Abstract: In a general aspect, a radio frequency (RF) measurement device includes first and second mode converters, each configured to convert a mode of the RF electromagnetic wave between a first RF waveguide mode and a second RF waveguide mode. The RF measurement device also includes an internal cavity between the first and second mode converters that contains a vapor or a source of the vapor. The vapor includes Ryberg atoms or molecules. The RF measurement device additionally includes an RF waveguide extending between the first and second mode converters and configured to establish a circular polarization of the RF electromagnetic wave in the internal cavity.

Abstract: A photonic device includes a substrate and a tantala ring resonator on the substrate. The tantala ring resonator has at least one of (i) a quality factor exceeding three million and (ii) a threshold power less than one hundred milliwatts. A frequency-comb generation method includes sweeping the output frequency of a laser coupled to a tantala ring resonator that has at least one of (i) a quality factor exceeding three million and (ii) a threshold power less than one hundred milliwatts.

Abstract: A tantala ring resonator includes a tantala ring resonator formed by ion-beam sputtering of tantalum pentoxide and exhibiting an optical quality factor in excess of 3×106, and a substrate to which the tantala ring resonator is attached. A method for fabricating nonlinear photonic devices includes depositing tantalum pentoxide with ion-beam sputtering to form a tantala layer onto a substrate, annealing the tantala layer, and etching the tantala layer to form a photonic device.