Toward an experimental test for the finite-time wave function collapse

In spite of recent conceptual and experimental advances, the ultimate nature of the wave function in quantum theory remains nebulous. Is it a mathematical device that describes our knowledge on a physical system or is it a physical field in any tenable sense? In this work we report on significant experimental steps toward a test on the reality of the wave function, which does not address the duality “epistemic versus ontic” in the usual way. Instead of considering inequalities derived from the partial indistinguishability of non-orthogonal states, we focus on the possibility of discriminating instantaneous reduction from finite-time wave function collapse. The former can only be associated with a Bayesian update, while the latter would be compatible with realistic interpretations. We employ non-maximally entangled photons which must be finely synchronized before detection. The required partial entanglement of the pairs is characterized by the violation they produce in the Clauser-Horne-Shimony-Holt inequality and the synchronization is obtained via time-scan measurements. This realization allows us to better understand the necessary instrumental conditions to the execution of a final, conclusive test.