| Quantum Key Distribution (QKD) is an important research direction in quantum information, whose security is guaranteed by the basic principles of quantum mechanics. QKD protocols are able to provide unconditional secure keys and have broad application prospects in varieties of fields such as military and economic. Over the last few years, several experiments via optical fiber and free-space channels have achieved great successes. These developments push forward the ultimate goal of the future global quantum network.Due to the technical challenge of realizing single photon sources, weak coherent pulses (WCPs) are widely used in QKD systems. In practice, the full width at half maximum (FWHM) and the frequency of the WCPs become the key factors to the quantum bit error rate (QBER) and the system performance. In this thesis, we make use of the gain switching technique to design a semiconductor laser driver with FWHM of 80ps at a repetition frequency of 120MHz. We use four semiconductor lasers and eight driver circuits to encode four BB84 states "H", "V", "+", and "-". The repetition rate of four photon source is 100 MHz, which is distributed to signal state, decoy state and vacuum state with a proportion of 2:1:1, respectively. This photon source has been successfully applied in many experiments, such as a 40km free-space channel demonstration, a floating hot air balloon at 20km, and over a 96km link with about 50dB loss. The experiments provide direct and full scale verification towards ground-satellite QKD.The polarization encoding scheme is mostly popular in free-space QKD experiments. A pockels cell is a common electro-optic module, often used to rotate photon polarizations in high speed. In this thesis, we make use of power MOSFET high speed modulation technique to design an electro-optic modulator with high repetition rate, high output voltage pulse, rapid rise and fall time.We design four different electro-optic modulators to satisfy the light seconds experiment, satellite-ground entanglement distribution experiment, DPS-QKD experiment and under water QKD experiment requirements.(1) The purpose of the light seconds experiment is to simulate the Bell inequality between the Earth and the Moon with freewill involved. The pockels cell modulator is used to control the polarization of the optical pulse. The duration of the high output voltage is up to several seconds.(2) DPS-QKD scheme uses differential phases of sequential pulses as an information carrier. A pockels cell modulator is used to control the time delay of the sequential pulses. The half wave voltage is proportional to the wavelength of the optical source and it is 2kV according to the 1550nm optical source. The repetition frequency is up to 45 kHz.(3) The modulator is also used to realize active basis choice entanglement distribution experiment. The repetition frequency is up to 100 kHz.(4) The goal of under-water decoy state QKD is to realize the communication with the submerged submarines through sea water channels. The electro-optic modulator is used to generate decoy state light source. The repetition rate is 1 MHz and the pulse voltage is up to 800V. In the experiment of 0.5m water sink decoy QKD, we obtained final secure key length of 12.87 Kbit and quantum bit error rate of 4.26%, with QKD system loss is 28dB.The key innovations of this thesis are listed as follows:1. The gain switching technique is successful applied in the QKD experiment, thus greatly improves the integration and reliability of the system. The light pulse HWHM is 80ps at a repetition rate of 120 MHz, which is obtained under gain switching scheme with direct current bias far below the threshold. We also optimize the design of decoy state light source, so greatly reduce the numbers of semiconductor lasers.2. The power MOSFET high speed modulation technique is applied in QKD experiment to design high speed electro-optic modulator, which can generate high voltage square pulse with high repetition rate, rapid rise and fall times. The modulators have been used in demonstrations of light seconds experiment, DSP-QKD, which has the property of high reliability, flexible design, low cost, relatively simple design.3. High-speed electro optic modulator is used to modulate the 532 nm solid state laser and generate a decoy state source with repetition rate of 1MHz and extinction ratio is 20dB. The decoy state QKD system has been designed, and implemented in a 0.5m water sink decoy QKD experiment. The outfield experimentwas tried, for the follow-up work smoothly laid a certain foundation. |
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