1983 October 17, Randall G. Hulet and Daniel Kleppner, “Rydberg Atoms in "Circular" States”, in Physical Review Letters, volume 51, →DOI, page1430:
Wereport the productionof apopulationofatomsin aRydberg state with , where and are the magnetic and principal quantum numbers, respectively. We designate these states as "circular" because in the classical limit they describe an electron in a circular orbit. Among all the states with a given principal quantum number, the circular states have the largest magnetic moment, smallest Stark effect, and longest radiative lifetime.
2021 August 6, Sam R. Cohen and Jeff D. Thompson, “Quantum Computing with Circular Rydberg Atoms”, in PRX Quantum, volume 2, →DOI, page030322: