Modeling a measurement as unitary

[The_Duck]

By "moving wave packets" I assume you mean something like coherent states?

I mean a wave packet like $\psi(x) = \exp(i k x - x^2/2\sigma^2)$.

 

[atyy]

Although I think it is only collapse that is necessarily non-unitary, and since measurement and collapse can be used as state preparation, I don't think MWI has to solve the problem of state preparation separately from the non-unitarity of collapse. But since we discussed time-dependent Hamiltonians above, here are some references about the extent to which unitary operations with time-dependent Hamiltonians enable one to move between any two states.
http://arxiv.org/abs/quant-ph/0106128
http://arxiv.org/abs/quant-ph/0108114

 

[stevendaryl]

Although I think it is only collapse that is necessarily non-unitary, and since measurement and collapse can be used as state preparation, I don't think MWI has to solve the problem of state preparation separately from the non-unitarity of collapse. But since we discussed time-dependent Hamiltonians above, here are some references about the extent to which unitary operations with time-dependent Hamiltonians enable one to move between any two states.
http://arxiv.org/abs/quant-ph/0106128
http://arxiv.org/abs/quant-ph/0108114

I don't understand the comments about MWI. I thought the whole point of MWI was to get rid of collapse, so there is nothing non-unitary going on.

 

[atyy]

I don't understand the comments about MWI. I thought the whole point of MWI was to get rid of collapse, so there is nothing non-unitary going on.

Yes. What I'm saying is that if MWI is able to convincingly get rid of collapse, then it doesn't have to solve state preparation as a separate problem.

The other point I was making is that although collapse can prepare a state, state preparation within Copenhagen is not necessarily non-unitary, but collapse is necessarily non-unitary.