Feynman path integral formalism was used to quantify contributions from non-classical paths in quantum interference experiments which provide a measurable deviation from a naive application of the superposition principle. With reasonable parameters, the deviation from naive superposition principle is found to be significant.
The maximum deviation was expressed as a simple analytical formula. Further, the result from path integral formalism was shown to agree with the results obtained from numerical solutions of Maxwell Equation.
Experimentally, deviation from superposition principle was obtained for the first time for microwave domain. We have a controllable parameter that can be varied in its contribution such that the effect due to these non-classical paths, which we will refer to as sub-leading paths, can be increased or diminished at will.