Code-based quantum key distribution system

Abstract

Quantum Key Distribution (QKD) is a cryptography method that makes two parts share the same keys over quantum channel. These keys are employed to one-time-pad encryption, which is proven to provide secure communication. Recently, QKD has drawn interest as a popular research topic because it guarantees unconditional security based on the laws of physics instead of mathematical complexity. Improvement for security of QKD means increase of key generation rate or(and) transmission distance. Many protocols are proposed to enhance the aforementioned two factors since BB84 protocol has been proposed. For example, a QKD system using subcarrier-multiplexing can improve a secure key rate by encoding photons with several wavelengths. There is also a new protocol extending communication distance by adding two quantum states to original BB84 protocol. In recently proposed multi-qubit protocol, it encodes multi-qubit information on a single photon and conducts sifting and post-processing by multi-qubit unit. As a result, it increases error tolerance bound, the maximum error rate QKD system can stand. There are few researches to implement a multi-qubit protocol. In this thesis, we propose a practical QKD system to realize a multi-qubit protocol. We divide a single photon pulse into a number of short pulses called as chips by phase-modulating in complex domain and each chip is modulated to 0, 90, 180 or 270 degree based on predetermined codes. We describe 2-chip coding can implement to BB84 protocol and find that increasing the number chips can make the system to be vulnerable to an eavesdropper if we use a single photon as an one-qubit state. To solve the problem, we encode multi-qubit information on a single photon depending on the number of chips. Additionally, we suggest a new detection model to complement implementation problems of our proposed protocol such as degraded detection efficiency of single photon detectors.