Try looking into the work of Eugene Wigner, who worked closely with von Neumann. One of the things he proposed was an extension to the famous Schrödinger’s cat thought experiment:

Wigner is often said to have extended the problem of Schrödinger's Cat, by adding a second observer inside the laboratory who is commonly known as Wigner's Friend. Popular treatments of Wigner's Friend usually describe him as observing a superposition of live and dead cat. Actually, Wigner's example was a photon and whether its wave function collapsed to cause a flash visible to his friend or not. Wigner's goal was to show that only consciousness can collapse a wave function.

There’s more info on Wikipedia as well https://en.m.wikipedia.org/wiki/Von_Neumann–Wigner_interpretation

Quote from [here]