# What impact would have introducing the quantum switch effect in classical computing?

This divulgation article by Prof. Brukner talks about the possibility of creating a situation where

"A causing B" and "B causing A" which we call a quantum switch. Such a setup is similar to some predator–prey relationships, in which predator numbers influence prey numbers, yet prey numbers also influence predator numbers. Following work that Ognyan Oreshkov, Fabio Costa, and I published in 2012, we now know that the quantum switch is just one example of an indefinite causal structure, in which it is not defined whether event A is a cause or an effect of event B, or whether the two are independent.

The work link leads to a paper that explains with great detail how to achieve that, and it also includes some applications, like to prove that two no-signalling channels that are not perfectly distinguishable in any ordinary quantum circuit can become perfectly distinguishable through the quantum superposition of circuits with different causal structures

I am a regular programmer without a background in quantum computing and don't quite understand the implications of such articles. What I get, and find very exciting is the possibility of having such expanded causality models as a tool. My question is: how/where could quantum switch fit in the classical computing landscape? Be it low-level hardware design or the (surely upcoming) else-then-if software pattern

• isn't "A causing B" and "B causing A" an entanglement, in quantum computing? – Adelin Mar 30 '18 at 5:41
• By the linked article nope, entanglement would be a non-signaling correlation, and switch a signaling one (not sure about the implications, or how a "channel" is defined though) – fr_andres Mar 30 '18 at 5:49
• @Adelin I asked for that, in case you are interested – fr_andres Apr 5 '18 at 12:58

Indeed, the interest of techniques like the quantum switch$^\dagger$ (aside from the purely fundamental aspects) is that they provide methods to create non-classical correlations, and more generally encoding quantum information (e.g. qubits) into "causal" degrees of freedoms, as nicely shown in Figure 1 here.
$^\dagger$ It is worth mentioning that there have also been at least two (to my knowledge) experimental demonstrations of indefinite causal orders: 1608.01683 and 1712.06884.