Quantum key distribution (QKD) offers information-theoretically secure communication between two parties, Alice and Bob, in the presence of an eavesdropper. When an adversary, Eve, attempts to steal information from the quantum channel, she also inevitably introduces disturbances in the channel and reveals herself. Since the first proposal by Bennett and Brassard in 1984, there have been a variety of QKD protocols both proposed and implemented. In
this seminar, we focus on two differential phase encoded QKD protocols - Differential phase shift (DPS) QKD and Differential phase shift measurement device independent (DPS-MDI) QKD. K. Inoue et. al proposed a DPS-QKD protocol, which is simple, easy to implement and robust against slowly varying environmental fluctuations. In DPS-QKD systems, the key bit information is encoded by phase differences between two adjacent pulses, in a train of WCPs. We compare the security of this pulse train DPS protocol with a variant, wherein Alice prepares a photon in a superposition of 3 paths or time-bins before carrying out the differential encoding. We also show that the time-bin superposition scheme is easier to implement and control, and can be
scaled without any additional hardware complexity. However, the DPS-QKD protocol is vulnerable against detector side-channel attacks. Hence, we propose a differential phase encoded MDI-QKD protocol using single photons in a linear superposition of three orthogonal time-bin states, for generating the key. We prove unconditional security by demonstrating an equivalent protocol involving shared entanglement between the two trusted parties. We show that the secure key rate for our protocol compares well to existing protocols in the asymptotic regime. Finally, we estimate the bit error rate and the phase error rate, in the finite key regime.