As other commenter said, the magic happens not just between photons, but between photons and non-linear optical materials, in which these photons travel. While

As other commenter said, the magic happens not just between photons, but between photons and non-linear optical materials, in which these photons travel. While not directly related to computing, I was fascinated to learn that lasers routinely use crystals to cut wavelength of the light by half ([1], [2]). 1. https://en.wikipedia.org/wiki/Second-harmonic_generation https://en.wikipedia.org/wiki/Second-harmonic_generation 2. https://en.wikipedia.org/wiki/Potassium_dideuterium_phosphate https://en.wikipedia.org/wiki/Potassium_dideuterium_phosphat...