1. Terminology here is fluid: except when quoting or paraphrasing others, this entry uses “quantum theory” to label a genus of which quantum field theories as well as non-relativistic quantum mechanics are species.

2. SIC abbreviates Symmetric, Informationally Complete positive operator-valued measure (POVM): see Renes, Blume-Cohout, Scott, and Caves 2004.

3. With a time-like world-line, an agent has no experience of space-like separated events.

4. They have been shown to exist at least in spaces of dimension \(d=2-53\); with high numerical precision for \(d=2-193\); and also for many higher values of \(d\): see Bengtsson 2020; Horodecki, Rudnicki, & Życzkowski 2022; Bengtsson, Grassl, & McConnell 2024.

5. Although several recent no-go theorems show that this conflicts with widely held assumptions in extensions of the scenario of Wigner’s friend: see Schmid, Ying & Leifer 2023.

6. Interview of Niels Bohr by Thomas S. Kuhn, Leon Rosenfeld, Aage Petersen, and Erik Rudinger on 1962 November 17, Niels Bohr Library & Archives, American Institute of Physics, College Park, MD USA: see the link in the Other Internet Resources.