The study of causal relations has recently been applied to the quantum realm, leading to the discovery that not all physical processes have a definite causal structure. While indefinite causal processes have previously been experimentally shown, these proofs relied on the quantum description of the experiments. Yet, the same experimental data could also be compatible with definite causal structures within different descriptions. In this talk, I will present the first demonstration of indefinite temporal order outside of quantum formalism. I will show experimental outcomes that are incompatible with a class of generalised probabilistic theories satisfying the assumptions of locality and definite temporal order. To this end, I will introduce physical constraints (in the form of a Bell-like inequality) on experimental outcomes within such a class of theories. I will then show experimental data that invalidate these theories by violating the inequality using entangled temporal order. This will provide experimental evidence that there exist correlations in nature which are incompatible with the assumptions of locality and definite temporal order.

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