By developing a `two-crystal' method for color erasure, we can broaden the scope of chromatic interferometry to include optical photons whose frequency difference falls outside of the 400 nm to 4500 nm wavelength range, which is the passband of a PPLN crystal. We demonstrate this possibility experimentally, by observing interference patterns between sources at 1064.4 nm and 1063.6 nm, corresponding to a frequency difference of about 200 GHz.
@article{arxiv.2009.08217,
title = {Chromatic interferometry with small frequency differences},
author = {Luo-Yuan Qu and Lu-Chuan Liu and Jordan Cotler and Fei Ma and Jian-Yu Guan and Ming-Yang Zheng and Quan Yao and Xiu-Ping Xie and Yu-Ao Chen and Qiang Zhang and Frank Wilczek and Jian-Wei Pan},
journal= {arXiv preprint arXiv:2009.08217},
year = {2020}
}