Quantum interference beyond the fringe

Thirty years ago, Len Mandel, together with his students Jeff Ou and C. K. Hong, published a description of a remarkably simple experiment (1), the consequences of which have had dramatic implications for quantum science and technology. The experiment consisted of sending two elementary particles of light, photons, onto opposite sides of a piece of glass that had been coated with a thin film to give it a reflectivity of 50% (see the figure). They observed that the two photons always left by the same side at the output, though it was not possible to determine beforehand which side that would be. This is completely surprising if one considers the behavior of particles as indivisible, identical entities obeying the laws of classical physics. It is easy to see that in such a circumstance, one might expect there to be four equally likely output configurations, only two of which consist of the particles occupying the same outputs. The phenomenon is a beautiful manifestation of the interference of a quantum field; in this case, the bosonic field associated with photons. The effect would be entirely the opposite with fermionic fields associated with electrons—both particles would always leave by different sides.