In a recent paper uploaded to arXiv, physicists have questioned whether an observer's choice can be considered absolute within the framework of quantum mechanics. The Wigner’s Friend paradox—where one observer measures a system while another observes the first—leads to the conclusion that free choices may actually be perspective-dependent.
A team of researchers from several European institutions analyzed a scenario in which Wigner’s friend performs a spin measurement, while Wigner himself describes that same system as a superposition. According to the preprint published in May 2024, the absoluteness of choice is lost if both observers treat each other as quantum systems. This is no mere thought experiment, but a rigorous calculation employing the formalism of quantum reference frames.
Imagine two people simultaneously deciding which way to turn at a crossroads, where their decisions only align when one perceives the other’s choice as a still-unresolved possibility. This is precisely how the mechanism functions in this model: one observer’s measurement remains indeterminate to the other until a coherence-breaking interaction occurs.
The results fundamentally change our understanding of what constitutes "free choice" in quantum cryptography protocols and Bell tests. If choices are not absolute, devices that rely on independent settings might provide false security guarantees. The team points out that these findings are currently theoretical and will require experimental verification using photonic or ionic setups.
The research relies on a mathematical framework developed in recent years to describe multiple observers within a single quantum system. The authors demonstrate that, under certain conditions, two observers can arrive at incompatible conclusions about a measurement outcome, even though both have strictly followed the laws of quantum theory.
Consequently, even the most basic assumptions about measurement independence prove to be context-dependent when the observers themselves are integrated into the quantum description.
