Bidirectional and efficient conversion between microwave and optical light

Converting low-frequency electrical signals into much higher-frequency optical signals has enabled modern communication networks to leverage the strengths of both microfabricated electrical circuits and optical fibre transmission, enabling information networks to grow in size and complexity. A microwave-to-optical converter in a quantum information network could provide similar gains by linking quantum processors through low-loss optical fibres and enabling a large-scale quantum network. However, no current technology can convert low-frequency microwave signals into high-frequency optical signals while preserving their fragile quantum state. Here we demonstrate a converter that provides a bidirectional, coherent and efficient link between the microwave and optical portions of the electromagnetic spectrum. We use our converter to transfer classical signals between microwave and optical light with conversion efficiencies of ∼10%, and achieve performance sufficient to transfer quantum states if the device were further precooled from its current 4 K operating temperature to temperatures below 40 mK.