The Englert–Greenberger duality relation relates the visibility, V, of interference fringes with the definiteness, or distinguishability, D, of the photons’ paths in quantum optics. As an inequality:

    D^2 + V^2 <= 1 

The relationship was first experimentally shown by Greenberger and Yassin in 1988. It was later theoretically derived by Jaeger, Shimony, and Vaidman in 1995, and over a year later it was also mentioned by Englert, in 1996.

— Wikipedia on Englert–Greenberger duality relation

2012.04.11 Wednesday ACHK