Einstein's 1927 gedanken experiment: how to complete it and measure the collapse time of a spatially spread photon

In the famous Solvay 1927 conference, Einstein discussed a gedanken experiment involving a single photon diffracted at an aperture and impinging on a screen. He devised the example to support De Broglie’s hypothesis of the pilot wave, and his own ideas on the incompleteness of the description of physical reality provided by Quantum Mechanics. Partial realizations of Einstein’s example have been performed, but the complete experiment has not been attempted (for good practical reasons) yet. Here I describe how to do it with accessible means. The setup will make possible to test Hellwig and Kraus postulate of covariant reduction of the quantum state.