Abstract
We report microwave to optical upconversion in isotopically purified erbium-doped yttrium orthosilicate in a Fabry-Pérot resonator at millikelvin temperatures. This follows on from investigations made at higher temperatures and with natural isotopic ratios for the erbium dopants. In these previous investigations the highest efficiency was seen only for moderately strong microwave powers. The removal of the unwanted erbium-167 which has hyperfine structure and provides unwanted background optical absorption, and the lower temperatures has removed this problem. We now see efficiencies still increasing as the microwave power is decreased when we reach the smallest input powers for which we could measure an output. Efficiencies of 2×10−6 were observed and we discuss potential improvements, including better optical cavity frequency stability and better thermalisation of the erbium spins.