Cryogenic Wavelength-Multiplexed Eight-Channel Optical-to-Microwave Converter.

Next-generation superconducting quantum computers face scalability limitations due to the increasing number of required interconnects from room temperature into the cryogenic environment. Scalability can be improved by transitioning to photonic links, and even further scaling can be obtained by employing wavelength multiplexing. This would allow to replace multiple RF cables by a single optical fiber. However, experimental demonstration of a cryogenic multichannel optical-to-microwave converter, required for demultiplexing incoming optical signals and performing conversion to microwave signals inside the cryostat, is still missing. Here, we demonstrate the first cryogenic eight-channel optical-to-microwave converter. It combines an arrayed waveguide grating in silicon photonics for wavelength demultiplexing with eight waveguide-integrated plasmonic graphene photodetectors. The device has a compact footprint of 1.75 mm × 0.71 mm, enabling scalable and dense integration. We characterized all eight channels at 4 K and found electrical crosstalk below 30 dB, high bandwidth >45 GHz, subnanoampere dark current, and 4-5× increased responsivity compared to room temperature. Finally, as a proof of concept, we simultaneously transferred two RF signals modulated on two different optical carriers into the cryostat and achieved crosstalk isolation of >50 dB in the best case. Therefore, our device demonstrates the potential of wavelength-multiplexed photonic links for mitigating the cryogenic interconnect bottleneck.