Tunable Rydberg-Rydberg transitions in helium with reduced sensitivity to dc electric fields by two-colour microwave dressing

The difference in the static electric dipole polarizabilities of the 1s55s3S1 and 1s56s3S1 Rydberg levels in helium has been eliminated by dressing the atom with a microwave field near resonant with the single-photon 1s55s3S1 & RARR;1s55p3PJ transition. For an 2.82mVcm-1 amplitude dressing field, detuned by 2 & pi;x10MHz from the zero-field 1s55s3S1 & RARR;1s55p3P2 transition frequency, the dc Stark shift of the two-photon 1s55s3S1 & RARR;1s56s3S1 transition between these states remained within & PLUSMN;15kHz for electric fields up to & SIM;60mVcm-1 . This transition was probed by single-color two-photon microwave spectroscopy, and by two-color two-photon spectroscopy with one strong additional dressing field and a weak probe field. For all measurements, the transition frequencies and Stark shifts were compared, and found to be in excellent quantitative agreement with the results of Floquet calculations of the energy-level structure of the Rydberg states in the presence of the dressing fields and applied dc electric fields. The two-color microwave dressing scheme demonstrated, with one field applied to null the differential polarizability of the Rydberg-Rydberg transition, and the second exploited to allow the two-photon transition to be employed to achieve tunable absorption of single-photons from a weak probe field, will facilitate improved coherence times and tunable single-photon absorption in hybrid cavity QED experiments with Rydberg atoms and superconducting microwave circuits.