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Applied Optics

High-speed free-space quantum key distribution system for urban daylight applications

María-José García-Martínez, Natalia Denisenko, D. Soto, David Arroyo, Amalia Beatriz Orue, Verónica Fernández

Published May 6, 2013• 35 citations
Publisher

Abstract

We report a free-space quantum key distribution system designed for high-speed key transmission in urban areas. Clocking the system at gigahertz frequencies and efficiently filtering background enables higher secure key rates than those previously achieved by similar systems. The transmitter and receiver are located in two separate buildings 300 m apart in downtown Madrid and they exchange secure keys at rates up to 1 Mbps. The system operates in full bright daylight conditions with an average secure key rate of 0.5 Mbps and 24 h stability without human intervention.

DOI
10.1364/ao.52.003311

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We report a free-space quantum key distribution system designed for high-speed key transmission in urban areas. Clocking the system at gigahertz frequencies and efficiently filtering background enables higher secure key rates than those previously achieved by similar systems. The transmitter and receiver are located in two separate buildings 300 m apart in downtown Madrid and they exchange secure keys at rates up to 1 Mbps. The system operates in full bright daylight conditions with an average secure key rate of 0.5 Mbps and 24 h stability without human intervention.
High-speed free-space quantum key distribution located in urban environment offers an interesting alternative to public key encryption - whose security strength is yet to be mathematically proven. To achieve this, three main objectives need to be accomplished: both the emitter and receiver have to be capable of transmitting and receiving at high speed - with the selection of the source's wavelength and detectors being of especial importance - the error rate needs to be kept at a minimum, especially that due to solar background radiation; and finally, a fast automatic tracking system, capable of compensating for atmospheric turbulence effects, is needed. Regarding to sky background and atmospheric turbulence, two different tracking techniques involving the beam wander compensation in the emitter or the receiver are presented and one of them is selected for our system, based on the link propagation distance and the atmospheric turbulence regime.
Quantum key distribution exploits the principles of quantum mechanics to create and distribute cryptographic keys. The quantum communication channel can be either fiber-based or free space-based. Fiber-based quantum key distribution has evolved from lab demonstrations to commercially available systems, but its infrastructure dependency and limitations on achievable distance, due to optical fiber loss, do not allow for the realization of a global-scale quantum communication network. Free space quantum key distribution addresses this problem with features such as low loss, high data rates, and ease of deployment. This paper presents an experimental demonstration of free space quantum key distribution using the differential phase shift protocol in an urban location during daylight hours. We established a secure link, achieving a key rate of 10 kbps.
We developed a high-speed quantum key distribution (QKD) system integrated with a compact self-developed silica-based planar lightwave circuit chip and a micro-optics-based polarization module. Also, we demonstrated that our system can generate average secure keys at the rate of 701.22 kbps over a free-space link of 275 m in daylight by using effective noise filtering systems in the temporal, spectral, and spatial domains. Real-time beam tracking system was adopted to stabilize the fluctuation of the beam position.
Summary form only given. The Los Alamos free-space quantum key distribution (QKD) system operates at a 1-MHz clock rate. On each clock cycle a /spl sim/1-ns "bright timing pulse" is emitted from an array of /spl sim/1550-nm lasers at the transmitter ("Alice"). After a /spl sim/100 ns delay one of four temperature-controlled "data" diode lasers emits a /spl sim/1-ns, 772-nm optical pulse that is attenuated to the single-photon level. We present results from operations over multi-kilometer atmospheric ranges. The spectral, spatial and temporal filtering allow practical secret bit rates even in full daylight. We also present a study of a satellite-to-ground QKD system, and an analysis of the expected secret bit rate.
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.