And since the late '80s, another application for P-N junctions has been discovered that may, in the end, prove even more revolutionary. When an N layer is sandwiched between two Ps, a kind of trap is created that attracts electrons into the middle layer and doesn't let them out. This is a useful trait all by itself and leads to a couple of exotic variants on the standard transistor. But if the N layer is really thin - about 10 nanometers (equal to 0.00001 millimeters or 50 atoms) - then something weird starts to happen. The size of the trap approaches a quantum-mechanical transition point: the de Broglie wavelength of a room-temperature electron.
The result? Along the vertical axis of the trap, the excess electrons can no longer move and propagate in the Newtonian way. Their positions and velocities take on an uncertain, probabilistic nature. They become waves rather than particles.
These P-N-P devices, known as quantum wells, are easy and cheap to produce and have the interesting property of generating photons of very precise wavelength, which means they can be used to make laser beams. Quantum wells find practical use in computers, fiber-optic networks, and those cute little $7 laser pointers you can buy for your keychain.
Like the meat inside a sandwich, a quantum well's electrons are confined in a two-dimensional layer. But if the meat and the top bread layer are cut away on two sides, leaving a narrow stripe of P-N sandwich sitting on a slice of P bread, the electrons take on wavelike behavior along an additional axis. This structure is called a quantum wire and is used to produce intense laser beams that can be switched on and off much more rapidly than quantum well lasers can - up to 10 gigahertz, or 10 billion times per second. Quantum wires can also be used as precision waveguides, and of course as actual wires.
The electrons trapped in a quantum dot arrange themselves as if they were part of an atom. With a big difference: This particle has no nucleus.
But quantum wires are only a stepping-stone here. They lead us to a final configuration: etching away the ends of the stripe to leave a tiny square of meat and bread atop the lower layer, producing a "quantum dot" that confines the electrons in all three dimensions. Unable to flow, unable to move as particles or even hold a well-defined position, the trapped electrons must instead behave as de Broglie standing waves, or probability-density functions, or strangely shaped clouds of diffuse electric charge. Strangely shaped because, even as waves, the negatively charged electrons will repel one another and attempt to get as far apart as their energies and geometries permit.
If this sounds familiar, it's because there's another, more familiar place where electrons behave this way: in atoms. Electrons that are part of an atom will arrange themselves into orbitals, which constrain and define their positions around the positively charged nucleus. These orbitals, and the electrons that partially or completely fill them, are what determine the chemical properties of an atom - such as what other sorts of atoms it can react with, and how strongly.
This point bears repeating: The electrons trapped in a quantum dot will arrange themselves as though they were part of an atom, even though there's no atomic nucleus for them to surround. Which atom they resemble depends on the number of excess electrons trapped inside. What's more, the electrons in two adjacent quantum dots will interact just as they would in two real atoms placed at the equivalent distance, meaning the two dots can share electrons between them - they can form connections equivalent to chemical bonds. Not virtual or simulated bonds, but real ones.
Amazing, right? If you're not amazed, go back and read the last three paragraphs again.
Now we'll take it a step further: Quantum dots needn't be formed by etching blocks out of a quantum well. Instead, the electrons can be confined electrostatically by electrodes whose voltage can be varied on demand, like a miniature electric fence around a corral. In fact, this is the preferred method, since it permits the dots' characteristics to be adjusted without any physical modification of the underlying material. We can pump electrons in and out simply by varying the voltage on the fence.
This type of nanostructure is called an artificial or designer atom, because it can be manipulated to resemble any atom on the periodic table. It's not a science-fictional device, but a routine piece of experimental hardware used in laboratories throughout the world.
Where do artificial atoms come from? The poetic answer is that they arise from the sweat and dreams of human beings such as Marc Kastner and Charlie Marcus, and the dozens of eager grads and undergrads and postdocs and visiting fellows who work for and with them. There are a few dozen labs worldwide engaged in this research, and the atmosphere among them is laid-back and clubby. This is basic research, geared toward methodical discovery rather than near-term commercial payoff. Collaborations and data sharing are the norm. Marcus, one of the youngest and most energetic of the lot, is always in motion, always on an errand, always engaged in jovial conversation with someone, with everyone. Laughing about it, he tells me, "Why bother hogging glory when there are so many hard problems still to be solved? The greatest threat out here is dying of loneliness."
The less poetic answer is that artificial atoms usually come from the same sort of semiconductor laboratories that produce exotic computer chips. This isn't surprising, since these labs are already expert at creating precision nanostructures, and since shrinking electronics are pushing their way into the mesoscale anyway. The marriage of the two fields is a natural: easy to explain, to fund, to justify.
For the price of coffee, any condensed-matter physicist will happily explain the future of computing. Quantum dots can serve as really, really small transistors, whose logic is many times more efficient than that of today's transistors and operates on single electrons. They can also serve as cellular automata - spreadsheet-like systems in which each "cell" contains a formula that defines its state as a function of the states of neighboring cells (a great way to simulate weather or fluid mechanics). In addition, scientists have shown that an array of quantum dots passing around excess electrons is, mathematically speaking, a kind of neural network. Quantum-dot "neurons" may one day display some of the same traits as biological ones, despite being orders of magnitude smaller.