Interfacing Rydberg atoms with chip-based superconducting microwave resonators using AC stark shifted single-photon transitions*

Hybrid cavity-QED with Rydberg atoms and superconducting microwave circuits offers opportunities for networking neutral atom and superconducting quantum processors. In this setting, a single Rydberg atom positioned 10–50 µm above a superconducting coplanar waveguide (CPW) resonator is expected to exhibit vacuum Rabi frequencies of 2π(1–10) MHz. Currently, work at these interfaces has focused on managing stray electric fields caused by adsorbates and charge buildup on the cryogenically cooled superconducting chip surfaces. These efforts led to the first demonstrations of coherent coupling between Rydberg atoms and superconducting CPW resonators [1-4]. However, so far, this coupling has relied on the use of either two-photon Rydberg-Rydberg transitions, or single-photon transitions in stray DC electric fields with electric dipole transition moments of ~30ea₀.

Here, we demonstrate the coupling of a strong single-photon transition between the low-ℓ triplet 50s and 50p Rydberg states in helium, to the 11.752 GHz second harmonic mode of a quarter-wave niobium nitride CPW resonator, in near-zero DC electric field. This represents the first demonstration of coupling the microwave field in a superconducting CPW resonator to a single-photon transition between Rydberg states with a large electric dipole transition moment (~1500 ea₀). To achieve this, a microwave dressing field, detuned from the 50p–50d transition, was used to tune the 50s–50p transition into resonance with the resonator mode using the AC Stark effect. This approach allowed the microwave power in the resonator to be reduced by more than 3 orders of magnitude compared to previous work, moving this hybrid quantum system closer to the single-photon strong-coupling regime.

[1] A. A. Morgan et al. Phys. Rev. Lett. 124, 193604 (2020) 

[2] D. Walker et al. Appl. Phys. Lett. 117, 204001 (2020)

[3] M. Kaiser et al. Phys. Rev. Research 4, 013207 (2022)

[4] L. L. Brown et al. Phys. Rev. A 110, 022615 (2024)