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How do you simulate a quantum gauge theory in a gauge with negative norms on a quantum computer? There are some gauges with negative norms. It's true that if restricted to gauge invariant states, the norm is always positive semidefinite, but how do you simulate that on a quantum computer?

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Why do you ask about numerical simulation on a quantum computer? This seems like a red herring. – user1504 Jun 27 '12 at 13:44
@user1504: It's not a red herring. There is a theorem that you can efficiently simulate any quantum system on a quantum computer (and it is known, and all but proven with Shor factoring, that you can't do this with a classical computer), but the computer is restricted to positive norm states (obviously), so the question is whether you can simulate the negative norm states, or whether you are restricted to simulating in physical gauges. It's a strange and potentially deep question. – Ron Maimon Jun 28 '12 at 1:36
...and that's why it doesn't hurt to spend a sentence or two on background material. I suppose the question is really whether it's possible to ensure that the time evolution, at least approximately, doesn't violate the choice of gauge. – user1504 Jun 28 '12 at 1:47

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