Quantum key distribution (QKD) offers the promise of secure communication between two parties, Alice and Bob, in the presence of an eavesdropper, Eve. Differential-phase encoded QKD protocols, a type of distributed phase protocol, are robust against slowly varying environmental fluctuations. This work analyzes the security of two differential-phase encoded (DPS) QKD protocols. We quantify the effects of the number of optical delays or time-bin superpositions on the secure key rate of DPS-QKD. However, DPS-QKD is prone to detector (measurement device) attacks. Hence, we present a differential-phase encoded measurement-device-independent quantum key distribution (DPS MDI-QKD) and establish its security in the asymptotic and finite-key regimes.

Though secure against detector side-channel attacks, the MDI protocols are still affected by experimental imperfections. Hence, we study the effects of pulse-width mismatch on the visibility of Hong-Ou-Mandel (HOM) interference. As many MDI-QKD protocols rely on HOM for secure key generation, our study thus helps in quantifying the effects of channel asymmetry, one of the primary sources of pulse-width mismatch, on the security of MDI-QKD. Furthermore, we propose a plug-and-play architecture of DPS MDI-QKD to reduce the effects of pulse-width mismatch and channel fluctuations on the key rate.

Finally, we study the effects of amplitude-damping noise on the security of QKD protocols. We propose an encoded BB84 protocol and show that the dual-rail BB84 implementation outperforms the conventional BB84 in the presence of damping noise, both theoretically and through implementations on IBM quantum computers.