Let's further suggest that, for maximum flexibility, each quantum dot is controlled by 16 electrodes with independent voltage sources. This means 16 separate conductor traces feeding into the chip for each of our several billion dots. That's a lot of wires, and a lot of independent voltage sources. Impractical? An obvious simplification is to break the grid up into smaller "tiles," say groups of 8 by 8, or 64, quantum dots. Each dot on a tile would be controlled independently of the others, but each tile of 64 would behave the same as every other tile. The 16 voltages controlling any given dot are also passed along to the same location in the neighboring tiles. Thus, only 1,024 different voltages (16 by 64) are needed to control a tile-floor chip of arbitrary size.
This may sound like a limitation, but if each electrode can be set, for example, to 256 discrete voltages, each designer atom will have 25616, or 3.4 x 1038 possible states. Compared with the 92 states of the periodic table, this is a staggering number, and if we place three designer atoms together, the number climbs to 10 quadrillion googol, or 1.02 x 10115 - higher than most calculators can count. So an 8 by 8 grid - more than 21 times as large - represents an absurd and downright spooky wealth in undreamed-of materials. Finding needles in that cosmic-scale haystack will be the work of lifetimes. Controlling the chip itself, however, is relatively easy: With only 1,024 signals to worry about, our only problem is splitting and routing these to the individual quantum dots.
Laying a few hundred of these chips side by side will result in exactly what I promised earlier: a TV screen that changes substance as easily as it changes color. With minuscule power consumption, it could easily switch from lead to gold and back again, many times a second. And since it isn't limited to the 92 natural elements, it would be capable of taking on characteristics that natural substances can't. It's a reasonable bet that there'll be better superconductors than today's yttrium barium copper oxides, better reflectors than the mercury and silver we use today, better photoelectric converters than silicon. In fiction, I've even posited the existence of programmable matter "superreflectors" and "superabsorbers," which process light in a given frequency band with 100 percent efficiency.
They won't change mass or shape. But with the flick of a bit, artificial atoms will change from one miraculous pseudosubstance to another.
Really, such chips would be capable of doing and being so many things that it's easier to start from the other end and list their limitations. They can't change their mass. They can't change their shape, although they can be mounted on the surface of something that can. They also can't self-replicate, although they can presumably be mass-produced by a sufficiently advanced nanotechnology. Also, while their chemical properties are real, they're not straightforward - the atoms of the semiconductor substrate don't simply go away. At best, you'll have an atomically thin programmable layer sitting on a bed of silicon or gallium arsenide. At worst, you'll have discrete programmable islands jutting up from the substrate like stones in a Japanese garden.
Too, since their electron orbitals are about 50 times larger than those of natural atoms, they won't interact with natural atoms in a natural way. Clever choice of quantum dot settings could allow bonding between artificial and natural atoms, but even so, the spacing of the dots is a major limitation. For example, we could tile the chip's surface with ersatz glucose molecules, but these would be so big our taste buds wouldn't recognize them. Still, if we really want the chip to taste sweet, or sour, or like filet mignon, future engineers may find some dot settings to approximate it.
Wellstone: A Logical Endpoint
A final, important shortcoming of this technology is its lack of 3-D structure. The programmable layer is a nanoscopically thin veneer on the surface of the chip, capable of mimicking only two-dimensional molecules. This rules out the vast majority of organic substances, inorganic crystals, and nanomachine components. You can't command a diamond coating to appear on the chip, or even a quartz one. Fortunately, this limitation also has a solution: We roll the chip into a long, thin fiber. With the P-N-P layers of the quantum well, the conducting traces on top of them, and the memory and insulation layers beneath, this fiber would have a minimum diameter of about 60 to 80 nanometers (300 to 400 atoms), meaning we could fit 10 to 13 artificial atoms around its circumference and a potentially infinite number along its length.
Once we have these fibers, we can string them up in a 3-D lattice not unlike the skeleton of a building, or else weave them together tight as basket wicker. This is a tough nano-assembly job either way, but once it's complete we have artificial atoms bumping right up against one another, able to bond with neighbors on the same fiber and/or adjacent fibers. Now we can create not only a thin film of goldlike pseudomatter but a three-dimensional solid with the mass of wickered silicon but the physical, chemical, and electrical properties of an otherwise-impossible gold/silicon alloy. Or mixtures of other metallic or nonmetallic substances, including the "unnatural" and "impossible" ones discussed above. And with the flick of a bit, the voltages on the quantum dots can be altered, to change the solid from one miraculous pseudosubstance to another.
In a discussion on this subject in the summer of 1998, Gary E. Snyder of Pioneer Astronautics and I coined the name "quantum wellstone" (or simply "wellstone") to describe this hypothetical but plausible form of programmable matter. It's a term that has served me well in my fiction.
As for the material's mechanical strength, the bonding between quantum dots on the same fiber is limited by their spacing, which is a function of how small we can reasonably make the fences. These bonds will be weak: at best, about .0025 percent as strong as a 5-ev carbon bond. If you programmed in a bunch of carbonlike atoms, you'd get back a sort of watered-down diamondoid crystal. The bulk properties of such a material are difficult to estimate, but they're probably quite different from both silicon and carbon, in the same way fiberglass is different from bulk SiO2 and polymer resin. And remember, we have an infinity of electron patterns to play with, so if we don't like the properties we end up with, we can simply adjust them.