Delayed-Choice Experiments

The phenomenon of ► “wave-particle duality” is at the heart of quantum mechanics, indeed has been described as “the one real mystery” of the subject. If we consider the standard Young's slits setup shown in Fig. 1, we may imagine for definiteness that the experiment is done with electrons (► Double-slit Experiment), then in the absence of “inspection” the probability of arrival of an electron on the final screen shows the usual interference pattern — the electron appears to behave as a wave. If on the other hand we arrange to inspect which path is followed (e.g. by shining light on the intermediate slits as in the Heisenberg “gamma ray microscope” thought experiment ► Heisenberg microscope; which-way experiments), then the electron is always found, like a classical particle, to take one route or the other, and under these conditions no interference occurs at the final screen. If we replace the ► electrons with photons (► light quantum), we expect a similar duality to manifest itself; however, in this case, since it is very difficult to detect a photon without destroying it, it is more convenient to try to display the “particle” aspect by removing the final screen and replacing it by a pair of detectors as indicated in Fig. 2; again we will find that one detector or the other clicks, never both.