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I hope some of you are more proficient in juggling with bra-kets...

I am wondering if/when the density operator commutes with other operators, especially with unitaries and observables.

1. My guess is, that it commutes with unitaries, but I am not sure if my thinking is correct. It's like this:

[itex]\langle x|\rho U^{\dagger}|x\rangle=\langle x|U^{\dagger}\rho U U^{\dagger}|x\rangle=\langle x|U^{\dagger} \rho|x\rangle

[/itex]

Is that ok?

2.How do I determine, if a state commutes with its observable? I don't see a physical meaning in this, I'm not sure what to do...

Thanks

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# Commutator of the density operator

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