Indefinite causal order and quantum reference frames

          @misc{ pirsa_PIRSA:24090099,
            doi = {10.48660/24090099},
            url = {https://pirsa.org/24090099},
            author = {de la Hamette, Anne-Catherine},
            keywords = {Quantum Foundations, Quantum Information},
            language = {en},
            title = {Indefinite causal order and quantum reference frames},
            publisher = {Perimeter Institute},
            year = {2024},
            month = {sep},
            note = {PIRSA:24090099 see, \url{https://pirsa.org}}
          }
          

Abstract

Recent research on quantum reference frames (QRFs) has shown that whether a system is in a superposed state of locations, momenta, and other properties can depend on the quantum reference frame relative to which it is being described. Whether an event is localized in spacetime or not can change under QRF transformations, in that case so-called quantum-controlled diffeomorphisms. This raises a critical question: can quantum reference frame transformations render indefinite causal order definite? In this talk, I propose a relativistic definition of causal order based on worldline coincidences and proper time differences, establishing it as an operationally meaningful observable in both general relativity and quantum mechanics. Using this definition, we can analyse the indefiniteness of causal order in the optical and gravitational quantum switch on equal footing. This analysis suggests an operational rather than a spacetime-based understanding of events. I will compare these findings to other recent results and conclude with broader implications for events in non-classical contexts