Witnessing superpositions of causal orders by weak measurements at a given time

Witnessing superpositions of causal orders by weak measurements at a given time

Onur Pusuluk, Zafer Gedik, Vlatko Vedral
University of Oxford – Physics Department

Preprint

ABSTRACT

The questions we raise in this letter are as follows: What is the most general representation of a quantum state at a single time? Can we adapt the current representations to the scenarios in which the order of quantum operations are coherently or incoherently superposed? If so, what is the relation between the state at a given time and the uncertainty in the order of events before and after it? By establishing the relationship of two-state vector formalism with pseudo-density operators, we introduce the notion of single-time pseudo-state which can be constructed by either ideal or weak measurements. We show that the eigenspectrum in the latter case enables us to discriminate between the coherent and incoherent superpositions of causal orders in which the pre- and post-selection measurements are interchanged with a non-zero probability. Finally, we discuss some of the possible experimental realizations in existing photonic setups.