Experimental implementation of quantum-coherent mixtures of causal relations

          @misc{ pirsa_PIRSA:16090056,
            doi = {10.48660/16090056},
            url = {https://pirsa.org/16090056},
            author = {Spekkens, Robert},
            keywords = {Quantum Foundations},
            language = {en},
            title = {Experimental implementation of quantum-coherent mixtures of causal relations},
            publisher = {Perimeter Institute},
            year = {2016},
            month = {sep},
            note = {PIRSA:16090056 see, \url{https://pirsa.org}}
          }
          

Abstract

Understanding the causal influences that hold among the parts of a system is critical both to explaining that system's natural behaviour and to controlling it through targeted interventions. In a quantum world, understanding causal relations is equally important, but the set of possibilities is far richer. The two basic ways in which a pair of time-ordered quantum systems may be causally related are by a cause-effect mechanism or by a common cause acting on both. Here, we show that it is possible to have a coherent mixture of these two possibilities. We realize such a nonclassical causal relation in a quantum optics experiment and derive a set of criteria for witnessing the coherence based on a quantum version of Berkson's paradox. (Joint work with Katja Ried and Kevin Resch)