X-Message-Number: 7602 Date: Thu, 30 Jan 1997 09:53:21 -0800 (PST) From: John K Clark <> Subject: Quantum Computers -----BEGIN PGP SIGNED MESSAGE----- I sent my post on Quantum Computers to comp.ai.philosophy, this is what Hans Moravec (Mind Children,) has to say on the subject. Moravec by the way is a fan of Uploading. John K Clark ============================================================================ Date: Thu, 30 Jan 1997 04:54:51 -0500 From: Hans Moravec <> Organization: Carnegie Mellon Robotics Newsgroups: comp.ai.philosophy Cc: , , Subject: Re: Quantum Computers John K Clark writes: > For me It's still hard for me for it to sink in that an > otherworldly object like a Quantum computer could actually > exist in our worldly universe, but it's beginning to look > like one can really be built. If so the world will > never be the same. David Longley asks: > Why? Wouldn't this "just" suggest that computers and > storage devices could be made smaller (and thus faster) > and even more cost effectively? Is there any reason to > expect anything other than that? No, this is not quantum-scale integrated circuitry, but something exponentially wilder: Quantum mechanics describes unobserved systems as existing in a "superposition" of all possible states, each weighted by an amplitude and a phase. A particle whose position is known has unknown momentum. Its future path is then described by a wavefront of all possible directions it could go. This wavefront is very real, as can be demonstrated by putting a barrier with two slits in its path. After doing the experiment repeatedly, a banded pattern of hits builds up on a screen beyond the barrier, that can be explained as the interference between the two possible paths each particle could have taken to its hit on the screen. Each path has a phase and an amplitude, and for some screen positions the phases are opposite, causing the two possibilities to partially or completely cancel. Other places, in phase, they mutually reinforce. Quantum computing creates n "qbits", each in an independent unobserved state of superposition of 1 and 0. Together, they constitute a machine with all 2^n possible combinations of states superimposed. By manipulating things like external fields, this giant superposition is then stepped through a sequence of computations, without being observed, producing a superposition of 2^n answers. When the superposition is finally observed to get an answer, the 2^n possibilities interfere, cancelling some outcomes, making others more probable. By cleverly designing the computation, the interference can be tailored to filter a desired result. Quantum computing became really exciting about two years ago when Peter Shor showed how a quantum computer with a few times n qbits could be used to factor n bit numbers in little more than linear time. On a classical computer, which can be in only one state at a time, factoring seems to take time exponential in the size of the number. In some limited circumstances, quantum computing gives the effect of a parallel computer with 2^n processors, in a single machine with n quantum bits. If it were possible to push n to hundreds of qbits, a quantum computer could complete computations beyond a classical atomic-scale computer filling the universe. The catch may be that things like the noise-to-signal ratio of the final measurement, or the difficulty of maintaining the coherence of the qbits (i.e. of isolating them from any possible observation via interaction with the outside world) goes up exponentially with the number of qbits. But these limitations may be just properties of particular implementations. As the Science article showed, the fun has just begun, and dramatically better approaches are still being created and developed. One of the first approaches, using the spin states of individual laser-trapped atoms to represent qbits, lost its coherence on the time scale of milliseconds. The magnetic-resonance approach described in Science, which uses the spin of atomic nuclei, shielded from the world by their fuzzy balls of orbiting electrons, can maintain coherence for an hour! -- Hans Moravec CMU Robotics -----BEGIN PGP SIGNATURE----- Version: 2.6.i iQCzAgUBMvDdQn03wfSpid95AQE8fwTwgpykSFCpg+tazrK4q4roRE1V1VUdas4p QU4zYw7kaA+ZvASErKeGc6SHdt39MqUIark4ycMYnPlDpPL9cKhsAfuaDhdO+G0h KQ3bJy062g1nSiEqGq79rLsuxuOyGbNyR6QPPeqFQ4as6E77/yDIbfyKZhvj7yn8 QJBQix62e/lIisFMdOL6nvEZiSwkMZ+T8hs3uAmZrMFntsIpMH4= =BpPZ -----END PGP SIGNATURE----- Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=7602