Approaching Tsirelson's Bound in a Photon Pair Experiment.

We present an experimental test of the Clauser-Horne-Shimony-Holt Bell inequality on photon pairs in a maximally entangled state of polarization in which a value S=2.82759±0.00051 is observed. This value comes close to the Tsirelson bound of |S|≤2sqrt[2], with S-2sqrt[2]=0.00084±0.00051. It also violates the bound |S|≤2.82537 introduced by Grinbaum by 4.3 standard deviations. This violation allows us to exclude that quantum mechanics is only an effective description of a more fundamental theory.