Quantum mechanics allows identical particles to lose their individuality completely and become truly indistinguishable. This property of indistinguishability gives rise to exclusive quantum phenomena such as two particle interference. Photon indistinguishability is crucial in realizing many quantum information protocols. This manuscript covers the concepts and applications related to indistinguishable photons.

Free-space quantum communication assumes importance as it is a precursor for satellite-based quantum communication needed for secure key distribution over longer distances. Prepare and measure quantum key distribution (QKD) protocols like BB84 consider the satellite as a trusted device, which is fraught with security threat looking at the current trend for satellite-based optical communication. Therefore, entanglement-based protocols must be preferred, so that one can consider the satellite as an untrusted device too. The current work reports the effect of atmospheric aerosols on the key rate obtained with BBM92 QKD protocol, an entanglement-based QKD protocol over 200 m distance, using an indigenous facility developed at Physical Research Laboratory (PRL), Ahmedabad, India. Our results show that concentration and extinction coefficient of atmospheric aerosols play a major role in the observed sift key rate, and eventually, the secure key rate. Such experiments are important to validate the models to account for the atmospheric effects on the key rates achieved through satellite-based QKD.

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