Quantum mechanics is extraordinarily successful in its predictions, yet its interpretation remains conceptually unsettled. One of its deepest challenges is the measurement problem: what counts as a measurement, and when does a definite outcome emerge? These questions become especially sharp in Wigner’s friend scenarios, where different observers may disagree on whether a measurement has occurred and whether an outcome has become definite.

Our group studies the conceptual limits of quantum theory through such scenarios, in which observers perform measurements while themselves being described as quantum systems by other observers. This research probes the tension between unitary evolution, collapse, observer-dependence, and the status of measurement records, and has led to important no-go results showing that assumptions such as locality and observer-independent facts cannot both be maintained together with the predictions of quantum theory.

Further Reading: 

1. Č. Brukner, On the quantum measurement problem, in "Quantum [Un]speakables II", Eds. R. Bertlmann and A. Zeilinger (The Frontiers Collection, Springer, 2017).

2. Č. Brukner, A no-go theorem for observer-independent facts, Entropy 20, 350; doi:10.3390/e20050350 (2018).

3. V. Baumann, Č. Brukner, Wigner’s Friend as a Rational Agent, in: Hemmo M., Shenker O. (eds) Quantum, Probability, Logic, Jerusalem Studies in Philosophy and History of Science (Springer, Cham, 2020)

4. C. Brukner, News & Views: Facts are relative, Nature Physics,16, 1172–1174 (2020).

5. P. Allard Guérin, V. Baumann, F. Del Santo and Č. Brukner, A no-go theorem for the persistent reality of Wigner’s friend’s perception, Communications Physics 4, 93 (2021)

6. Č. Brukner, Wigner's friend and relational objectivity, Nature Reviews Physics, 4, 628–630 (2022)

7. V. Baumann, and Č. Brukner, Wigner’s friend’s memory and the no-signaling principle, Quantum 8, 1481 (2024).

8. F. Del Santo, G. Manzano, and C. Brukner, Wigner’s friend scenarios: On what to condition and how to verify the predictions, Phys. Rev. Research 7, 033279 (2025)