[EMAIL PROTECTED] (John J. Lee) writes: > > Building larger ones seems to > > have complexity exponential in the number of bits, which is not too > > Why?
The way I understand it, that 7-qubit computer was based on embedding the qubits on atoms in a large molecule, then running the computation procedure on a bulk solution containing zillions of the molecules, then shooting RF pulses through the solution and using an NMR spectrometer to find a peak at the most likely quantum state (i.e. the state which had the most of the molecules in that state). To do it with 8 qubits instead of 7, you'd have to use twice as much solution, so that particular technique doesn't scale. What we want is a way to calculations on single molecules, not bulk solutions. But no one so far has managed to do even 7 qubits that way. > > It's not even known in theory whether quantum computing is > > possible on a significant scale. > > Discuss. <wink> The problem is maintaining enough coherence through the whole calculation that the results aren't turned into garbage. In any physically realizeable experiment, a certain amount of decoherence will creep in at every step. So you need to add additional qubits for error correction, but then those qubits complicate the calculation and add more decoherence, so you need even more error correcting qubits. So the error correction removes some of your previous decoherence trouble but adds some of its own. As I understand it, whether there's a quantum error correcting scheme that removes decoherence faster than it adds it as the calculation gets larger, is an open problem in quantum computing theory. I'm not any kind of expert in this stuff but have had some conversations with people who are into it, and the above is what they told me, as of a few years ago. I probably have it all somewhat garbled. -- http://mail.python.org/mailman/listinfo/python-list
