Elitzur-Vaidman
Elitzur-Vaidman Experiment
The Elitzur-Vaidman "bomb testing" experiment, proposed by Avshalom Elitzur
and Lev Vaidman
in 1993, is one of the most fascinating experiments that demonstrates the weird nature of quantum physics. Consider the above interferometer, consisting of two 50/50 beam splitters (the cubes), two mirrors (plates), and two single photon detectors (red cylinders). The interferometer is perfectly adjusted in such a way that if a classical, perfectly coherent laser would pass through it, one has perfect destructive interference at one exit (no light hits the detector in the back of the scene) and perfect constructive interference in the other exit (max light at the detector on the right). Quantum mechanics now tells us that even if one sends in only single photons, this will still work: One will never observe that a photon is detected by the detector in the back, but will only observe photon detections on the detector to the right. In the animation, a photons is depicted as a small moving ball, the red and blue color indicating the phase of the electromagnetic field oscillation. Photon splittings shall symbolize the many simultaneous paths it can travel, in the sense of Feynman's multiple path interpretation of quantum mechanics, see above.
Now consider the situation when a pebble ("the bomb") is placed in the back arm of the interferometer. The thought experiment of Elitzur and Vaidman was that the "bomb" is light sensitive and explodes as soon as a photon hits hit. When we send in a photon, there is a 50% chance that it will hit the pebble and the game is over. But there is now also a 25% chance that it will be detected by the detector in the back: In that moment, we know that something is blocking one of the arms of our interferometer, because otherwise we would never see any detection at the back detector (see panel left), but we also know that the detected photon could not have traveled along the blocked path, because then it would have never made it to the detector. Thus, such a detection event unambiguously tells us that the pebble is there although our photon could never have interacted with it. Isn't that weird?
The animation here shows only the two situations where the photon is not
absorbed by the pebble but detected by either one or the other detector. Generally, in 50% of cases, our machine explodes, in 25% of cases we detect a photon at the right, which does not help, and in only 25% of cases we detect a photon at the back, which gives useful information about the presence of the pebble.