The idea is that Alice directs a photon at the first beam splitter and Bob has a detector placed behind that device that he can switch on if he wishes to try and detect the photon. But with the detector switched on, the photon’s wavefunction is forced to collapse and it follows just one of the two paths. The second beam splitter is placed at the point where the two paths meet and Bob has a second detector behind that device. Using this set-up Bob is able to tell Alice he has all of his detectors switched on without any photons passing between himself and Alice. He says that a key challenge will be in quantifying precisely how many photons or fractions of photons are actually transferred from Bob to Alice.