Free-space Quantum Communication Experiments In Remote Light In Line With The Electronics System Development

Quantum information is an interdiscipline between quantum physics and information science. Because of the superior advantage and the bright foreground, quantum information has been widely emphasized and developed. Quantum communication is the most developed field, including through fiber and free space mediums. For effects of the light loss, dispersion and depolarization through fiber, the communication distance is limited. In free space, the loss, decoherence effects are small, the atmosphere equivalent thickness is only five kilometers. If the communication distance can been broken, satellite-based global quantum communication will be possible. So the free-space quantum communication becomes a new focus research field.The free-space quantum communication is based entanglement source. Usually we generate polarization entangled photon pairs by type II parametric down-conversion. One of difficulties of the experiment is to figure out the entangled photon pairs. The task of electronic system is to figure out the entangled photon pairs by time coincidence and the main performance index is the minimal width of time coincidence window. In the finished experiment of free-space distribution of entangled photon pairs over 13 kilometers, the synchronous light offline coincidence scheme is used, and the minimal width of time coincidence window is about 20ns. In order to complete the 20 kilometers free-space quantum teleportation experiment, designed an online remote long-distance photon coincidence electronic system, and the system's minimal width of time coincidence window is 2ns. It can substantially reduce accidental coincidence and improve measurement accuracy.In the work of this thesis, to establish the online remote long-distance photon coincidence system, we designed four general purpose NIM modules, including discriminator module, delay module, counter module, coincidence module. The discriminator module's performance: four-channel, each channel three NIM output, output pulse width is 3ns~200ns adjustable, time jitter is less than 35ps. The delay module's performance: four-channel, each channel maximum delay is about 1us, smallest step is about 25ps, NIM output pulse width is 3ns~200ns adjustable, time jitter is less than 50ps. The counter module's performance: eight-channel, maximum count rate is 170MHz, minimum input pulse width is about 2ns, input signal can be positive or negative. The coincidence module's performance: two-channel, each channel can finish four coincidence or anti-coincidence, NIM output and TTL output, response time is less than 2ns.To finish real-time unitary transformation in the 20 kilometers free-space quantum teleportation experiment, we need to control pockels cells. Pockels cell is a variable wave plate controlled by voltage, the control voltage is about 800V. To drive pockels cell needs to design a specialized high-voltage driver module. In the work of this thesis, the designed high-voltage driver module's largest repetition rate is up to 100KHz, meeting the local Bell state measurement result's count rate requirement in the experiment.In the 20 kilometers free-space quantum teleportation experiment, to establish classic channel and finish the figure out of the entangled photon pairs, we designed a coder and decoder system to transform the local Bell state measurement result's time flag information and channel code information to remote end. Coder and decoder use parity code and hamming code to reduce the error code brought by the free space transmission.In order to validate the online remote long-distance photon coincidence electronic system can be used in the 20 kilometers free-space quantum teleportation experiment, we completed the 800 meters free-space quantum teleportation experiment in Tsinghua university, and the whole online remote long-distance photon coincidence electronic system can work well.Most of key innovations are listed as follows:(1). The first time, an online remote long-distance photon coincidence electronic system in quantum communication experiment were established, and using the system we completed the 800 meters free-space quantum teleportation experiment in Tsinghua university.(2). The first time, Bell state measurement result's nanosecond time flag information and channel code information in quantum communication experiment were transformed in free space.(3). The first time, hamming code was introduced to quantum communication experiment for data transform in free space, which can correct one bit error code brought by atmospheric jitter.(4). The first time, the real-time unitary transformation was realized in free-space quantum communication experiment.