Multiphoton-dressed Rydberg excitations in a microwave cavity with ultracold Rb atoms

We investigate magneto-optical trap-loss spectroscopy of Rydberg excited 85Rb (66 n 68 S1/2) atoms, placed inside a tailored microwave cavity. The cavity frequency at 13.053 GHz is in resonance with the 67S1/2 -* 66P3/2 transition, inducing a ladder multiphoton microwave Rydberg absorption and emission. The observed spectra are modeled with an extended Jaynes-Cummings formalism that accounts for nonlinear multiphoton absorption from and emission into the cavity, the loss from the trap due to Rydberg excitation, and cavity imperfection. We calculate the average photons in each spectral feature and find evidence for fractional photon emission into the cavity modes within the loss spectra. The microwave cavity Rydberg spectroscopy in this work provides key insights for advancing Rydberg-based sensors, quantum gates in hybrid systems, and the broader development of quantum technologies.