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arXiv:0806.3085 · New Journal of Physics

Practical long-distance quantum key distribution system using decoy levels

Danna Rosenberg, C G Peterson, J W Harrington, P R Rice, N Dallmann, K T Tyagi, K P McCabe, S Nam, B Baek, Robert H. Hadfield, R J Hughes, J E Nordholt

Published Apr 30, 2009• 84 citations
arXivHTMLPDF

Abstract

Quantum key distribution (QKD) has the potential for widespread real-world applications. To date no secure long-distance experiment has demonstrated the truly practical operation needed to move QKD from the laboratory to the real world due largely to limitations in synchronization and poor detector performance. Here we report results obtained using a fully automated, robust QKD system based on the Bennett Brassard 1984 protocol (BB84) with low-noise superconducting nanowire single-photon detectors (SNSPDs) and decoy levels. Secret key is produced with unconditional security over a record 144.3 km of optical fibre, an increase of more than a factor of five compared to the previous record for unconditionally secure key generation in a practical QKD system.

DOI
10.1088/1367-2630/11/4/045009

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Quantum key distribution (QKD) has the potential for widespread real-world applications. To date no secure long-distance experiment has demonstrated the truly practical operation needed to move QKD from the laboratory to the real world due largely to limitations in synchronization and poor detector performance. Here we report results obtained using a fully automated, robust QKD system based on the Bennett Brassard 1984 protocol (BB84) with low-noise superconducting nanowire single-photon detectors (SNSPDs) and decoy levels. Secret key is produced with unconditional security over a record 144.3 km of optical fibre, an increase of more than a factor of five compared to the previous record for unconditionally secure key generation in a practical QKD system.
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.
Advances in single-photon sources (SPSs) and single-photon detectors (SPDs) promise unique applications in the field of quantum information technology. In this paper, we report long-distance quantum key distribution (QKD) by using state-of-the-art devices: a quantum-dot SPS (QD SPS) emitting a photon in the telecom band of 1.5 μm and a superconducting nanowire SPD (SNSPD). At the distance of 100 km, we obtained the maximal secure key rate of 27.6 bps without using decoy states, which is at least threefold larger than the rate obtained in the previously reported 50-km-long QKD experiment. We also succeeded in transmitting secure keys at the rate of 0.307 bps over 120 km. This is the longest QKD distance yet reported by using known true SPSs. The ultralow multiphoton emissions of our SPS and ultralow dark count of the SNSPD contributed to this result. The experimental results demonstrate the potential applicability of QD SPSs to practical telecom QKD networks.
Quantum key distribution (QKD) allows two parties to communicate in absolute security based on the fundamental laws of physics. Up till now, it is widely believed that unconditionally secure QKD based on standard Bennett-Brassard (BB84) protocol is limited in both key generation rate and distance because of imperfect devices. Here, we solve these two problems directly by presenting new protocols that are feasible with only current technology. Surprisingly, our new protocols can make fiber-based QKD unconditionally secure at distances over 100km (for some experiments, such as GYS) and increase the key generation rate from O(η2) in prior art to O(η) where η is the overall transmittance. Our method is to develop the decoy state idea (first proposed by W.-Y. Hwang in "Quantum Key Distribution with High Loss: Toward Global Secure Communication", Phys. Rev. Lett. 91, 057901 (2003)) and consider simple extensions of the BB84 protocol. This part of work is published in "Decoy State Quantum Key Distribution", . We present a general theory of the decoy state protocol and propose a decoy method based on only one signal state and two decoy states. We perform optimization on the choice of intensities of the signal state and the two decoy states. Our result shows that a decoy state protocol with only two types of decoy states—a vacuum and a weak decoy state—asymptotically approaches the theoretical limit of the most general type of decoy state protocols (with an infinite number of decoy states). We also present a one-decoy-state protocol as a special case of Vacuum+Weak decoy method. Moreover, we provide estimations on the effects of statistical fluctuations and suggest that, even for long distance (larger than 100km) QKD, our two-decoy-state protocol can be implemented with only a few hours of experimental data. In conclusion, decoy state quantum key distribution is highly practical. This part of work is published in "Practical Decoy State for Quantum Key Distribution", . We also have done the first experimental demonstration of decoy state quantum key distribution, over 15km of Telecom fibers. This part of work is published in "Experimental Decoy State Quantum Key Distribution Over 15km", .
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.
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.