Afshar's experiment revisited: a demonstration of Bohr's complementary principle

Bohr's principle of complementarity states that every quantum system has mutually incompatible properties which cannot be simultaneously measured [1]. This principle is commonly illustrated by considering single particles in a two-way interferometer where one chooses either to observe interference fringes, associated to a wave-like behavior, or to know which path of the interferometer has been followed, according to a particle-like behavior. In such an experiment, any attempt to obtain some which-path information (WPI) unavoidably reduces interference, and reciprocally. The incompatibility between these two measurements is then ensured by the complementarity inequality V 2 +D 2 ≤1 which puts an upper bound to the maximum values of independently determined interference visibility V and path distinguishability D, the parameter that quantifies the available WPI on the quantum system.