Physical principles that single out quantum theory

4 problems

• Can a consistent theory beyond quantum mechanics have three-path interference?

  Partially resolved

  Quantum theory forbids interference that needs three paths at once. The question is whether any other self-consistent probabilistic theory allows it, which fixes what a positive three-slit test would mean.

• Do almost-quantum correlations satisfy information causality?

  Open

  The almost-quantum set is a slightly larger family than the quantum correlations that obeys nearly every principle proposed so far. If it also obeys information causality, that principle cannot single out quantum theory.

• Does information causality exclude every post-quantum box with two binary inputs?

  Partially resolved

  In the simplest Bell test each side picks one of two measurements with two outcomes. A 2026 result shows a strengthened information-causality principle reproduces the quantum boundary when each local outcome is equally likely, but whether it excludes every non-quantum box, including ones with biased local outcomes, is open.

• Which principle singles out exactly the quantum correlations?

  Open

  No-signalling allows superstrong correlations, such as the Popescu-Rohrlich box, that quantum mechanics forbids. Proposed principles remove many of them, but none so far gives exactly the quantum set.