Fig. 1: Wigner’s friend experiment.
From: Reply to: Quantum mechanical rules for observed observers and the consistency of quantum theory
The experiment concerns two quantum mechanics, Alice and Wigner. Alice measures a system S and records an outcome x. Alice applies the Heisenberg cut around system S: she treats S as a quantum system but regards herself, her notebook, and the outcome x as classical. Wigner analyses the situation from the outside, applying the Heisenberg cut around Alice’s entire isolated lab: he models Alice, her notebook, her measurement instruments, and everything else in her lab as quantum systems undergoing a global reversible evolution.
