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# Readout-efficient superconducting nanowire single-photon imager with orthogonal time–amplitude multiplexing by hotspot quantization

## Abstract

Scaling superconducting nanowire single-photon detectors into a large array to obtain imaging capability is desired for applications in photon-starved conditions, considering the outstanding performance already demonstrated on a single detector. However, this is challenging because the ultralow operation temperature only allows specific electronics of ultralow power dissipation to work. Here we develop a kilopixel imager by introducing an orthogonal time–amplitude-multiplexing method. This readout is solely built in the superconducting nanowire by geometrically designing the nanowire structure to manipulate its hotspot growth and microwave propagation after photon detection. As a result, pixel locations are encoded on both time and amplitude domains of the output pulses. This dual multiplexing method overcomes the previous limitation of a time-multiplexing readout, where the time measurement uncertainty deteriorates the spatial resolution and scalability. Experimentally, with two readout lines, we have demonstrated a 32 × 32 imager with an average readout pixel fidelity of 97% and an average temporal resolution of 67.3 ps. The performance of this imager is further verified by single-photon imaging experiments at a low photon flux down to one detected photon per pixel. This orthogonal time–amplitude-multiplexing readout and corresponding nanowire designs give the most efficient readout compared with previous methods, which would speed up the development of large-scale single-photon imagers for quantum measurements, remote sensing, astronomical telescopes and so on.

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## Data availability

The data that support the findings of this study are available from the corresponding authors upon reasonable request.

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## Acknowledgements

We thank the other RISE members for assistance in nanofabrication, measurements and providing instruments. This work was supported by the National Key R&D Program of China Grant (2017YFA0304002), the National Natural Science Foundation (nos. 62071214, 61571217 and 11227904), the Program for Innovative Talents and Entrepreneur in Jiangsu, the Fundamental Research Funds for the Central Universities, the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and the Jiangsu Provincial Key Laboratory of Advanced Manipulating Technique of Electromagnetic Waves.

## Author information

Authors

### Contributions

L.-D.K. conceived the idea, designed and fabricated the device, implemented the experiments and developed the analysis methods. H.W. deposited the superconducting film. L.-D.K. and J.-W.G. performed the optical simulations. H.W., X.-C.T., L.-B.Z., X.-Q.J. and L.K. helped with the device fabrication. Y.-H.H., H.H. and S.C. helped with the cryogenic setup. Q.-Y.Z., J.C. and P.-H.W. supervised the work. L.-D.K. and Q.-Y.Z. wrote the manuscript.

### Corresponding authors

Correspondence to Qing-Yuan Zhao or Jian Chen.

## Ethics declarations

### Competing interests

L.-D.K. and Q.-Y.Z. applied for a Chinese patent (no. 202111220932.3). The remaining authors declare no competing interests.

## Peer review

### Peer review information

Nature Photonics thanks the anonymous reviewers for their contribution to the peer review of this work.

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## Supplementary information

### Supplementary Information

Supplementary Figs. 1–7 and discussion.

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Reprints and Permissions

Kong, LD., Wang, H., Zhao, QY. et al. Readout-efficient superconducting nanowire single-photon imager with orthogonal time–amplitude multiplexing by hotspot quantization. Nat. Photon. 17, 65–72 (2023). https://doi.org/10.1038/s41566-022-01089-6

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• DOI: https://doi.org/10.1038/s41566-022-01089-6

• ### Scaling up single-photon detector arrays

• Daniel F. Santavicca

Nature Photonics (2023)