Abstract
Optimising the interconnection between free-space and fibre links will be necessary for future quantum communication networks. In daylight free-space quantum communication based on direct detection, the required Field Of View (FoV) of the receiver system needs to be minimised to reduce solar background noise coupling into the detectors. Reducing the FoV requires minimising beam wander effects caused by atmospheric turbulence through active optics. We implement a fine tracking system designed to correct tip and tilt wavefront aberrations, using two feedback loops; each of them consisting of a quadrant detector and a fast steering mirror for stabilising the beam in the whole optical axis of the receiver. We test the performance of the tracking system with different optical fibres in order to evaluate the reduction in the quantum bit error rate (QBER) caused by solar background noise. A reduction of 75% for single mode fibre was obtained, and 45% reduction for a 25 µm core diameter fibre, both cases for strong turbulence (Cn2~10-12 – 10-13 m-2/3) and 100 m propagating channel. These results look promising for enabling free-space Quantum Key Distribution (QKD) in wireless networks for realistic/adverse conditions such as daylight and strong turbulent regimes.
Highlights
Quantum safe communications is recommended by national securities agencies worldwide
In daylight free-space quantum communication based on direct detection, the required Field Of View (FoV) of the receiver system needs to be minimised to reduce solar background noise coupling into the detectors
Since our study focuses in analysing the performance of a tracking system for a Quantum Key Distribution (QKD) link, the QKD emitter was approximated by a single diode laser source connected to a beam collimator emitting a 10 mm diameter beam
Summary
Quantum safe communications is recommended by national securities agencies worldwide. It is vital to reduce this noise, which can be achieved by reducing the Field of View (FoV) of the receiver through using small core optical fibres that collect and guide the quantum signal to the detectors in the receiver. This must be performed in conjunction with active beam tracking techniques to avoid signal losses. We test a fine tracking system designed for improving the free-space to fibre interface for QKD systems, in high turbulence regimes as those typically observed in urban areas in daylight. The paper is organized as follows: section 2 introduces the experimental set up implemented to test the tracking system; section 3 briefly describes how the characterization of the turbulence conditions is performed; section 4 shows the experimental results obtained in different days, and both the estimated reduction in QBER and increase in the Secret Key Rate (SKR)
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