Wigner's friend

Wigner's friend is a thought experiment proposed by the physicist Eugene Wigner; it is an extension of the Schrödinger's cat experiment.

The thought experiment posits a friend of Wigner who performs the Schrödinger's cat experiment after Wigner leaves the laboratory. Only when he returns does Wigner learn the result of the experiment from his friend, that is, whether the cat is alive or dead. The question is raised: was the state of the system a superposition of "dead cat/sad friend" and "live cat/happy friend," only determined when Wigner learned the result of the experiment, or was it determined at some previous point?

The standard algebraic description of the experiment is by means of a tensor product of three states: the state of the cat, the state of Wigner's friend, and the state of Wigner himself. Let ${\displaystyle |1\rangle _{C}}$ represent the state of the living cat, and ${\displaystyle |0\rangle _{C}}$ represent the dead cat. The initial cat state is then the quantum superposition

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