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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.

A study of the influence of building modelling when predicting the signal attenuation due to over-rooftop multiple diffraction in urban environments is presented. A straightforward formulation expressed in terms of Uniform Theory of Diffraction (UTD) coefficients is used for the above-mentioned analysis assuming a sphericalwave incidence over the array of buildings and considering the latter modelled as series of different representations such as knife edges, double knife edges, wedges and rectangular sections. The results clearly indicate the importance of an appropriate building modelling in obtaining realistic path-loss predictions in urban environments when multiple diffraction has to be considered.

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