Alladi Ramakrishnan Hall

Trading causal order for locality

Ravi Kunjwal

Centre for Quantum Information and Communication (QuIC), Université libre de Bruxelles (ULB), Brussels, Belgium

I will discuss a recent result on an intimate link between two *a priori* distinct phenomena: quantum nonlocality without entanglement and classically-achievable indefinite causal order. The first phenomenon refers to a multipartite scenario where the parties are unable to perfectly discriminate orthogonal product states drawn from an ensemble of quantum states by using local operations and classical communication (LOCC). The second (hypothetical) phenomenon refers to a multipartite scenario where the parties can communicate classically but the local operations of each party are in the future of some other party, i.e., they cannot be ordered causally. Specifically, I will show how three separated parties with access to a classical process exhibiting indefinite causal order---the AF/BW process---can perfectly discriminate the states in an ensemble---the SHIFT ensemble---that exhibits quantum nonlocality without entanglement. Time permitting, I will discuss the generalization of this result beyond the tripartite case and comment on its connection with separable operations that are outside LOCC. Based on joint work with Ämin Baumeler, arXiv:2202.00440