ABSTRACT
It is a common belief that both the sender and receiver’s laboratory will be secured which will limit the feasibility of Quantum key distribution (QKD). Due to unavailability of a perfect single photon device, Eve can intercept the data during key generation process as well as distribution at sender’s laboratory. This imperfect photon source usually generates some multi-photons (decoy state) which create an open door for eavesdropping. These decoy states, which are more realistic than single photon states, are vulnerable using Photon Number Splitting Attack. This limitation in a long distance secure communication can be overcome by conceiving the idea of a perfect device independent protocol. This chapter deals with the various decoy state protocol methods and their limitations. The chronological developments of decoy state protocol for secure communication using various medium like fiber, free space, sea water, etc., has been discussed here.
