JohnBarchak said:
Some people did not seem to understand my comment about superposition never having been observed. I was not questioning how you get information out of a quantum computer. I was questioning the validity of the superposition concept itself. How can you convince anyone of the validity of a concept for something that has never been observed. We know that an electron has a magnetic moment through observations made on electrons. What observations tell us that superposition exists? QM is advertised to be based on "observables." What observable tells us that a particle is in a state of superposition?
All the best
John B.
Agreed. People seem to have forgotten that the
only evidence we have that superposition is real is the Bell test experiments, and these, as we know, have loopholes that allow for local realist explanations. As Marshall et al said in their seminal 1983 paper,
T W Marshall, E Santos and F Selleri, F, “Local Realism has not been Refuted by Atomic-Cascade Experiments”, Physics Letters A, 98, 5-9 (1983),
"Any serious examination of the realist alternative must involve challenging assumptions based on three generations of immersion and acculturation by quantum concepts".
Some people here are arguing that there is all sorts of other evidence for superposition. Let them specify it! Back in 1972 John Clauser (of the CHSH Bell test) wrote an interesting paper:
Clauser, J E, “Experimental limitations to the validity of semiclassical radiation theories”, Physical Review A 6, 49 (1972)
He was able to give "semi-classical" explanations for all the phenomena claimed to be best explained by QM apart from one little area, that of entanglement of separated particles -- precisely the area that is tested in the Bell tests. He, unfortunately, he was under the impression that experiment already showed that in this area QM was correct. The above paper was written before the first actual Bell test (his own, with Freedman, also published in 1972) had been done, and the data he was going on was that from the Kocher-Commins 1967 expt.. I have reason to believe that, like Marshall et al in 1983, he was unknowingly trying to explain data that had been adjusted by subtraction of accidentals. Had he allowed for this, he would have been able to explain this too, using his same semi-classical methods.
Anyway, you are asolutely right, John: the superposition principle has never been proved to be true. Real wave systems can, of course, exist with many frequencies at once -- a bell can ring producing a complicated spectrum of sounds -- but these are just mixtures, not quantum-theoretical "superpositions" of states. Since our general experience of the world suggests that quantum superposition is physically impossible and so does not happen, I think we can safely bet that quantum computers will never achieve any improvement over classical ones. I forsee many
simulations of quantum computers, but actual ones few and far between, with unimpressive performance.
Hopefully, all the effort put into them will not have been entirely wasted! Perhaps the algorithms developed will prove useful in their own right.
Caroline
http://freespace.virgin.net/ch.thompson1/
