@article {1568598, title = {Universal interferometric signatures of a black hole{\textquoteright}s photon ring}, journal = {Science Advances}, volume = {6}, number = {12}, year = {2020}, pages = {eaaz1310}, url = {https://ui.adsabs.harvard.edu/abs/2020SciA....6.1310J/abstract}, author = {M. Johnson et al.} }