Which-Way or Welcher-Weg-Experiments

The issue of the ► wave-particle duality of light and matter is commonly illustrated by the ► double-slit experiment, in which a quantum object of relatively well defined momentum (such as a photon, electron, neutron, atom, or molecule) is sent through a diaphragm containing two slits, after which it is detected at a capture screen. It is found that an interference pattern characteristic of wave behaviour emerges as a large number of similarly prepared quantum objects is detected on the screen. This is taken as evidence that it is impossible to ascertain through which slit an individual quantum object has passed; if that were known in every individual case and if the quantum objects behaved as free classical particles otherwise, an interference pattern would not arise.

The notion that a description of atomic objects in terms of definite classical particle trajectories is not in general admissible is prominent in Werner Heisenberg's seminal paper [1] of 1927 on the ► Heisenberg uncertainty principle; there he notes: “I believe that one can fruitfully formulate the origin of the classical ‘orbit’ in this way: the ‘orbit’ comes into being only when we observe it.” In the same year, in his famous Como lecture, Niels Bohr introduced the ► complementarity principle, which entails that definite particle trajectories cannot be defined or observed for atomic objects because according to it their spatiotemporal and causal descriptions are mutually exclusive [2]. Bohr cited the uncertainty relation as a symbolic expression of complementarity but recognized that this relation also offered room for approximately defined simultaneous values of position and momentum. Still in the same year, at the 1927 Solvay conference, Albert Einstein questioned the impossibility of determining the path taken by an individual particle in a double-slit interference experiment [21]; he proposed an experimental scheme wherein he considered it possible to infer through which slit the particle passed, without thereby destroying the interference pattern by measuring the recoil of the double-slitted diaphragm. This was the first instance of a welcher-weg or which-way experiment. As Bohr reported in his 1949 tribute to Einstein [3], he was able to demonstrate that Einstein's proposal was in conflict with the principles of quantum mechanics.