Research Of The Control System Of Continuous Variable Quantum Key Distribution

With the development of society and technology, information security is widespread concerned. In the future national defense and finance, the importance of information security will be particularly highlighted. The conventional encryption is mainly based on computational complexity. Such as RSA algorithm based on the difficulty of prime decomposition of large numbers, but the unconditional safety has not been proven, moreover it is easily eavesdropped without notice in the classic channel. Quantum Key Distribution is a new discipline developed in recent years. It can provide unconditional security key and eavesdropping is possible to be detected. QKD has huge application prospects in military and commercial and it quickly raised people’s attentions. The government invest a lot of resources to explore and research it. There are a variety of transport protocols on quantum key distribution currently. Its safety has been improved constantly. Quantum key distribution will become more and more mature in the future.CVQKD has a simple light source, high detector efficiency and is well compatible with existing fiber-optic communication systems, so CVQKD has been widely studied. This paper introduces the basic knowledge of continuous variable quantum key distribution and conduct experimental and theoretical research on key issues affecting the long-term stable operation of CVQKD. The main contents include:1. The amplitude modulator used in CVQKD is a lithium niobate Mach-Zehnder Modulator. This modulator is widely used in optical fiber communication. However, its bias operating point vary with the time, temperature, vibrations and other factors. We propose a bias control technique for lithium niobate Mach Zehnder modulator. This technique has a high precision and it’s ditherless. It can be well applied to continuous variable quantum key distribution.2. Due to the temperature, thermal stress, mechanical stress, vibration, polarization state in long-distance single-mode optical fiber has changed. We precisely control polarization state of the light field using dynamic polarization controller based on simulated annealing algorithm. The results show that the polarization extinction ratio can be more than 25dB.3. The absolute 50/50 fiber coupler does not exist and splitting ratio changes as the temperature changes accordingly. The quantum efficiency of the diode is not identical. These factors will result in all-optical time-domain pulsed balanced homodyne detector arms can not maintain the delicate balance and long-term stability. We use a fiber attenuation based on fiber bending to make detector arms perfectly balanced and control temperature of 50/50 fiber coupler to eliminate effects of temperature on homodyne detector. These measures improve the long-term stability of the detector.