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IEEE photonics journal

Suppressing Dark Counts of Multimode-Fiber-Coupled Superconducting Nanowire Single-Photon Detector

Chengjun Zhang, Weijun Zhang, Lixing You, Jia Huang, Hao Li, Xing-Qu Sun, Heqing Wang, Chaolin Lv, Hui Zhou, Xiaoyu Liu, Zhen Wang, Xiaoming Xie

Published Aug 26, 2019• 10 citations
Publisher

Abstract

Large active-area superconducting nanowire single-photon detectors (SNSPDs) coupled with multimode fibers (MMFs) can provide high light-gathering capacity, which is essential for free-space detection applications in photon-starved regimes. However, MMF-coupled SNSPDs often suffer from large system dark count rates (DCRsys) over kHz due to blackbody radiation of the MMF at room temperature. Such large DCRsys would significantly degrade signal-to-noise ratio (SNR) of the receiving system. This paper reports an MMF-coupled large-active-area SNSPD system with low DCRsys by using a homemade cryogenic MMF filter bench. The filter bench, which consists of lenses and optical filters, can provide a high transmittance of about 80% at the central wavelength of the passband (1550 ± 12.5 nm) and a wide blocking range from 500 nm to over 6000 nm at 40 K. With using the filter bench, the DCRsys of an MMF-coupled 9-pixel SNSPD array with an active area of 50 μm in diameter is greatly suppressed by 23 dB with 1 dB loss of system detection efficiency (SDE). The detector demonstrates an SDE of 51% at a DCRsys of 100 Hz for 1550 nm photons. Thus, SNR of the detector is enhanced by about 160 times and the noise equivalent power is improved to 3×10-19W/Hz1/2.

DOI
10.1109/jphot.2019.2937537

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Large active-area superconducting nanowire single-photon detectors (SNSPDs) coupled with multimode fibers (MMFs) can provide high light-gathering capacity, which is essential for free-space detection applications in photon-starved regimes. However, MMF-coupled SNSPDs often suffer from large system dark count rates (DCRsys) over kHz due to blackbody radiation of the MMF at room temperature. Such large DCRsys would significantly degrade signal-to-noise ratio (SNR) of the receiving system. This paper reports an MMF-coupled large-active-area SNSPD system with low DCRsys by using a homemade cryogenic MMF filter bench. The filter bench, which consists of lenses and optical filters, can provide a high transmittance of about 80% at the central wavelength of the passband (1550 ± 12.5 nm) and a wide blocking range from 500 nm to over 6000 nm at 40 K. With using the filter bench, the DCRsys of an MMF-coupled 9-pixel SNSPD array with an active area of 50 μm in diameter is greatly suppressed by 23 dB with 1 dB loss of system detection efficiency (SDE). The detector demonstrates an SDE of 51% at a DCRsys of 100 Hz for 1550 nm photons. Thus, SNR of the detector is enhanced by about 160 times and the noise equivalent power is improved to 3×10-19W/Hz1/2.
Multimode fiber (MMF)-coupled superconducting nanowire single-photon detectors (SNSPDs) are critical for free-space quantum communication and sensing, yet their performance is severely limited by polarization-dependent detection efficiency and high dark count rate (DCR) arising from thermal photons. Here, we present a transformative solution that addresses both challenges simultaneously. We demonstrate a polarization-insensitive SNSPD based on a dielectric-compensated bilayer nanowire structure, achieving a system detection efficiency (SDE) of 88% at 1.5 μm with a DCR of ∼4.8×10 4 cps for MMF-coupled signals. To mitigate the elevated thermal noise inherent to multimode coupling, we integrate a compact, dual-stage on-fiber filter with a suppression ratio exceeding 60 dB. The packaged device delivers an SDE of 81% with a DCR of only 4 cps, corresponding to an over 11,000-fold SNR enhancement relative to the unfiltered MMF-coupled configuration.
Abstract Superconducting nanowire single-photon detectors (SNSPDs) with both high system detection efficiency (SDE) and low dark count rate (DCR) play significant roles in quantum information processes and various applications. The background dark counts of SNSPDs originate from the room temperature blackbody radiation coupled to the device via a fiber. Therefore, a bandpass filter (BPF) operated at low temperature with minimal insert loss is necessary to suppress the background DCR. Herein, a low-loss BPF integrated on a single-mode fiber end-face was designed, fabricated and verified for the low temperature implement. The fiber end-face BPF was featured with a typical passband width about 40 nm in the 1550 nm telecom band and a peak transmittance of over 0.98. SNSPD with high SDE fabricated on a distributed Bragg reflector was coupled to the BPF. The device with such a BPF showed an SDE of 80% at a DCR of 0.5 Hz, measured at 2.1 K. Compared the same device without a BPF, the DCR was reduced by over 13 dB with an SDE decrease of <3%.
High-performance superconductor nanowire single-photon detectors (SNSPDs) coupled by 50-μm multimode fiber were designed and fabricated for the purpose of space applications. Microlens coupling between multimode fiber and detector chip was proposed, resulting in coupling efficiency of 86% at the chips with detection area of 15 x 15 μm2. An optical structure for enhancing the photons absorption and superconductor nanowires with low critical current of 3 μA were fabricated on the chips. A cryogenic amplifier was adopted to increase the signal-to-noise ratio and time jitter of output pulse. The SNSPD exhibited system efficiency (1550 nm) of 56% at a dark count rate of 100 cps, 50% at a dark count rate of 5 cps, and 40% at a dark count rate of 1 cps. The time jitter was 46 ps at full-width at half-maximum. The multimode fiber coupling system with high performance will promote its applications in free space.
The dark-count rate (DCR) is a key parameter of single-photon detectors. By introducing a bulk optical band-pass filter mounted on a fiber-to-fiber optical bench cooled at 3 K and blocking down to 5 μm, we suppressed the DCR of a superconducting nanowire single-photon detector by more than three orders of magnitude. The DCR is limited by the blackbody radiation through a signal passband of 20-nm bandwidth. The figure of merit, system detection efficiency, and DCR were 2.7×10(11), 2.3%, and 10(-3) Hz, respectively. Narrowing the bandwidth to 100 GHz suppresses the DCR to 10(-4) Hz, and the figure of merit increases to 1.8×10(12).
Suppressing the dark count rate (DCR) is one of the main issues in the development of the superconducting nanowire single-photon detector. Here, we study the effect of suppressing DCR using the various optical bandpass filters cooled at 3 K. The DCR, which is dominated by the background blackbody radiation at room temperature through an optical fiber, can be strongly suppressed by introducing the cold filter. Using the bulk commercial filter with the bandwidth of 10 nm and the high transmission above 85%, the DCR is improved about 29 dB with 2.4 dB of decrease in the system detection efficiency (η). The noise equivalent power reaches 1.5 × 10-18and the figure of merit reaches 1.8 × 109. These values are compared to the results using other type of filters.
Satellite-ground quantum communication requires single-photon detectors of 850-nm wavelength with both high detection efficiency and large sensitive area. We developed superconducting nanowire single-photon detectors (SNSPDs) on one-dimensional photonic crystals, which acted as optical cavities to enhance the optical absorption, with a sensitive-area diameter of 50 μm. The fabricated multimode fiber coupled NbN SNSPDs exhibited a maximum system detection efficiency (DE) of up to 82% and a DE of 78% at a dark count rate of 100 Hz at 850-nm wavelength as well as a system jitter of 105 ps.