Quantum chicken-or-egg experiment blurs the distinction between before and after

In the everyday world, events occur in a definite order—your alarm clock rings before you wake up, or vice versa. However, a new experiment shows that when fiddling with a photon, it can be impossible to say in which order two events occur, obliterating our common sense notion of before and after and, potentially, muddying the concept of causality. Known as a quantum switch, the setup could provide a useful new tool in budding quantum information technologies.

http://www.sciencemag.org/news/2018/08/quantum-chicken-or-egg-experiment-blurs-distinction-between-and-after

https://www.sciencenews.org/article/quantum-switch-superposition-physics