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### How to take Statevector for subsystem?

In general, a subsystem could be entangled with the rest of the system and we can not describe it using a state vector. A general method that can be always used to describe the state of a subsystem is ...
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In case anyone else gets caught up on this detail: I spoke to Soonwon Choi and he explained that the "linear" cross-entropy is not a linearization of the cross-entropy. Rather it is called ...