Experimental Higher-Order Interference in Quantum Mechanics Induced by Optical Nonlinearities

The double-slit experiment is a direct demonstration of interference between quantum objects. Of course, interference is also observed in experiments using multiple slits. However, according to quantum theory, the interference patterns produced using such multi-slits can all be explained in terms of interference between pairs of slits [1] , [2] . Interestingly, there are proposed versions of quantum theory—so-called higher-order theories—wherein this is not possible [3] . In such theories the "third-order" interference (with three or more slits open) is genuinely different from second-order interference. Here we show that standard quantum mechanics actually allows for apparent high-order interference, if one considers nonlinear effects. Note, that another mechanism for this has also recently been shown based on so-called looped trajectories [4] – [6] .