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

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.