All Fiber Four-State Discrete Modulation Continuous Variable Quantum Key Distribution

Modern society has stepped into information age. The security of information is becoming more and more important. Cryptography has attracted more and more attention, which can protect the security of information. The application of cryptography has penetrated into the daily life of people in various fields. Quantum key distribution is based on the basic principles of quantum mechanics, which make both legitimate communicating parties to get a group of random key with unconditional security. The key can be used for encryption and decryption of information, thus achieving a secure communication between the two parties, any third party eavesdropping can be found by communicating parties. Unconditional security of quantum key distribution makes the theory and experiment enter a period of rapid development. In the future Quantum key distribution will have a broad application prospects in the fields such as defense, finance, networking and communications. In the continuous variable quantum key distribution field, quadratures of the optical field are used as a kind of information carrier, optical source is easy to prepare, and detector has a high efficiency. The whole system has a good compatibility with optical communication network. Thus the field has been of great concern in recent years. The theory and experiment obtain a rapid development. Coherent state continuous variable quantum key distribution is divided into Gaussian modulation and non-Gaussian modulation according the way of modulation. Four-state discrete modulation belongs to non-Gaussian modulation. The protocol has a simple modulation method, high reconcilication efficiency. A distance above hundred kilometers quantum key distribution can be achieved in theory with the protocol. The dissertation gives a detail analysis of the all fiber continuous variable quantum key distribution system in theory and experiment. The dissertation firstly gives a review of the development of continuous variable quantum key distribution. Then some fundamental knowledge in this field is introduced. An unconditional security analysis of four-state modulated continuous variable quantum key distribution based on homodyne detection is given in detail. The balanced homodyne detector is developed specially to use in this field. The various characteristics of the detector are analyzed in detail. At last an introduction to our system is given in detail. The continuous variable quantum key distribution based on it has been realized. The distance is30km, and the secret key rate is1kbits/s.The main content of the dissertation includes three parts.1. Two models of four-state discrete modulation coherent state continuous variable quantum key distribution are analyzed in theory. They are prepare-and-measure model and Entanglement-based model. The encoding rules of the protocol are introduced in prepare-and-measure model. After encoding each side can obtain a set of associated binary numbers. The evolution of the signal field and excess noise in phase space is also given in this model. In the Entanglement-based model the unconditional security of the protocol is analyzed. A variety of noise in the system is introduced firstly. The excess noise of Alice’s setup is equivalent to the noise produced by Fred. Then a method of calculating the mutual information between Alice and Bob is given. Bob measures the quadratures of signal field with balanced homodyne detector. Then amount of information that Eve can achieve is limited by the Holevo boundary. The process to calculate the Holevo boundary is analyzed in detail. At last a method of calculating the secret key rate and excess noise is also been given. Excess noise is the key factor that determines the distance of key distribution and the magnitude of secret key rate. In the proof of unconditional security, it is assumed that Eve has any advanced equipment, but her attacks are not contrary to the principles of quantum mechanics and she also knows nothing of the parameters of Bob’s experiment setup in realistic mode. When Eve knows the quantum state of Fred, the excess noise produced by experimental setup of Alice is equivalent to the excess noise due to the channel.2. The fiber-based time domain homodyne detector designed by us is applied to the domain of quantum communication. The repetition rate of pulses can be up to2MHz. The gain of the detector is3.2microvolts per photon. The common mode rejection ratio is76dB. The signal to noise ratio is above20dB. The total efficiency of the detector is66%. The characteristics of the detector are analyzed in detail. They include common mode rejection ratio, shot noise limited, and stability of the detector. To obtain a high common-mode rejection ratio, we not only select two photodiodes which have the same response characteristics as same as possible, but also make sure that the intensities of the photocurrent generated by the two photodiodes are same and the arrival time of the photocurrent is synchronized. The rising and falling edges of the photocurrent should be flat. Three methods are used to measure the characteristic of shot noise limited. The pulse resolved ability of the detector is tested by measuring the correlation coefficient of the output electrical pulse’s peaking value. After measurement of vacuum shot noise fluctuations at different local powers, we plot a linear response curve to verify the detector’s linear response characteristic. After acquiring the output pulses’ peaking values, we do a fast Fourier transform to them and obtain a flat noise power spectrum. The total quantum efficiency of the detector can be easily calculated with the linear response curve. In the optical path a variable attenuator based on pigtail of the coupler and stable connectors are used. In the electrical circuit, a reasonable wiring is taken to achieve a high signal-to-noise ratio, and both ends of each photodiode has a stable bias voltage. Thus we can ensure the intensities of photocurrents generated by two photodiodes have good stability and the arrival time of the photocurrents is also stable. When the detector has a higher signal-to-noise ratio, we can decrease the local power of device to weak the external disturbance factors such as temperature, vibration. The stability of the detector is measured with Allan variance, and the detector’s measurement window without calibration is100seconds. 3. The quantum key distribution based on the experimental setup developed by us is realized with a secret key the rate of1kbits/s. The distance is30km, and the repetition rate is500kHz. Firstly, a detailed introduction to the operating principle of the system is given. Alice does a modulation to the coherent state in accordance with the protocol. Modulated coherent state is sent to Bob through the quantum channel. Bob does a random measurement to the conjugated quadratures of coherent state with pulsed balanced homodyne detector. Pulsed coherent states containing information are transmitted in the form of frame which is composed of blocks. The block is the basic unit to calculate the relative phase which is used to lock the relative phase with the way of compensation. In order to make sure the locked phase has a good accuracy, the time of the block must be far smaller than the free-running cycle of the relative phase, and the number of test pulses must be sufficient. In order to ensure the frames are transmitted in a stable environment, the stabilities of the variable attenuator and tricyclic polarization controller are analyzed specially. Then the experimental parameters and results are analyzed, the methods to measure and calculate the excess noise are also given. After analyzing the phase-locked results under three different optical paths, it is found that the main reason to affect phase locking accuracy is the polarization maintaining fiber used for time division multiplexing. A method to calculate the excess noise due to the disturbance of relative phase is introduced in detail. The method to measure the Local field’s leakage to signal field is proposed. A measurement is done to measure the magnitude of the disturbance. Through decreasing the power of local field, the disturbance due to Rayleigh scattering from Local field will be reduced.