changing the voltage of the electricity flowing through the dot, the artificial atom can mimic any one of the more than 100 elements appearing in the periodic table, such as hydrogen, magnesium, carbon, or potassium. It can also make elements that never yet existed by simply adding extra electrons to the mix.

The quantum dot's chameleon quality occurs because it "traps" electrons inside its structure. Depending on how many electrons it traps, it roughly assumes the characteristics of an element. It is easy to speculate about the future use of artificial atoms as manufacturing materials, once they are released from their super-low temperature cages. But using quantum dots as switches and components in electrical circuits could also be the basis of a new kind of quantum computing, says Marcus. Such quantum machines, also known as nanocomputers, would make today's most powerful computers look like prehistoric counting sticks.

If the basic paradox of chaos and order can be overcome.

Quantum mechanics' uncertainty principle says that before an atomic particle is measured, it exists in all possible states, all superimposed on one another. An electron, for example, is best described by what physicists term a "probability wave function," a mathematical expression that describes the chance that the electron is traveling at a range of speeds over a range of places. Once you measure its speed or position, quantum reality decoheres, the indefinite wave function "collapses," and either its speed or its position becomes definite. But not both at once.

Marcus says, "Once a measurement has been made, then all of the possible ways that things could have come out vanish, leaving only the way in which things did come out."

Re-enter the quantum dot, which connects the classical and quantum worlds. Inside the dot, electrons can be trapped and controlled for certain amounts of time. The theory of quantum computing shows that if information is stored in the dot's trapped electrons, before the electrons are measured all of the superimposed possibilities form an ultra- complex database.

If quantum computing comes about, less space will hold more information.

Think of it this way: Today's transistors, or microswitches, can be controllably switched to either state 0 or state 1 -- the either/or phenomenon that makes electronic computing possible. The 0s and 1s are coded bits of classical information. Computing capacity depends on how many switches can be built and interconnected.