Multi-photonic microwave excitation of cold Rydberg atoms held inside a microwave cavity*

We investigate the energy spectrum associated with the excitation of Rydberg states in cold 85Rb atoms confined within a standard magneto-optical trap (MOT) placed inside a microwave cavity. Rydberg atoms are generated through a two-step process. Initially, two optical photons at 780nm and 480nm connect the ground state 5S1/2 to the intermediate Rydberg state nS1/2 in a ladder configuration. Subsequently, microwave photons within the cavity's volume can drive multi-photonic microwave Rydberg transitions, either higher or lower in energy. Experimental characterization of the microwave cavity reveals a central resonance frequency peak of 13.053 GHz with a standard deviation of 5 MHz, precisely aligning with the coupling of the 67S1/2 Rydberg state to the nearby 66P3/2 excited state. Continuous monitoring of fluorescence from the cycling trap transition light (780nm) is achieved using sensitive photodetectors. The trap-loss spectrum is reconstructed by measuring the reduction in fluorescence resulting from the transfer of atoms from the MOT to the Rydberg state as a function of the 480nm laser frequency. Employing a timed sequence acquisition allows for the isolation of the atomic background vapor gas trap-loading dynamics. Our observations reveal robust multi-photonic microwave Rydberg excitation, demonstrating high excitation efficiencies across a range of energies. The results align closely with a multi-level Jaynes-Cumming formalism employed for modeling trap-loss spectroscopy, considering cavity imperfections.