Photonic link from single-flux-quantum circuits to room temperature

Broadband, energy-efficient signal transfer between a cryogenic and room-temperature environment has been a major bottleneck for superconducting quantum and classical logic circuits. Photonic links promise to overcome this challenge by offering simultaneous high bandwidth and low thermal load. However, the development of cryogenic electro-optic modulators—a key component for the photonic readout of electrical signals—has been stifled by the stringent requirements of superconducting circuits. Rapid single-flux-quantum circuits, for example, operate with a tiny signal amplitude of only a few millivolts, far below the volt-level signal used in conventional circuits. Here we demonstrate one of the first direct optical readouts of a rapid single-flux-quantum circuit without additional electrical amplification enabled by a novel superconducting electro-optic modulator featuring a record-low half-wave voltage Vπ of 42 mV on a 1-m-long superconducting electro-optic modulator. Leveraging the low ohmic loss of superconductors, we break the fundamental Vπ–bandwidth trade-off and demonstrate an electro-optic bandwidth up to 17 GHz on a 0.2-m-long superconducting electro-optic modulator at cryogenic temperatures. Our work presents a viable solution towards high-bandwidth signal transfer between future large-scale superconducting circuits and room-temperature electronics.