Optically heralded microwave photons - Part 1*

Future quantum networks of distant superconducting microwave quantum processors demand the use of optical photons for low loss and low noise interconnects. Bridging the energy gap between microwave and optical photons, however, requires the use of a frequency converter. Here, we demonstrate a frequency converter that can add a single photon to the microwave field conditioned on the detection of an optical photon. The device consists of a hybrid lithium niobate - silicon gigahertz nanomechanical resonator acting as an intermediary between a microwave and optical channel via strong piezoelectric and optomechanical interactions. For pulsed microwave photon generation, we achieve a heralding rate of 15 Hz. Thermal noise added by optical absorption in the device amounts to less than 2 microwave photons. Additionally, we measure a microwave-to-optical conversion efficiency of 5% under continuous wave operation. Finally, we discuss necessary steps for increasing the heralding rate and reducing thermal noise. Improving the device performance and system efficiency should pave the way for entangling two distant microwave-frequency quantum nodes through joint measurement on optical photons from a pair of transducers.