Performance monitoring for quantum key distribution systems

• Main Author: Yu, Timothy Shengrong
• Other Authors: Lim Han Chuen
• Format: Theses and Dissertations
• Language: English
• Published: 2015
• Subjects: DRNTU::Engineering::Electrical and electronic engineering::Optics, optoelectronics, photonics
• Online Access: https://hdl.handle.net/10356/65360
• Institution: Nanyang Technological University

In 1946, C. E. Shannon proved that One-Time Pad is truly unbreakable. However, stringent conditions pose difficulties such as the key distribution problem which limited its practicability. Fortunately, public key cryptosystem widely used today was developed to solve the key distribution problem. Its security is based on the assumption that an adversary has limited computational power to factor a number with large prime factors. With the increase in computational power and technological advancement, this security may one day be compromised. In contrast, quantum key distribution (QKD) offers a platform for secure key distribution and everlasting secrecy. Unlike conventional cryptography, quantum key distribution’s security is governed by the laws of quantum mechanics. The basic principle of quantum key distribution is to encode classical binary bit information onto the properties of quantum states such as the polarisation of a photon. Because of the quantum no-cloning theorem, an eavesdropper is unable to simply duplicate these photons. Moreover, by intercepting these photons, the eavesdropper will leave detectable trace which will reveal its presence. In 1989, the first experimental demonstration of QKD using the polarisations of single photons based on the BB84 protocol occurred through 32cm of air. Since then, QKD over hundreds of kilometres of optical fibre and free space have been reported. Key generation rate of a few mega-hertz have also been demonstrated. However, photons travelling through optical fibres are subjected to random polarisation drifts. Moreover, clock drifts often causes inaccuracy during the detection of these photons. If these performance issues were not addressed, the reliability and availability of QKD systems and their cryptographic keys will be affected. Therefore, polarisation recovery schemes have been implemented to mitigated polarisation drifts. However, these schemes often limit the transmission distance, slow down or even disrupt the key generation process for polarisation recovery. These limitations lead to the proposed development of a polarisation-encoded QKD system based on an adaptive polarisation state monitoring and recovery scheme that adapts the system to the existing polarisation drift condition in the transmission link to enhance its reliability and availability. On the other hand, current high-speed single-photon detection schemes are often designed to work with idealised parameters such as fixed gating rate and operating temperature. Therefore, such schemes are unable to accommodate changes in gating frequency induced by clock drifts which results in the reduction of detection efficiency. Hence, a proposed robust high-speed single-photon avalanche diode with tunable sinusoidal gate frequency was developed to mitigate the effect caused by clock drift in order to maintain the detection efficiency over varying operating conditions.