Abstract
Free Space Optical Quantum Communication (FSOQC) is a new wireless approach that allows users to communicate in a secure and faster manner. The working mode of quantum communication is based on quantum physics and is somewhat probabilistic in nature. In order to carry quantum signal in a scattered environment, it is needed to derive the probability distribution of transmittance for a specified region at a desired communication link distance. To understand the performance of FSOQC under adverse weather condition, the effects of various atmospheric parameters such as fog, rain, and turbulence need to be analyzed. In the present study, an attempt is made to predict transmittance for a tropical location, Kolkata, India in real-time conditions, and Keyrate is estimated based on the predicted transmittance. The diurnal, monthly and yearly variation of transmittance is analyzed which is required for effective implementation of a quantum communication system. Two alternative situations, with and without sunlight are considered for Keyrate computation. A combination of two techniques, Difference Frequency Generation and Spontaneous Parametric Down Conversion has been applied to increase Keyrate under adverse weather condition. The operating wavelength is 800 nm, and combined technique is developed around that wavelength.