Experimental Violation of Bell's Inequality for Temporal Orders

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].