  Quantum Key Distribution is a practically implementable information-theoretic
secure method for transmitting keys to remote partners performing quantum
communication. After examining various protocols from the simplest such as QC
and BB84 we move on to describe BBM92, DPSK, SARG04 and finally MDI from the
largest possible communication distance and highest secret key bitrate. We
discuss how any protocol can be optimized by reviewing the various steps and
underlying assumptions proper to every protocol with the corresponding
consequence in each case.
