Wednesday, October 15, 2014

The people who said QC was impossible

MIT Complexity theorist Scott Aaronson just gave a talk on his favorite topics in quantum computing complexity, and his slides concluded with:
Single most important application of QC (in my opinion): Disproving the people who said QC was impossible!
Dream on!

At least he admits that no one has disproved us yet. Quantum computing is impossible. If I am right, then the majority of what he says is meaningless.

His title was "When Exactly Do Quantum Computers Provide A Speedup?". The answer would be never. Everyone should just ignore this field until someone actually demonstrates some sort of speedup.

He tries to explain that quantum computers have been overhyped as being able to compute all possible inputs at once, but that is parallelism, and not how quantum computer algorithms really work. On his blog, he says this is caused by common misunderstandings:
Over the years, I’ve developed what I call the Minus-Sign Test, a reliable way to rate popularizations of quantum mechanics.  To pass the Minus-Sign Test, all a popularization needs to do is mention the minus signs: i.e., interference between positive and negative amplitudes, the defining feature of quantum mechanics, the thing that makes it different from classical probability theory, the reason why we can’t say Schrödinger’s cat is “really either dead or alive,” and we simply don’t know which one, the reason why the entangled particles can’t have just agreed in advance that one would spin up and the other would spin down.  Another name for the Minus-Sign Test is the High-School Student Test, since it’s the thing that determines whether a bright high-school student, meeting quantum mechanics for the first time through the popularization, would come away thinking of superposition as

(a) one of the coolest discoveries about Nature ever made, or
(b) a synonym used by some famous authority figures for ignorance.

Despite the low bar set by the Minus-Sign Test, I’m afraid almost every popular article about quantum mechanics ever written has failed it
This is a stupid test, because there are no "positive and negative amplitudes". The wave function for an electron is a complex-coefficient spinor function. Only in his imaginary world of hypothetical qubits are things so simple.

I do emphasize the interference. There is a always a diagram of it on the side of this blog.

But the interference is a lousy explanation for "why the entangled particles can’t have just agreed in advance that one would spin up and the other would spin down." The particles cannot agree in advance because spin (up or down) is not a discrete property of particles. We only get a discrete up or down when we do a measurement, like in a Stern-Gerlach detector. The spin has wave-like properties, like everything else in quantum mechanics, whether or not any interference is taking place.

Aaronson is writing a new book on this subject, because he complains that no one understood these points in his previous books.

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