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International Conference on Quantum Information

Towards a Short-range Free-space GHz-clocked Quantum Key Distribution System

María-José García-Martínez, Natalia Denisenko, D. Soto, Verónica Fernández

Published Jan 1, 2011• 0 citations
Publisher

Abstract

A free-space quantum key distribution system that operates at a wavelength of 850 nm is presented. The system is designed to implement the B92 protocol at high transmission rates between two locations in urban areas.
DOI
10.1364/icqi.2011.qmi17

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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.
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In recent years quantum information research has lead to the discovery of a number of remarkable new paradigms for information processing and communication. These developments include quantum cryptography schemes that offer unconditionally secure information transport guaranteed by quantum-mechanical laws. Such potentially disruptive security technologies could be of high strategic and economic value in the future. Two major issues confronting researchers in this field are the transmission range (typically <100km) and the key exchange rate, which can be as low as a few bits per second at long optical fiber distances. This paper describes further research of an approach to significantly enhance the key exchange rate in an optical fiber system at distances in the range of 1-20km. We will present results on a number of application scenarios, including point-to-point links and multi-user networks. Quantum key distribution systems have been developed, which use standard telecommunications optical fiber, and which are capable of operating at clock rates of up to 2GHz. They implement a polarization-encoded version of the B92 protocol and employ vertical-cavity surface-emitting lasers with emission wavelengths of 850 nm as weak coherent light sources, as well as silicon single-photon avalanche diodes as the single photon detectors. The point-to-point quantum key distribution system exhibited a quantum bit error rate of 1.4%, and an estimated net bit rate greater than 100,000 bits-1 for a 4.2 km transmission range.
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.