We demonstrate the novel effect of gain-induced spectral shifts in the type-II parametric down-conversion (PDC) process, which results in a transition from degenerate to non-degenerate PDC with increasing parametric gain. This effect, originating from the second-order dispersion terms, significantly alters the properties of PDC in the high-gain regime, where it leads to increased distinguishability of the generated photon pairs. The effect is established by evaluating a rigorous theoretical model, which is based on solving a system of coupled integro-differential equations for monochromatic operators. The widely used spatially-averaged approximate model fails to reproduce this important effect.
@article{arxiv.2603.01656,
title = {Gain-induced spectral non-degeneracy in type-II parametric down-conversion},
author = {Behnood Taheri and Denis Kopylov and Manfred Hammer and Torsten Meier and Jens Förstner and Polina Sharapova},
journal= {arXiv preprint arXiv:2603.01656},
year = {2026}
}