The Study Of Continuous-Variable Coherent Optical Encoding Scheme Based On Electro-optic Modulation Technology

With the rapid development of various technology, the security of classicalcryptography based on complex mathematical algorithm, is confronted with a greatchallenge. However, quantum cryptography, which is absolutely safe and is able to bedetected eavesdropper, overcomes these problems well and has its own advantages. Inthe system of quantum cryptography, the continuous-variable coherent opticalquantum cryptography can surmount the shortcoming of single photon quantumcryptography, and is more practical. Therefore, we devote ourselves to study thetheory and experiment of the continuous-variable quantum cryptography.In this paper, the method of homodyne coherent detection and the principle ofelectro-optical phase&litude modulation (PM&AM) were analyzed at first, andthe whole optical system of studying continuous-variable coherent optical encodingwas designed following. Then, the relevant control system was designed accordingto the encoding scheme and the optical system. Finally, system platform was set up,then the sending of Gaussian random encoding information, the synchronizationcontrolling of each part, data acquisition, data storage and data processing wereachieved.According to the design idea of hardware and software collaborative design andIP core reuse, gaussian random encoding was achieved by the embedded systemwhich is based on Xilinx FPGA. The design, simulation and verification of gaussianrandom logic module were completed by using the tools, such as ISE, et al. Thismodule was packaged as IP core by EDK. The whole encoding system wasaccomplished, which was used to connect the IP core to the microprocessorMicroBlaze soft-core. Meanwhile, the DAC circuit used AVR master control wasdesigned to realize the seamlessly connection between gaussian random encoding andthe electro-optic modulation. Thus, the process of sending quantum information basedon AM and PM is mastered. It is one of key technology in continuous-variable quantum key distribution (CV-QKD) experiment.Synchronous control and phase lock are the premise of correct data acquisition.The information synchronized transmission and the synchronous data acquisition wererealized by optical synchronization scheme, the phase was locked by adapting PIDalgorithm to control PZT.The entire measurement and control system based on the effective combinationof homodyne coherent detection system and virtual instrument (VI) system wasrealized by using LabVIEW graphical programming language. The acquisition andprocessing scheme, which was suitable for our system platform, was designed bycollecting data with DAQmx technology and carrying on data processing,quantization decoding and histogram analysis with signal processing module, et al.On this system platform, the information sending and data acquisition&processing were realized by AM at first. Then they were accomplished by PM.Finally it attempted to finish the AM and PM union modulation.By means of testing the system platform and analyzing the data, the result showsthat the system platform and the encoding scheme are feasible and achieve the designgoals. CV-QKD protocol can be simulated and demonstrated, and continuous-variablequantum cryptographic key distribution is able to be further studied, provided theprecision is improved and the interference reduced.