Double-Loop Wavefront Tilt Correction for Free-Space Quantum Key Distribution

Abstract

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.

Inflight demonstrator of quantum key distribution between CubeSats of Q-ANSER program

“…The QKD system for Q-ANSER will be based on the high speed free-space prepare-and-measure system developed by ITEFI-CSIC 24 and the recent improvements implement on it. 25,26 Most satellite solutions so far implement entanglement-based QKD which do not need to trust the satellite; however this sacrifices considerably the key rate, since developing high-speed quantum entangled photon sources remains a challenge. Prepare-and-measure protocols, in contrast, allow the use of high clock rates in the transmitter which can operate at several GHz clock rates.…”