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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Quantum key distribution at GHz transmission rates

Alessandro Restelli, Joshua C. Bienfang, Alan Mink, Charles W. Clark

Published Dec 2, 2008• 7 citations
Publisher

Abstract

Quantum key distribution (QKD) channels are typically realized by transmitting and detecting single photons, and therefore suffer from dramatic reductions in throughput due to both channel loss and noise. These shortcomings can be mitigated by applying telecommunications clock-recovery techniques to maximize the bandwidth of the single-photon channel and minimize the system's exposure to noise. We demonstrate a QKD system operating continuously at a quantum-channel transmission rate of 1.25 GHz, with dedicated data-handling hardware and error-correction/privacy amplification. We discuss the design and performance of our system and highlight issues which limit our maximum transmission and key production rates.

DOI
10.1117/12.809461

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Quantum key distribution (QKD) channels are typically realized by transmitting and detecting single photons, and therefore suffer from dramatic reductions in throughput due to both channel loss and noise. These shortcomings can be mitigated by applying telecommunications clock-recovery techniques to maximize the bandwidth of the single-photon channel and minimize the system's exposure to noise. We demonstrate a QKD system operating continuously at a quantum-channel transmission rate of 1.25 GHz, with dedicated data-handling hardware and error-correction/privacy amplification. We discuss the design and performance of our system and highlight issues which limit our maximum transmission and key production rates.
The desire for quantum-generated cryptographic key for broadband encryption services has motivated the development of high-transmission-rate single-photon quantum key distribution (QKD) systems. The maximum operational transmission rate of a QKD system is ultimately limited by the timing resolution of the single-photon detectors and recent advances have enabled the demonstration of QKD systems operating at transmission rates well in to the GHz regime. We have demonstrated quantum generated one-time-pad encryption of a streaming video signal with high transmission rate QKD systems in both free-space and fiber. We present an overview of our high-speed QKD architecture that allows continuous operation of the QKD link, including error correction and privacy amplification, and increases the key-production rate by maximizing the transmission rate and minimizing the temporal gating on the single-photon channel. We also address count-rate concerns that arise at transmission rates that are orders of magnitude higher than the maximum count rate of the single-photon detectors.
ABSTRACT Quantum Cryptography has demonstrated the potential for ultra-secure communications. However, with quantum-channel transmission rates in the MHz range, typical link losses and signal-to-noise ratios have resulted in key-production rates that are impractical for continuous one-time-pad encryption of high-bandwidth communications. We have developed high-speed data handling electronics that support quantum-channel transmission rates up to 1.25 GHz. This system has demonstrated error-corrected and privacy-amplified key rates above 1 Mbps over a free-space link. While the transmission rate is ultimately limited by timi ng jitter in the single-photon avalanche photodiodes (SPADs), we find the timing resolution of silicon SPADs sufficient to operate efficiently with temporal gates as short as 100 ps. We have developed systems to implement such high-resolution gating in our system, and anticipate the attendant reduction in noise to produce significantly higher secret-key bitrates. Keywords: Single-photon detection, quantum key distribution, avalanche photodiode
Quantum key distribution (QKD) can produce secure cryptographic key for use in symmetric cryptosystems. By adopting clock-recovery techniques from modern telecommunications practice we have demonstrated a free-space quantum key distribution system operating at a transmission rate of 625 MHz at 850 nm. The transmission rate of this system is ultimately limited by the timing resolution of the single-photon avalanche photodiodes (SPADs), and we present a solution to take advantage of SPADs with higher timing resolution that can enable repetition rates up to 2.5 GHz. We also show that with high-repetition-rate sub-clock gating these higher-resolution SPADs can reduce the system's exposure to solar background photons, thus reducing the quantum-bit error rate (QBER) and improving system performance.
We report the first quantum-key-distribution (QKD) systems capable of delivering sustainable, real-time secure keys continuously at rates exceeding 10 Mb/s. To achieve such rates, we developed high-speed postprocessing modules, achieving maximum data throughputs of 60 MC/s, 55 Mb/s, and 108 Mb/s for standalone operation of sifting, error correction, and privacy amplification modules, respectively. The photonic layer of the QKD systems features high-speed single-photon detectors based on self-differencing InGaAs avalanche photodiodes, phase encoding using an asymmetric Mach-Zehnder interferometer, and active stabilization of the interferometer phase and photon polarization. An efficient variant of the decoy-state BB84 protocol is implemented for the security analysis, with a large dataset size of 108bits selected to mitigate finite-size effects. Over a 2-dB channel, a record secure key rate of 13.72 Mb/s has been achieved averaged over 4.4 days of operation. We confirm the robustness and long-term stability on a second QKD system continuously running for 1 month without any user invention.
In free-space single-photon quantum key distribution (QKD), the error rate due to daytime background photons can be reduced with strong temporal filtering. In this case, the improvement in performance is determined by the receiver's ability to resolve signal-photon arrival times. We use fast clock recovery and commercially available single-photon detectors with timing resolution enhanced by additional electronic circuitry to implement temporal gating down to 50 ps in a free-space QKD system. The single-photon channel operates at 850 nm, and the improved timing resolution enables transmission rates of 1.25 GHz. We observe daytime quantum bit error rates of 0.04, which is less than one-third of the ungated error rate. We present the design and performance of the system and demonstrate its benefit to free-space QKD.
Free-space Quantum key distribution (QKD) has shown the potential for the practical production of cryptographic key for ultra-secure communications. The performance of any QKD system is ultimately limited by the signal to noise ratio on the single-photon channel, and over most useful communications links the resulting key rates are impractical for performing continuous one-time-pad encryption of today's broadband communications. We have adapted clock and data recovery techniques from modern telecommunications practice, combined with a synchronous classical free-space optical communications link operating in parallel, to increase the repetition rate of a free-space QKD system by roughly 2 orders of magnitude over previous demonstrations. We have also designed the system to operate in the H-alpha Fraunhofer window at 656.28 nm, where the solar background is reduced by roughly 7 dB. This system takes advantage of high efficiency silicon single-photon avalanche photodiodes with <50ps timing resolution that are expected to enable operation at a repetition rate of 2.5 GHz. We have identified scalable solutions for delivering sustained one-time-pad encryption at 10 Mbps, thus making it possible to integrate quantum cryptography into first-generation Ethernet protocols.