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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.

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Fig. 1: SEOM and its application in a cryogenic-to-room-temperature link.
Fig. 2: Low-drive-voltage operation of SEOM.
Fig. 3: SEOM bandwidth: modelling and experimental characterization.
Fig. 4: Photonic link from an RSFQ circuit to room temperature.
Fig. 5: Projected SEOM transduction efficiency.

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Data availability

The data that support the findings of this study are included in this Article. Source data are provided with this paper.

Code availability

All relevant computer codes supporting this study are available from the corresponding author upon reasonable request.


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This project was funded by IARPA’s SuperCables program through the ARO grant W911-NF-19-2-0115, and DOE Office of Science, National Quantum Information Science Research Centers, Co-design Center for Quantum Advantage (C2QA), contract no. DE-SC0012704. We thank the Office of Naval Research for providing funding support in the construction of the RF interface through grant N00014-20-1-2134. Y.Z. acknowledges support from the Yale Quantum Institute. We extend our gratitude to B. Liu for his assistance with the RSFQ circuit module installation and operation. We would also like to acknowledge HYPRES for their contributions to the RSFQ circuit design and for granting permission to use the micrograph of the RSFQ circuit chip in this Article. Special thanks go to D. Kirichenko and D. Gupta for their insightful discussions. We are grateful to M. Gehl, B. Palmer, S. Nam, D. V. Vecheten, W. Mayer and W. Harrod for providing valuable technical and administrative support throughout this project. Finally, we thank Y. Sun, S. Rinehart, L. McCabe, K. Woods and M. Rooks for assistance in the device fabrication.

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H.X.T. and M.S. conceived the idea and experiment. M.S. fabricated the device and performed the experiment. J.X. and Y.X. helped with the fabrication and experiments. S.W., R.C., W.F. and Y.Z. helped with the device packaging and instrumentation. M.S. wrote the manuscript, and all authors contributed to the manuscript. H.X.T. supervised the work.

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Correspondence to Hong X. Tang.

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Nature Photonics thanks Paolo Pintus and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Shen, M., Xie, J., Xu, Y. et al. Photonic link from single-flux-quantum circuits to room temperature. Nat. Photon. 18, 371–378 (2024).

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