Instrumentation limitation on a polarization-based entangled photon source

Free-space optical communication is hindered by turbulence resulting in spatial modal dispersion of the optical beam. Here we mimic in the laboratory the far-field turbulence effects on entangled photons in the polarization basis. We make use of a diffractive optical element to simulate turbulence distortions and measure the entanglement as a function of the turbulence strength. We show that the standard method of coincidence detection using single-mode-fiber coupling to single-photon counters results in spatial mode dependence of entanglement even though it is not measured with spatial modes. We find that the overall coupling inefficiency of the detected coincidence caused by spatial mode dispersion in free space can be corrected by the use of a multimode fiber. Our results suggest that care is required in the choice of a detection system for free-space quantum communication systems. (C) 2017 Optical Society of America