A Research Of Multi-channel Quantum Coincidence Counter With High Precision

Quantum coincidence counter is a statistical system that measures the time correlation between multiple entangled photons. It is widely used in the fabrication of quantum entanglement source, photon interference experiments, quantum teleportation, quantum key distribution and so on. With the rapid development of quantum communication and quantum computation, how to improve the precision of the quantum coincidence counting device has become one of the most important issues in coincidence measurement field. This paper is aimed to design an eight channel quantum coincidence counting system with high precision. In order to improve the precision of quantum coincidence counter, time interpolation method is presented in this paper. The work of quantum coincidence counter design mainly includes hardware design and logic design, and the logic design is largely focused.First of all, in terms of hardware design, multiple entangled photons are converted into electrical pulses by single photon detector. And then timing discriminator detects the rising edge of each electrical pulse. Through comparing the merits and defects of different timing discriminator, we ultimately chose the constant fraction timing discriminator based on low-pass filter circuit. Then with the preprocessing of pulse width modulation circuit and time delay compensation circuit, the pulses are eventually sent to FPGA for coincidence measurement. And the coincidence raw data will be sent to computer through Ethernet. Differential routing method and emitter-coupled logic level interface are used to improve the hardware performance of quantum coincidence counting system.Furthermore, in terms of logic design, the coincidence method based on time measurement with coarse data and fine data is adopted in this design. At first, 128 levels fast carry chains are built in FPGA for timing interpolation. And then the signals are latched by the fast clock of 200 MHz. Latched codes records the fine time data, and the same time, clock counts records the coarse time data. In order to improve the precision of fine time data, we present the error correction method based on adjacent bits. Then with the method of division and sum, the codes are decoded into fine data. Due to the randomness of photon arrival in the coincidence measurement occasion, we suppose the first arrival signal as the starting signal, then the starting signal switching method and the timeout mechanism are presented to get the reasonable starting point of measurement. Finally the measured time and coincidence window are compared to get the coincidence result.With repeated statistical measurement, an eight channel quantum coincidence counting system with a resolution of 200 ps and up to 100 MHz input frequency is realized in this design. At the final of this paper, Bell inequality is verified with the help of quantum coincidence counting system and double photon entanglement source.