Causal Nonseparability

Schrödinger's cat is a famous thought experiment that explains how a quantum object can be in two incompatible states at the same time. Now, research that was recently published in the New Journal of Physics has shown that it's possible to have a superposition of incompatible states and incompatible orders of events. Ultimately, this marks the first time that scientists have discovered a physical quantum process that clearly demonstrates causal nonseparability.

Causal nonseparability is  essentially a phenomenon where events don’t occur in just a single definite order; instead, they happen in both different orders. In this respect - A occurs before B, and B occurs before A - all at the same time. Coauthor of the study Cyril Branciard, at CNRS and the Universite Grenoble Aples in Grenoble, France, outlined the significance of the research in a press release, "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."

Quantum Switch

The specific quantum process of causal nonseparability is called "quantum switch." The process was recently proposed to improve the efficiency of quantum computers. For comparison, causally separable processes involve performing the orders “A then B” and “B then A”, and then comparing the results. Causally nonseparable processes, like the quantum switch, involves performing both orders at the same time within a quantum superposition and solving the problem in a single step. The process is similar to that of quantum entanglement, which is the simplest example of causal nonseparability.

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