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Circular Rydberg states with $n=70$ have been prepared in helium using amodified version of the crossed-fields method. This approach to the preparationof high-$n$ circular Rydberg states overcomes limitations of the standardcrossed-fields method which arise at this, and higher, values of $n$. Theexperiments were performed with atoms traveling in pulsed supersonic beams thatwere initially laser photoexcited from the metastable 1s2s$\,^3$S$_1$ level tothe 1s73s$\,^3$S$_1$ level by resonance-enhanced two-color two-photonexcitation in a magnetic field of 16.154 G. These excited atoms were thenpolarized using a perpendicular electric field of 0.844~V/cm, and transferredby a pulse of microwave radiation to the state that, when adiabaticallydepolarized, evolves into the $n=70$ circular state in zero electric field. Theexcited atoms were detected by state-selective electric field ionization. Eachstep of the circular state preparation process was validated by comparison withthe calculated atomic energy level structure in the perpendicular electric andmagnetic fields used. Of the atoms initially excited to the 1s73s$\,^3$S$_1$level, $\sim80$\% were transferred to the $n=70$ circular state. At these highvalues of $n$, $\Delta n = 1$ circular-to-circular Rydberg state transitionsoccur at frequencies below 20 GHz. Consequently, atoms in these states, and thecircular state preparation process presented here, are well suited to hybridcavity QED experiments with Rydberg atoms and superconducting microwavecircuits.

Preparation of circular Rydberg states in helium with $n \geq 70$ using a modified version of the crossed-fields method