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Rice Contracted to Provide NASA's Quantum Wire
geekman writes "NASA is paying Rice University $11 million to build a prototype quantum wire that can conduct electricity 10 times better than traditional copper cables at one-sixth the weight. Rice has four years to build a one-meter-long quantum wire, which will be made out of carbon nanotubes. Seems like a lot of money for a little wire, but then again, all the rocket scientists at Los Alamos have only ever been able to put together a four-centimeter nanotube."
I believe this refers to the ballistic conduction that takes place in carbon nano-tubes and is a quantum phenomenon. Basically electrons experience a small resistance entering and leaving a nano-tube, but then near zero resistance travelling along them.
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These things could be the next revolution after fiber optics for network communication, so there is reason to be excited. I wonder if there would be too much interference to run these things in a twisted pair configuration.
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Yes, they are.
A metallic carbon nanotube carries 4 quanta of current (4 charge carriers at a time): 2 conducting channels, 2 spins per channel. That's what NASA is referring to as a quantum wire.
Most of the resistance in such a wire is due to the fact that only a very few number of charge carriers can be transmitted at any time. The electrons going through the wire do not lose any energy in the wire, as there are no available lower energy states for them scatter into, and only two possible directions of motion (foward and backward). Thus, a perfect nanotube can be thought of as a "ballistic" conductor. There is some resistance to putting current into it and getting it back out, but in between, there is no resistance in the normal sense. (Although this sounds a little like superconductivity, it is definitely not.)
In a real nanotube, there are defects, contact resistances, impurities and environmental factors which act as transmission barriers, raising the probablility that an injected electron will reflect back to the source and not make it all the way through. It will be interesting to see how the Rice guys plan on annealing or growing their meter long wire to maintain the desired properties (and that's where the money comes in). Simply weaving a bunch of small nanotubes together is not going to cut it.
Hell, even the theory of how they conduct electricity is younger that superconductors, and just see how many of those we have around.
As an aside, superconductivity is now very well understood. It's just that the race for a room-temperature superconductor has stalled out. In those fields where they can afford to keep the superconductors below critical temperature (e.g. NMR/MRI machines), superconductors are very widely used.
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