Single-photon implementation of an indefinite causal order
Lorenzo M. Procopio, University of Vienna
Investigating the role of causal order in quantum mechanics has recently revealed that the temporal distribution of events may not be a-priori well-defined in quantum theory. Although this has triggered a growing interest on the theoretical side, the existence of processes without a causal order is an experimental question. In this talk, I will present an optical implementation of an indefinite causal-order structure called quantum switch for two purposes; to execute an algorithm and to verify a causally non-separable process. For the first purpose, we superimposed two unitary quantum gates in two different orders in order to determine if two quantum gates commute or anti-commute. Our implemented protocol uses fewer resources than the standard quantum protocols use. For the second purpose, we superimposed two more general quantum operations in order to verify that the quantum switch does not have a definite causal order. In order to do that, we measured the recent introduced mathematical object called causal witness to demonstrate that our created process cannot be described as a causally ordered process. Finally, we give our conclusions and discuss some interesting possibilities to further causal-order experiments.