Fig. 1.

Wigner’s and Deutsch’s arguments. Agent F measures the spin S of a silver atom in the vertical direction, obtaining outcome z. From F’s perspective, S is then in one of the two pure states ψ_S given in (1). Agent W, who is outside of F’s lab, may instead regard that lab, including the agent F, as a big quantum system L (orange box). Wigner argued that, having no access to z, he would assign a superposition state Ψ_L of the form (3) to L². Deutsch later noted that agent W could in principle test this state assignment by applying a carefully designed measurement to L⁶