Practical Continuous Variable Quantum Key Distribution System with ‘Local’ Local Oscillator
Quantum key distribution (QKD) has been proven to promise unconditionally secure data communication between remote legitimate users，based on the fundamental laws of quantum mechanics. QKD has great potential to be commercialized in the market and implemented in securing critical institutions, such as financial infrastructures, smart Grid, and national defence. Most theoretical security analyses of QKD protocols are based on several idealistic assumptions. However, the practical limitations, such as device imperfections, may bring security loopholes and leave QKD systems vulnerable to malicious eavesdropping attacks. Since QKD systems will be used as the practical applications in the real-world, it is of great importance to study the practical security and practical implementations of QKD protocols. Compared with dedicated single-photon based discrete variable (DV) QKD protocols, continuous variable (CV) QKD systems have attracted more attentions in recent years, as they benefit from their compatibility with commercial off-the-shelf (COTS) telecommunication components and high key rate over metropolitan networks. However, the requirement of a co-transmitted local oscillator (LO) brings a number of issues that compromises secure key generation and becomes a major obstacle to the practical implementation of CVQKD. To overcome these problems, a new CVQKD protocol with a locally generated LO (LLO) at Bob has been proposed. Based on previous studies, this thesis presents a novel quantum attack based on a discovered security loophole of the LLO CVQKD protocol, and demonstrates two high performance and low complexity practical LLO CVQKD systems.
- After exploring the phase recovery scheme and the phase noise model, the first quantum attack on LLO CVQKD system is proposed, that exploits the phase estimation error associated with the amplitude of the phase reference pulses, in a ‘reference pulse attack’. Under this attack, a new system noise model and a refined linear quantum states transmission model have been developed when the phase noise is partially trusted. The attack performance is evaluated in terms of excess noise manipulation and mutual information extraction over both trusted – referred to as realistic, and untrusted – referred to as paranoid, security models of phase noise. Effective countermeasures are also given to improve the practical security and performance of the LLO CVQKD system under the trusted phase noise model.
- Aside from the practical security analysis, two practical LLO CVQKD implementations developed in the laboratory are demonstrated here. The first practical LLO CVQKD system is featured as high speed (500 MHz) and low complexity, realizing the world’s fastest predicted key rate of 26.9 Mbps over a 15 km optical fibre link. To increase the signal repetition rate, and hence the potential secure key rate, our system is equipped with high-performance, wideband devices and components designed to support high repetition rate operations. To reduce the system complexity and correct for any phase shift during transmission, reference pulses are interleaved with quantum signals at Alice. Customized monitoring software has been developed allowing all parameters to be controlled in real-time without any modification to the physical setup. The system-level noise model analysis is introduced at high bandwidth and a new ‘combined optimisation’ technique is proposed to simultaneously optimize system parameters to high precision. It also has the potential for an even faster implementation.
- The second demonstrated system presents the feasibility of the first practical LLO CVQKD system. The system comprises integrated real-time data-processing units (DPUs) and commercial off-the-shelf (COTS) telecommunication components. Instead of using conventional expensive, high-specification, standalone laboratory equipment (e.g. oscilloscope and arbitrary waveform generator (AWG) used in most CVQKD systems), each DPU consists of a high-speed, multi-channel data acquisition module, and is embedded in a computer at both Alice and Bob to achieve real-time quantum data generation, acquisition and analysis. In addition, a hardware-based filter unit has been implemented to amplify and stabilise modulation signals, together with a system-level noise model analysis and a combined parameter optimisation method to enhance the system key rate. In laboratory conditions, the system has been shown to run continuously, and the asymptotic secure key rates are estimated as 1.37 Mbps and 509 kbps over 15 km and 25 km optical fibre links, respectively, through stable excess noise measurements for over 10 hours at a 20 MHz repetition rate. The measurement results open up the possibility of building up a commercialized LLO CVQKD system.