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.
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.
Quantum Cryptography has demonstrated the potential for ultra-secure communications. However, with quantumchannel
transmission rates in the MHz range, typical link losses and signal-to-noise ratios have resulted in keyproduction
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 timing 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.
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.