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.
Wavefront tilt correction is necessary for reducing beam deviations caused by atmospheric turbulence in the receiver of an optical system. This reduction allows decreasing the receiver's field of view, which in the case of free-space quantum key distribution (QKD), reduces the solar background noise that reaches the detectors, and with this, the quantum bit error rate (QBER). A wavefront tilt correcting system that stabilizes the beam in two different points of the receiver's optical axis has been developed and characterised, and it is capable of operating under realistic conditions of strong turbulence with almost perfect ideal correction. The reduced area of beam deviations at the focal plane of the receiver after correction enables a reduction of the QBER of more than 80%, which paves the way to free-space quantum key distribution in daylight under strong turbulence.
Both free-space and optical fiber quantum key distribution (QKD) links will coexist in future quantum networks, requiring efficient coupling between both types of channels. However, wavefront distortions introduced by the atmospheric channel can severely affect this efficiency. Active mechanisms such as precise beam tracking or steering can correct for some of these wavefront distortions, considerably improving the signal-to-noise ratio of the received quantum signal in many scenarios. A tracking system that uses two, instead oftypically one, controlling loops for tilt correction, stabilizes the beam in the whole optical axis of the receiver relaxing the restrictions of the receiver's optical design, and reduces the area of beam fluctuations in the receiver's focal plane a 24% more than a single-loop configuration. The tracking system was characterized in a QKD system at a 300 meter-link in moderate to strong turbulent conditions (C 2 n − 10 −14 − 10 −13 m −2/3) and an improved coupling efficiency of a factor of 2.1 and 1.6 was obtained for a 9.5 µm-core diameter standard telecommunications Single Mode Fiber (SMF) and a 25 µm-core diameter multimode fiber (MMF), respectively. This reduces the quantum bit error rate (QBER) caused by solar background photons in a 52 % for the former and 39% for the latter, enabling an increase in the secret key rate ofmore than one order ofmagnitude for SMF and a factor of five for MMF. These results are promising for enabling QKD free-space links and their interconnection to fiber optic infrastructure in realistic scenarios of communication networks of high turbulence regimes and daylight conditions. INDEX TERMS Quantum key distribution, beam steering, beam tracking, free-space quantum communication, quantum cryptography, wavefront tilt correction. I. INTRODUCTION Recent research in free-space Quantum Key Distribution (QKD) has focused on increasing the feasibility of practical systems in real scenarios, which implies fulfilling several challenges like increasing the key rate, the propagating distance and making practical systems compact, robust, and low cost [1]-[7]. Many advances on stationary links have been reported in the last few years [8]-[11], whereas other potential scenarios for mobile systems, such as aircrafts [12], hotair balloons [13] and Unmanned Aerial Vehicles (UAVs) [14] have also been explored. In addition, after several tests and proofs to analyze their possibilities, links involving satellites have also been recently demonstrated [15]-[18]. The associate editor coordinating the review of this article and approving it for publication was Wei Huang.
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