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

High-speed photon counting techniques for broadband quantum key distribution

Daniel J. Rogers, Joshua C. Bienfang, Alan Mink, Barry Hershman, Anastase Nakassis, Li Ma, Xiao Hong Tang, David H. Su, Charles W. Clark, Carl J. Williams

Published Oct 18, 2006• 1 citation
Publisher

Abstract

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

DOI
10.1117/12.686188

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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
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We discuss high-speed electronics that support the use of single-photon avalanche diodes (SPADs) in gigahertz singlephoton communications systems. For InGaAs/InP SPADs, recent work has demonstrated reduced afterpulsing and count rates approaching 500 MHz can be achieved with gigahertz periodic-gating techniques designed to minimize the total avalanche charge to less than 100 fC. We investigate afterpulsing in this regime and establish a connection to observations using more conventional techniques. For Si SPADs, we report the benefits of improved timing electronics that enhance the temporal resolution of Si SPADs used in a free-space quantum key distribution (QKD) system operating in the GHz regime. We establish that the effects of count-rate fluctuations induced by daytime turbulent scintillation are significantly reduced, benefitting the performance of the QKD system.
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.
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.
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 implementation and commercialization of quantum cryptography technologies have to face some challenges related to the development of single-photon detectors operating at 1550 nm. The main requirements are: i) high detection efficiency; ii) low noise; iii) high count rate; iv) low timing jitter. Different technologies are currently available for single-photon detection at 1550 nm, but semiconductor devices (like Single-Photon Avalanche Diode, SPAD) offer a photon detection efficiency that is inherently higher than any photocathode employed in vacuum tube detectors. Additionally InGaAs/InP SPADs can detect single photons at 1550 nm with low noise when moderately cooled by means of thermo-electric coolers. Consequently, InGaAs/InP SPAD can be the enabling technology of practical quantum key distribution (QKD) systems, provided that the maximum count rate is increased above 1 Mcps. The main limit is the afterpulsing effect that usually sets too long (< 10 μs) a hold-off time after each avalanche ignition. We present the developments achieved in InGaAs/InP SPAD device design, fabrication technology and front-end electronics, aimed at decreasing the afterpulsing effect, while not impairing photon detection efficiency and timing jitter. The new InGaAs/InP SPADs provide count rates higher than 1 Mcps and temporal response with 60 ps Full-Width at Half Maximum and very short (30 ps time constant) tail. The front-end electronics includes a wide-band pulse generator able to gate the SPAD up to 133 MHz repetition rate. Eventually, a fast avalanche-quenching scheme minimizes quenching time to less than 1 ns, thus effectively reducing afterpulsing by decreasing the total charge flowing through the junction.