X-Message-Number: 6130 From: (Tad Hogg) Date: Tue, 30 Apr 1996 17:38:18 PDT Subject: SCI.CRYONICS Re: Quantum Computers In article <4lfhqm$>, John K Clark <> writes: > When a conventional 64 bit single processor computer performs an > operation, it does it on ONE 64 bit number at a time. When a 64 > bit (actually a 64 qubit) single processor QUANTUM computer > performs an operation it does it on ALL 64 bit numbers at the > same time, all 2^64 of them, more than a billion billion, > and any increase in the number of qubits the computer can handle > will increase it's already astronomical power exponentially. Even if these machines can be built reliably, there's the problem of how to program them. Operating simultaneously on ALL 64 bit numbers is the easy part for these machines! The real trick is to get a useful answer from the computation. This requires "measuring" the quantum state. Unfortunately this just gives you the result for ONE number, chosen randomly with probabilities determined by the final state. This measurement destroys the quantum state, so no further information is available on the rest of choices. Programs must use a second aspect of quantum computers, interference, to combine different ways to compute the each result. Properly done this gives a high probability to get a desired SINGLE answer from among ALL the choices. But this requires clever design of algorithms, e.g., to allow different ways of computing undesirable results to "cancel" each other out. So far as I know, only two specific algorithms using interference have been proposed -- one for factoring integers and another, more complex, one for general search problems. A lot more work is needed to invent more algorithms and see how well they might work when subject to errors and noise. Further information is available at ftp://parcftp.xerox.com/pub/dynamics/constraints.html#quantum including pointers to other quantum computing pages at Stanford and Oxford. While it is too soon to tell if quantum computers will be practical, they could substantially increase the computer power available for determining the correct way to repair frozen tissue. Additional discussion of the computational issues of repair is at http://merkle.com/merkleDir/cryptoCryo.html Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=6130