摘要： Entanglement is a prediction of quantum mechanics that suggests that our physical laws cannot be described by a local causal model. This has led to much debate, including Einstein’s famous suggestion that quantum mechanics is an incomplete theory, and we may one day discover a deeper theory that can be described by a locally causal model. However, loophole free violations of Bell inequalities demonstrate that this is not the case. Bell’s theorem is independent of quantum mechanics, and it shows that any theory attempting to supplant quantum mechanics must be somehow nonlocal. Since then, there have been many experimental violations of Bell inequalities using different physical systems, culminating in three loophole-free experiments [1-3]. Very recent theoretical [4, 5] and experimental techniques [6, 7] have enabled the study of a new phenomenon predicted by quantum mechanics. Namely, the existence of processes without a fixed causal order. Previous experimental tests of this effect, required the assumption that quantum mechanics can be used to correctly model the experimental results. However, just as the study of entanglement before Bell, these results could still be described by some other theory in which the causal order is definite. The question, then, naturally arises do such processes really exist, or are they an artefact of the quantum mechanics. Here we show that these effects are indeed real by constructing a Bell inequality for temporal orders [8]. The violation of this equality implies, independent of quantum mechanics, that the temporal order of the process under study must have been truly indefinite. We then show we can violate this inequality using a system consisting of two quantum SWITCHes [6,7] with entangled control qubits (see Fig. 1).