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IEEE Transactions on Applied Superconductivity

High Performance Superconducting Nanowire Single Photon Detectors for QKD Applications

Ciro Bruscino, Pasquale Ercolano, Daniela Salvoni, M. Di Giancamillo, Chengjun Zhang, Mikkel Ejrnaes, Hao Li, Lixing You, L. Parlato, Mario Martinelli, Giovanni Piero Pepe

Published Jan 19, 2024• 6 citations
Publisher

Abstract

The forthcoming era of quantum computers can be a threat to the conventional cryptography and data security. Quantum Key Distribution (QKD) provides unconditional security under real-life conditions with several protocols over long distances in fibre and free space communication. Superconducting Nanowire Single Photon Detectors (SNSPDs) are becoming a dominant technology for QKD thanks to their unique characteristics, such as a near-unity efficiency in the infrared, low dark counts and picoseconds time resolution. Where the detector is typically a weakness of QKD, these SNSPDs characteristics make exploitation difficult. In this work, we characterized NbN SNSPDs at 2.2 K, using a CW laser source at 1550 nm, varying both bias currents and input photon rates to prove their high efficiency at low dark counts with a high counting rate, consistent with the requirements for QKD over long distances or with a high secure key rate.

DOI
10.1109/tasc.2024.3355878

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The forthcoming era of quantum computers can be a threat to the conventional cryptography and data security. Quantum Key Distribution (QKD) provides unconditional security under real-life conditions with several protocols over long distances in fibre and free space communication. Superconducting Nanowire Single Photon Detectors (SNSPDs) are becoming a dominant technology for QKD thanks to their unique characteristics, such as a near-unity efficiency in the infrared, low dark counts and picoseconds time resolution. Where the detector is typically a weakness of QKD, these SNSPDs characteristics make exploitation difficult. In this work, we characterized NbN SNSPDs at 2.2 K, using a CW laser source at 1550 nm, varying both bias currents and input photon rates to prove their high efficiency at low dark counts with a high counting rate, consistent with the requirements for QKD over long distances or with a high secure key rate.
The recent advances in superconducting nanowire single-photon detector (SNSPD or SSPD) technology has enabled long distance quantum key distribution (QKD) over an optical fiber. We point out that the performance of SNSPDs play a crucial role in achieving a secure transmission distance of 100 km or longer. We analyze such an impact from a simplified model and use it to interpret results from our differential-phase-shift (DPS) QKD experiment. This allows us to discuss the optimization of the detection time window and the clock frequency given the detector characteristics such as dark count rate, detection efficiency, and timing jitter.
Single-photon sources and detectors are key enabling technologies for photonics in quantum information science and technology (QIST). QIST applications place high-level demands on the performance of sources and detectors; it is therefore essential that their properties can be characterized accurately. Superconducting nanowire single-photon detectors (SNSPDs) have spectral sensitivity from visible to beyond 2 μm in wavelength, picosecond timing resolution (Jitter <100 ps FWHM) and the capacity to operate ungated with low dark counts (<1 kHz). This facilitates data acquisition at high rates with an excellent signal-to-noise ratio. We report on the construction and characterization of a two-channel SNSPD system. The detectors are mounted in a closed-cycle refrigerator, which eliminates reliance on liquid cryogens. Our specification was to deliver a system with 1% efficiency in both channels at a wavelength of 1310 nm with 1 kHz dark count rate. A full width at half maximum timing jitter of less than 90 ps is achieved in both channels. The system will be used to detect individual photons generated by quantum-optical sources at telecom wavelengths. Examples include single-photon sources based on quantum dots (emitting at 1310 nm). The SNSPD system's spectral sensitivity and timing resolution make it suited to characterization of such sources, and to wider QIST applications.
We report the fabrication of a superconducting nanowire single-photon detector (SSPD or SNSPD) with an ultralow dark count rate. By introducing optical band-pass filters at the input of the SSPD and cooling the filters at 3 K, the dark count rate is reduced to less than 1/100 at low bias. An SSPD with 0.1 cps dark count rate and 5.6% system detection efficiency at 1550 nm wavelength is obtained. We show that a quantum key distribution (QKD) over 300 km of fiber is possible based on a numerical calculation assuming a differential phase shift QKD protocol implemented with our SSPDs.
Photon detectors connect optical and electrical systems by converting light to electrical signals. Single photon detectors detect single photons which measure ultra-weak light. Traditional semiconductor single-photon detectors suffer from high dark count rates, low infrared detection efficiency, and poor temporal resolution. Superconducting-nanowire single-photon detectors (SNSPDs) overcome these limitations using the hotspot effect. SNSPDs have created a solid foundation for optical technologies and become key in quantum information, LiDARs and Deep-Space Communications. This paper reviews working principles and performance metrics: system detection efficiency, dark count rate, recovery time, and timing jitter. The study summarizes applications in quantum information (quantum key distribution, optical quantum computing), single-photon LiDAR, and deep-space optical communication, replacing most single-photon avalanche diodes (SPADs). Recent experiments show that SNSPD-based systems can achieve meter-scale detection of sea fog, interplanetary laser communication over billions of kilometers, and boson sampling with 20 photons. Ongoing research is expected to improve performance in maximum count rate, system integration, and cryogenic operation, as well as to find applications in emerging fields such as environmental monitoring.
Single photon detection advanced rapidly in the last decade. The superconducting nanowire single-photon detector (SNSPD) is an emerging technology, which offers low dark count rates, broadband sensitivity from visible to mid-infrared wavelengths and very high quantum efficiency. All these features are appealing for future emerging applications in remote gas sensing, quantum key distribution and quantum information processing. By enclosing the detector in an optical cavity, tuned to the operating wavelength, the total system detection of the SNSPD is improved. Based on the cavity design, the detector absorption is boosted at the desired experimental wavelength. Devices have been fabricated and tested, in this thesis, with enhanced detection efficiency at near-infrared, infrared and mid-infrared wavelengths. NbTiN detectors fabricated on Ta2O5 / SiO2 distributed Bragg reflectors (DBR) for front-side coupling exhibit a system detection efficiency of ≈ 90% at λ = 940 nm for the best device measured. SNSPDs were also fabricated in NbTiN on metallic mirrors for back-side coupling for 1550 and 2333 nm experimental wavelengths. The first design exhibited a system detection efficiency of ≈ 84.5%, at λ = 1550 nm for the best device measured. The second exhibited low system detection efficiency since the device suffered from unexpectedly high dark count rate mainly due to blackbody radiation at 2.2 K. However, it was possible to measure the timing jitter of this device at 350 mK indicating a FWHM of 84 ps. Another technique to improve the total system detection efficiency of the SNSPD, is by fabricating a hairpin-shaped nanowire atop a single mode waveguide. In this design, photons travel through the waveguide and via evanescent coupling are absorbed in the nanowire. These SNSPD devices have been designed to thread along single mode Si ridge waveguides using a hairpin-shaped nanowire structure, which offers enhanced coupling and absorption efficiency via evanescent coupling. These ultra-thin superconducting MoSi detectors are precisely patterned on top of a single mode Si waveguide (600 nm) with alignment error less than 40 nm. Low temperature electrical and nano-optical characterization was carried out using a two-stage pulse tube (PT) coldhead, using an additional 4He / 3He sorption pump, low vibration characterization of SNSPD devices at 350 mK was carried out. Characterised devices exhibit low dark count rate <5 counts per second and timing jitter of ≈ 83 ps full width half maximum (FWHM) at wavelength (λ) = 1550 nm and at critical current ≈ 30 µA.
Two approaches to single-photon detection at telecom wavelengths have been investigated, using semiconductor and superconductor technologies. The low temperature operation of free-running InGaAs/InP negative feedback avalanche diodes (NFADs) was studied, demonstrating that they can operate with an extremely low dark count rate, which opens up a host of new applications. Secondly, superconducting nanowire single-photon detectors (SNSPDs) based on amorphous materials were developed, fabricated and characterized. These detectors are highly attractive because they can achieve excellent attributes simultaneously, such as high efficiency, low noise, good temporal resolution, no afterpulsing and high count rates. Finally, it was demonstrated that the use of NFADs can enable record distance quantum key distribution (QKD) over 307 km of optical fiber. A next generation QKD system has been proposed and numerically analyzed, aiming at extending the record distance by an additional 100 km and increasing the state-of-the-art secret key rate by over an order of magnitude.