Testing transient deviations from Quantum Mechanics’ predictions in spatially spread optical entangled states

One of the conflicts between Quantum Mechanics (QM) and classical physics is visible in the high correlation predicted by QM to exist between observations performed on remote regions of a spatially spread entangled state. It has been hypothesized that this correlation needs a time longer than L/c to build up, where L is the spatial spread of the entangled state and c is the speed of light. In shorter times, the level of correlation would be compatible with classical physics. This hypothesis is appealing, among other reasons, for it solves that conflict by minimally modifying the interpretation of QM. The hypothesis is difficult to test directly because the observable difference would vanish fast, but a stroboscopic observation is at hand. We present the results of such observation, performed on a specially designed optical Bell setup with a distance between stations up to L = 24 m in straight line. This completes the preliminary results for short L we recently reported in this Journal. No evidence of deviations from QM predictions is found. Yet, some additional assumptions are involved; they are detailed and discussed. If fair sampling is valid, then the hypothesis of transient deviations is refuted unless the stroboscopic approach with decay time shorter than 1.5 μs (≈ 19 L/c) is false.