Particles in a quantum interference experiment may follow exotic "non-classical paths" that must be accounted for when analysing such superposition experiments.
The double-slit at single photon or electron level is the ubiqutous quantum interference experiment with the path phase playing a key role. On the other hand, photon's orbital angular momentum (OAM) brings a new dimension to interference problems with the spiral phase degree of freedom. In this work, we use OAM's two dimensional properties to extended the double-slit to a triple-slit configuration in the shape of an equilateral triangle, obtaining a triangular latticed single-photon counting interference pattern. In order to understand this interference pattern in terms of wave-particle duality, we have to take into account that, in contrast with the parallel double and triple slit cases, the azimuthal phase also plays a fundamental role, being undistinguishable from path phase.
The Feynman Path Integral formalism has long been used for calculations of probability amplitudes. Over the last few years, it has been extensively used to theoretically demonstrate that the usual application of the superposition principle in slit based interference experiments is often incorrect. This has caveat in both optics and quantum mechanics where it is often naively assumed that the boundary condition represented by slits opened individually is same as them being opened together. The correction term comes from exotic sub leading terms in the Path Integral which can be described by what are popularly called non-classical paths. In this work, we report an experiment where we have a controllable parameter that can be varied in its contribution such that the effect due to these non-classical paths can be increased or diminished at will. Thus, the reality of these non-classical paths is brought forth in a classical experiment using microwaves, thereby proving that the boundary condition effect being investigated transcends the classical-quantum divide. We report the first measurement of a deviation (as big as 6%) from the superposition principle in the microwave domain using antennas as sources and detectors of the electromagnetic waves. We also show that our results can have potential applications in astronomy.