| Photons entangled in orbital angular momentum (OAM) propagating in atmospherechannel would be affected by turbulence generated by the variation of temperature, pressureand density of atmosphere. The performance of quantum communication system based onentangled OAM would be weakened due to decoherence. In this paper, the effects ofatmospheric turbulence on entangled OAM states generated by spontaneous parametric downconvertion are analyzed. Based on Rytov approximation and non-Kolmogorov spectrummodel of index-of-refraction fluctuation, we model the probabilities of detecting the signaland idler photons entangled in OAM as well as signal photons after propagating throughturbulent atmosphere. Using numerical simulation, we discuss the variations of detectionprobabilities with different values of OAM quantum number, propagation distance, the groundturbulence strength and channel zenith angle. The effect of non-Kolmogorov parameter isalso studied. Then, we analyze the impact of Gaussian-Schell beam’s transverse coherentwidth on the detection probability. Besides, based on Zernike-model expansion of turbulencephase aberrations we analyze the effects of low-order Zernike turbulence aberrations onentangled OAM states, that is turbulence Z-tilt aberration, defocus aberration, astigmatismaberration, coma aberration. Under the modulation of low-order aberration, the variations ofsignal photon detection probabilities with different values of OAM quantum number,propagation distance, ground turbulence strength, channel zenith angle and the diameter ofdetector are discussed. The main results are as follows:1. The photons entangled in OAM are less robust to non-Kolmogorov turbulence thansignal photons. The coincidence probability of entangled photons is lower than signal photondetection probability with same parameters. The detection probability decreases as OAMquantum number increases. Detection probability attenuates with the increase of propagationdistance. This attenuation is rapid when propagation distance is less than500m and is slowwhen propagation distance is more than500m. The larger of structure parameter on theground or channel zenith angle, the lower of detection probability. Detection probabilitydecreases as the non-Kolmogorov parameter increases from3to4.2. Using Gaussian-Schell beam as the pump beam of parametric down convertion, thebeam’s transverse coherent width plays a more important role in short distance.3. Among the four low-order turbulence aberrations, turbulence Z-tilt aberration is thedominant aberration, coma is the second and astigmatism is the third, but the defocusaberration has no impact on the detection probability. |
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