http://phys.org/news/2015-11-quantum-superposition-events.htmlIn a quantum superposition, a quantum object can be in two incompatible states at the same time, which is famously illustrated by Schrödinger's dead-and-alive cat. Recent research has shown that it's possible to have a superposition not only of incompatible states, but also of incompatible orders of events. We often think of events occurring in a definite chronological order, with event A happening (and causing) event B, or vice versa. But in certain quantum processes, events don't happen in a single definite order, but instead both orders (A before B, and B before A) occur at the same time. This counterintuitive superposition-like phenomenon is called "causal nonseparability." "In everyday life, we are used to experiencing one thing always happening after another, effects following their causes," Mateus Araújo at the University of Vienna and the Institute for Quantum Optics and Quantum Information in Vienna, Austria, told Phys.org. "So it is a bit unsettling to realize that deep down Nature doesn't work like this, that things can happen without a definite causal order, where we cannot say what is the cause and what is the effect."
Until now, causal nonseparability in quantum mechanics has been conceived only in a very abstract way, with no clear physical interpretation. But in a new paper published in the New Journal of Physics, Araújo and coauthors have described an example of a physical quantum process that demonstrates causal nonseparability. "The theory of relativity has already shaken the idea that there is an absolute, global time, and that everyone experiences the flow of time and time relations in the same way: two different observers in different reference frames may, for instance, disagree on which event happens before the other," said coauthor Cyril Branciard at CNRS and the Universite Grenoble Alpes in Grenoble, France. "Quantum theory, on the other hand, has shaken our understanding of reality by telling us that physical systems may not have well-defined properties, and may be in a 'superposition' of incompatible states. For example, a poor cat could be both alive and dead at the same time. Now we find that not just physical properties, but also causal relations (or causal orders) themselves can be undefined, and can be put in some kind of superposition—a phenomenon that had not been observed experimentally until very recently."
