Continuous Variable Quantum Cryptography Communication Techniques

Quantum cryptographic as a burgeoning discipline, has changed the conventional encryption which is only dependent on mathematical and logical process, and give birth to quantum physical secure. The potential foreground and unlimited capacity of quantum cryptography have attracted a lot of funds and scientists of all of the world. As an important branch of quantum cryptography communications, the continuous variable quantum cryptography technology appeared late. However, the continuous variable quantum cryptography, which use the classical coherent light source, has a low cost, a large capacity and a high secure key rate. So it has been attracted a lot of attention since its birth. The dissertation is focused on solving some practical diffi-culties in the continuous variable quantum communication. Moreover, several novel and operational modulation and detection schemes are proposed.What's more, an en-tire continuous variable quantum cryptography communication system prototype is realized.In order to complete the subject, the following work are presented in this paper.1. An ultra-low electronic noise and pulsed balanced homodyne detector is de-signed and implemented. The balance of homodyne detection is calibrated rig-orously. We have overcome the challenges imposed by the mismatch of pho-todiode response functions, insert loss and time-delay. We demonstrate that the quantum efficiency of this homodyne detector is 52.6% and CMRR is 70dB. The homodyne detector has a 17.517 dB shot-noise-to-electronic-noise ratio in the time domain at a-25dBm local oscillator. The results show that the homemade homodyne detection is quantum noise limited. The performance of this homo-dyne detector has reached the same order of magnitude of the best homodyne detectors reported. 2. A novel polarization diversity homodyne detection scheme is proposed. We analyze this scheme with quantum operator algebra and the results show that this scheme improves the efficiency of the quantum signal receiver and eliminates the effect due to the random fluctuation of the polarization in fibre channel.3. A continuous variables quantum key distribution system is implemented over 8km optical fibre based on the go& return configuration. A secret key rate of 0.81kb/s is obtained at 100KHz. In the experiment, a polarization-independent dual phase modulation scheme is proposed to substitute the AM&PM. The ex-perimental results show that dynamic extinction ratio of intensity modulation is 9.5dB, the modulation-noise-to-shot-noise ratio is-20dB. Simultaneously, the characterization of the co-existence of quantum signal and classical signal on dense wavelength division multiplexing network is discussed in detail, too.4. A one-way continuous variable quantum key distribution scheme is realized based on the double asymmetric Mach-Zehnder interferometers. This system achieves nearly lOkbps sifted key rate at 500KHz system clock. The security communication distance is 27.2Km, which is the longest distance reported by far. The influence of the loss, noise and environment factor on the sifted key rate of system is quantitatively discussed. The optimal modulation variance and the actual modulation accuracy are calculated in the dissertation. The channel real-time monitoring shows the system is secure practically.5. The practical security of quantum data stream cipher is discussed in detail. The channel model of quantum data stream cipher is proposed. In this model, the quantum channel connecting the two legal users is assumed to be lossy and the gaussian noise is imposed on the observed quadratures of the signal light field. The secure boundaries against beam splitter attacks are obtained based on the channel model, when Eve uses the homodyne detection and heterodyne detec-tion, respectively. The results show that the security of system depends on prac-tically physical parameters, then, the corresponding numerical curve is plotted. We proposes a method to use the weak coherent states to replace the classical light source in the secure data cipher system, This scheme is demonstration ex- perimentally.6. We have designed and realized an continuous variable quantum cryptography integrated platform. The system use continuous variable quantum techniques to implement key distribution, data cipher and identity authentication. The platform is able to support the cipher picture, cipher text and real-time cipher voice.