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

Alejandro Ocampos-Guillen,Natalia Denisenko,Jorge Gomez-Garcia,Veronica Fernandez

doi:10.1109/access.2019.2933694

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 n 2 - 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.

Highlights

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]
We present a double-loop tilt-correcting system and its characterization through the decrease in the receiver’s beam focal area and its effect on the quantum bit error rate (QBER) and secret key rate (SKR) of a QKD system
Beam tracking techniques generally use an automated control that implements a proportional-integrative- derivative (PID) feedback loop between the beam position measured in a position sensitive detector (PSD) and an actuator, capable of deflecting the beam in order to correct for the deviations and provide a position where the error is minimized

Summary

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]. Some will include only wireless links, such as in cases of communication among small-sized satellites, high altitude platforms and UAVs, whereas others will include fiber channels, such as in cases where communication must be sent to a ground station or relayed to a fiber-optic network. In the latter case the interconnection among both types of transmission channels, air and fiber, must be optimized to avoid signal losses. Considering that potential applications will likely involve networks with many users a solution that it is both simple and cheap is desired

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Conclusion