Long distance quantum communication

Fiber and free-space are the most used transmission mediums for Quantum Key Distribution, however, there are several challenges in establishing practical and secure networks. The challenges include device imperfections, detector noise, polarization-extinction-ratio, signal loss depending on the transmission medium. In the case of fiber-based QKD, the losses increase with distance, making it impossible to implement in large distances. Free-space QKD is one of the alternatives that offers extended coverage and flexibility but is still susceptible to losses caused by atmospheric turbulence, fog, and other environmental factors. Satellite-based QKD is considered another candidate for long-distance communication, however, it shares the same issues with free-space on top of the limitations of the operational timing window, non-agility and higher infrastructural costs. The solution to achieve practical quantum secure communication networks at large scales is to combine all transmission modes into an integrated network.