Quantum erasing the memory of Wigner's friend

Wigner's friend scenarios, in which observers measure one another, have generated considerable debate over the years. Most notable in recent times was the article by Frauchiger and Renner (Nature Communications 9, 3711 (2018)), which introduced a situation whereby different agents, applying quantum theory, arrive at inconsistent conclusions. We argue that the agents' differing lines of thought can be traced to distinct experimental contexts, which can never be realized simultaneously in an experiment, since they correspond to observables that do not commute. One of these contexts involves quantum erasing the memory of Wigner's friend. We provide a single-photon interferometer that maps onto the Frauchiger-Renner scenario and illustrates our point.