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Quantum Computing, Communication, and Simulation

Decoy-state quantum key distribution over long-distance optical fiber

Giulia Guarda, Domenico Ribezzo, Daniela Salvoni, Ciro Bruscino, Pasquale Ercolano, Mikkel Ejrnaes, L. Parlato, Cai Zhang, Hao Li, Lixing You, Ilaria Vagniluca, Claudia De Lazzari, Tommaso Occhipinti, Giovanni Piero Pepe, Alessandro Zavatta, Davide Bacco

Published Mar 12, 2024• 1 citation
Publisher

Abstract

Today’s society heavily relies on secure communications, which can be guaranteed by Quantum Key Distribution (QKD), the most mature quantum technology. However, achieving long-distance links without relying on trusted nodes is still challenging. An important limitation is the non-ideality of detection systems, where intrinsic dark counts can hinder key extraction. This work proposes using state-of-the-art superconducting nanowire singlephoton detectors (SNSPD) with ultra-low dark count rates (<1 Hz) to reduce the quantum bit error rate (QBER) and achieve a higher secret key rate. Together with a high-rate QKD transmitter and a self-stabilizing receiver, we enabled a key exchange over 55 dB, corresponding to 340 km over an ultra-low-loss optical fiber.

DOI
10.1117/12.3003698

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Today’s society heavily relies on secure communications, which can be guaranteed by Quantum Key Distribution (QKD), the most mature quantum technology. However, achieving long-distance links without relying on trusted nodes is still challenging. An important limitation is the non-ideality of detection systems, where intrinsic dark counts can hinder key extraction. This work proposes using state-of-the-art superconducting nanowire singlephoton detectors (SNSPD) with ultra-low dark count rates (<1 Hz) to reduce the quantum bit error rate (QBER) and achieve a higher secret key rate. Together with a high-rate QKD transmitter and a self-stabilizing receiver, we enabled a key exchange over 55 dB, corresponding to 340 km over an ultra-low-loss optical fiber.
Quantum key distribution represents one of the most mature and well-studied advances in quantum technologies. Many protocols have been implemented, breaking record distances both employing satellites [1] and fiber-based systems [2]. However, there still are open challenges in achieving long-distance links without implementing quantum repeaters. One of the main limiting factors comes from the non-ideality of detection systems, in particular from intrinsic dark counts. As the covered distances increase, the signal decreases in intensity and the intrinsic noise in the detector prevails, hindering the extraction of a secret key. In this work, we propose the use of high-performance state-of-the-art superconducting nanowire single-photon detectors (SNSPD) showing ultra-low dark count rates to reduce the quantum bit error rate (QBER) in the detection system. Overall we achieved a link distance of 55 dB equivalent to 275 km in ultra low-loss fiber.
Long distance quantum key distribution links are essential for the development of a widely spread fiber-based quantum network. For instance, it would allow for two remote users to communicate securely without the need for a trusted intermediate node. In this work, we focus on enhancing the receiver side of a quantum key distribution system, deploying state-of-the-art superconducting nanowire single photon detectors with ultra low dark count (< 1Hz) and very high efficiency (> 90%), achieving a secure key distribution over a 55 dB losses channel with 0.6 bps rate. This result opens new possibilities for long distance secure communications.
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.
Quantum key distribution (QKD) enables secure communication by harnessing the fundamental principles of quantum physics, which inherently guarantee information-theoretic security and intrinsic resistance to quantum computing attacks. However, the secure key rate of QKD typically decreases exponentially with increasing channel distance. In this work, by developing a novel polarization-state preparation method, an ultra-low time-jitter laser source and superconducting nanowire single-photon detectors, we demonstrate a 5-GHz integrated QKD system featuring ultra-low quantum bit error rates (QBERs). The system achieves secure key rates of 1.076 Mbps at 150 km and 105 kbps at 200 km over standard single-mode fiber channels, respectively. Our system substantially enhances the secure key rate, enabling high-resolution video calls with one-time-pad encryption over intercity backbone QKD links. This work represents a significant step forward in the development of high-performance practical QKD systems.
We report the first quantum key distribution (QKD) experiment over a 72 dB channel loss using superconducting nanowire single-photon detectors (SSPD, SNSPD) with the dark count rate (DCR) of 0.01 cps. The DCR of the SSPD, which is dominated by the blackbody radiation at room temperature, is blocked by introducing cold optical bandpass filter. We employ the differential phase shift QKD (DPS-QKD) scheme with a 1 GHz system clock rate. The quantum bit error rate (QBER) below 3% is achieved when the length of the dispersion shifted fiber (DSF) is 336 km (72 dB loss), which is low enough to generate secure keys.
We developed a superconducting nanowire single-photon detection (SNSPD) system based on Gifford-McMahon cryocooler for quantum communication applications. Environmental factors which may influence the system performance are intensively studied. Those factors include temperature fluctuations, the ambient magnetic field and the background radiation. By optimizing the bias circuit, the stability of SNSPD system to electrical noise and disturbance was effectively enhanced, thus making it more suitable for field application. A 4-channel SNSPD system with quantum efficiency higher than 4% at the dark count rate of 10 Hz for λ=1550 nm is integrated and applied into a quantum key distribution (QKD) experiment. QKD was successfully carried out over 100 km optical fiber with the final secure key rate of 1.6 kbps and the quantum bit error rate of less than 2%.