Correction of Wavefront Tilt Caused by Atmospheric Turbulence Using Quadrant Detectors for Enabling Fast Free-Space Quantum Communications in Daylight

Low photon-level signals used in most free-space quantum communication systems require a narrow field of view in the receiver to minimize the amount of background noise coupled into the single photon detectors. This can be achieved through beam tracking techniques, which compensate atmospheric effects, such as beam wander, in the receiver, reducing the long-term beam area. However, reducing the diameter of this area below a few microns, typically necessary to achieve a low level of solar background noise and successful daylight quantum transmission, require fine tracking precision and diffraction-limited optics. We demonstrate that this can be done with standard voice-coil fast steering mirrors and cheap commercially-available quadrant detectors. Two correcting strategies (open and closed loop) are experimentally tested and analyzed for their applicability in metropolitan (~km range) free-space quantum communications. The area containing the random fluctuations of the beam centroid caused by atmospheric turbulence at the focal plane of the receiver was reduced by a factor of 4 with an open-loop configuration, and up to a factor of nine with a closed loop configuration. This is equivalent to a reduction in the quantum bit error rate caused by background solar noise of up to one order of magnitude, which, combined with spectral filtering techniques, enable the possibility of fast daylight quantum key distribution.