Does quantum theory apply at all scales, including that of observers? New light on this fundamental question has recently been shed through a resurgence of interest in the long-standing Wigner’s friend paradox. This is a thought experiment addressing the quantum measurement problem—the difficulty of reconciling the (unitary, deterministic) evolution of isolated systems and the (non-unitary, probabilistic) state update after a measurement. Here, by building on a scenario with two separated but entangled friends introduced by Brukner, we prove that if quantum evolution is controllable on the scale of an observer, then one of ‘No-Superdeterminism’, ‘Locality’ or ‘Absoluteness of Observed Events’—that every observed event exists absolutely, not relatively—must be false. We show that although the violation of Bell-type inequalities in such scenarios is not in general sufficient to demonstrate the contradiction between those three assumptions, new inequalities can be derived, in a theory-independent manner, that are violated by quantum correlations. This is demonstrated in a proof-of-principle experiment where a photon’s path is deemed an observer. We discuss how this new theorem places strictly stronger constraints on physical reality than Bell’s theorem.

量子理论是否适用于所有尺度，包括观察者？最近，对长期存在的维格纳的朋友悖论的兴趣重新燃起了对这个基本问题的新认识。这是一个解决量子测量问题的思想实验，该问题是调和孤立系统的（统一，确定性）演化和测量后（非统一，概率性）状态更新的困难。在这里，通过建立一个由布鲁克纳引入的两个分离但纠缠的朋友的场景，我们证明如果量子演化在一个观察者的规模上是可控的，那么“无超确定性”，“局部性”或“观测事件的绝对性”之一“-每个观察到的事件绝对（而不是相对）存在-必须是错误的。我们表明，尽管在这种情况下违反Bell型不等式通常不足以证明这三个假设之间的矛盾，但是可以以理论无关的方式推导新的不等式，这些不等式被量子相关性所破坏。这在原理证明实验中得到了证明，其中光子的路径被视为观察者。我们讨论了这个新定理如何比贝尔定理对物理现实施加严格更严格的约束。