Guys, all this stuff about switches only having two positions and magnetic media only having two states and wires only carrying two voltages and transisitors only conducting or not is all rubbish.
Note that audio and video MAGNETIC tapes are still mostly analogue, as is radio circuitry (ever heard of a TRANSISTOR radio). Transistors are amplifiers, you put in a power supply and a small voltage signal to produce a bigger voltage signal. Not only do all these electronic systems work with analogue, they worked with analogue FIRST.
The reasons that digital electronics are used are for simplicity and reliability. Signals get distorted easily, especially if you try and run lots of them near to each other. A digital signal has only two states, as far away from each other as possible (typically 0-5 or 0-3 voltas seperation for computer hardware, falling to much less within the chip). Because of this there is mininal chance of a '1' being read as a '0' or vice versa, which would make your program crash, or worse.
For digital radio transmission, more than two states are frequently used. The binary signal is translated into 4 or 16 voltage levels (hex) and sent like that, but for this you ideally want high voltages (hence, high seperation) and also you need error correction code, i.e. redundant signal information to allow for detection/correction of the odd bit that goes astray (sometimes called parity).
This would all be highly innefficient on the scale of a modern cpu, and so is not used. Analogue chips do exist for certain applications, particularly radio as they are much faster, but using an analogue chip to process digital information (which computer info is bound to be, whatever base you use, unless you want to use 16.7 million voltage levels to represent each pixel of your 24bit picture faithfully) is inherantly inneficient.
Anyway, It's not like bus width is the primary factor holding back computer tech at the moment.
The immediate future of CPUs will probably lie in 3D chips (wires running in 3 dimensions instead of just on a flat surface as now), and beyond that maybe optical.
Quantum tech will happen, but it will not use 4-state electronics, the fact that qubits are quaternions is irrelevant because they still only represent 1 or 0 depending on their spin. The purpose of qubits is to do massive parallel processing by splitting tasks across parallel universes (don't believe me? look it up - check out <a href="http://www.qubit.org
</a> and click on FAQ). This won't speed up all tasks though, it's sadly not quite equivalent to having infinite processors in you computer, although that is the general idea. Oh, and don't expect to see one on the shelves for about 50 years either.
Did any of that make sense? The first bit at least i hope.