Quantum Causality Creates a Challenge to Define the Order of Events

Synopsis

In our daily life, events have a definite order of occurrence. As an example, your car starts to work when you turn the switch on and vice versa. But what happens if you set up two events with a photon? For such a case, unlike our usual way of thinking which gives the order of before and after for events, we cannot distinguish the sequence of them. Such behavior of photons leads to the concept of quantum causality which disrupts our conventional interpretations of causality. In this paper, the authors, propose a setup known as a quantum switch to model such events with photons to examine the order of outcomes. What does the result look like? Think about two events, A and B: A occurs before B and B occurs before A.

References:

Goswami, K., Giarmatzi, C., Kewming, M., Costa, F., Branciard, C., Romero, J., & White, A. G. (2018). Indefinite Causal Order in a Quantum Switch. Phys. Rev. Lett. 121, 090503 – Published 31 August 2018.