#### Hurkyl

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My learning style is rather erratic -- I can't usually point to someplace and say "I learned from here".Nice, thanks! I wasn't aware of this. Where did you learn this from? Where is it discussed most clearly?

I do know that a short course on quantum computing solidified most of the meager understanding I had of quantum mechanics before then. Learning about programming such a computer means setting up circuits to compute function of some input qubits and adding the result into an qubit in some fashion (or some other invertible manipulation of the output bits) -- and that is where I got the idea of such a thing being analogous to a measurement.

I'm sure that at least

*some*of what I have subsequently read about decoherence, particularly involving decoherence-based interpretations of QM, had similar ideas in mind. I couldn't really point to anything specific, except for one.

Rovelli's paper on Relational Quantum Mechanics was the next most significant thing I encountered. It wasn't the interpretation that impressed me, but the treatment of the situation where Alice is observing Bob observe a system.

Bob, in his analysis, places the von Neumann cut between himself and the system; he does his measurement, sees the result, then continues his study as if the system has collapsed into the corresponding state.

Alice, however, places the von Neumann cut between herself and Bob. Alice does her analysis by treating Bob+System as Bob does the measurement as a quantum system, evolving according to Schrödinger's equation. She may eventually perform a measurement herself to collapse Bob+System into a definite state.

(Alas, the discussion in section II doesn't take the next step to apply decoherence or anything of the sort)

My impression is that this shows the way you can have your cake and eat it too, regarding interpretations. We know that, so long as something unusual is going on, it doesn't matter where you place the von Neumann cut between the quantum and classical world.

From Alice's point of view, we see the consistency between treating Bob+System as if it collapses when Bob makes a measurement, and treating Bob+System as if it continues to evolve a là Schrödinger.

This is made even clearer if you suppose decoherence occurs in the latter treatment, (or you partial trace to extract Bob's state from the joint system), because the quantum state is now a mixture of all the possible collapsed states.

This I don't know. But then, I don't know how it can happen in classical mechanics either, so I'm content that QM is no more lacking than classical mechanics in that regard.Let us now inquire to which extend CNOT gates can serve as measurement devices. The whole ansatz works only if your assumption ''The initial state of the target line is |0>'' from post #64 holds for each CNOT gate.

So please tell me how - in your world of quasi-measurements without what you call ''metaphysical choices'' - the target can be objectively prepared in that state.