Choose the right path

In Roch's experiment, single photon pulses are emitted one at a time into an interferometer. As they leave a first beam splitter (BS1), they have the option of two 48-metre paths with equal probability, which eventually lead to two separate detectors. Just before the detectors, a second beam splitter (BS2) is randomly inserted or removed by a system that is synchronized with the emitter. With the beam splitter in place, a photon can reach either detector from the same path, preventing its path from being observed. When the beam splitter is removed, however, the detectors can observe a photon's path unambiguously.

Roch's team performed the experiment many times until they could confirm with certainty that unobserved photons behave like waves (i.e. interfere), while observed photons behave like particles (i.e. do not interfere). Crucially, however, they removed the possibility that the photons could somehow be informed of the system's decision, as the decision was only made after the photons had entered the interferometer.

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