Quantum cryptography, or more specifically, quantum key distribution (QKD), guarantees information theoretical security-the Holy Grail of communication se- curity based on the principles of quantum physics. Since the primitive BB84 QKD protocol, great strides have been made both in theory and experiments to develop quantum technologies for real-life applications. Even though QKD has reached this milestone, most security analyses are based on the assumption that the QKD system’s preparation and measurement devices are idealized. However, such as- sumptions can be fatal to the security of a QKD system. As a result, we relax some of these assumptions and prove the security of several QKD protocols by using techniques that closely emulate the imprecisions in preparation and measurement devices and the environment in which the QKD is operated. For instance, we apply the post-selection technique to demonstrate the security of the six-state SARG04 protocol against general attacks with finite resources. Through this approach, one can derive a secure key from a multi-photon source while also taking into account an optimal coherent attack. Moreover, we employ the reference frame independence (RFI) concept, which eliminates the need for active alignment of reference frames to derive the security bounds for several proposed QKD protocols. Our security analyses also incorporate the loss-tolerant method, which allows us to prove the security of QKD by considering imperfections in the state preparation. Further- more, we propose the variants of the measurement-device-independent (MDI) QKD protocol, which utilizes a measurement device controlled by a third party, who may be an eavesdropper, thus removing the possibility of detector side-channel attacks. We also exploit quantum resources, such as biphotons, that enable us to achieve high dimensional encoding in our proposed protocols. Also, we utilize vector vortex and scalar beams to study the security of proposed protocols through a turbulent xvi atmospheric link under diverse weather conditions such as rain or haze. The sim- ulation results demonstrate that the proposed protocols can achieve a significant secret key rate at reasonable transmission distances comparable to the key rates obtained under an ideal environment with perfect preparation and measurement devices. Therefore, proving the security of QKD while allowing intrinsic errors due to signal preparation and measurement devices makes a giant step toward realizing practical QKD implementations.

Thesis (MSc in Physics ---Botswana International University of Science and Technology, 2024