Multi-channel Coincidence Counter Based On FPGA

Coincidence counting plays an important role in Quantum Optics. It could be used in the violation Bell's inequality and measuring single photons interference, or indivisibility of single photons. The electronic equipment used to count coincidences is called coincidence counter. Since single photon avalanche diode (SPAD) has been greatly improved, now coincidence counting more depends on the performance of coincidence counters. Traditional multi-channel coincidence counter is based on time-amplitude converter and ASICs. They are expensive and not designed for quantum optics experiments. Simple AND gates is hard to measure coincidences more than three channels, because more discrete chips bring more delay time out of control. It may affect final coincidence result. To solve those problems, we use Field Programmable Gate Array (FPGA) to build multi-channel coincidence counter for quantum optics experiments. FPGA has the advantage of low cost and small sized. It could be used to realize smart coincidence counter with high performance.In this paper, the fundamental concept of coincidence counting is present at first, including coincidence and coincidence resolving time. Then, the principle of multi-channel coincidence counting module will be introduced. Final part of this paper will take an 11 channel coincidence counter for instance, which is designed by Alan Stummer in Toronto University. The calibration of coincidence time window will be introduced. Further test will be taken by using two single photon counting modules under different background light. This paper includes two parts followed:1) Element is used to generate a pulse which is delayed by a certain time compared to the input pulse. Delay time range could be from ns to us. The delayed pulse is taken as the switch signal of coincidence process in FPGA. Coincidence results will be stored in RAM of FPGA.2) Take a real multi-channel coincidence counter based on FPGA for instance to explain how a multi-channel coincidence counter works. Especially, the calibration of coincidence time window or coincidence resolving time will be displayed by testing the coincidence with different coincidence time window. Coincidence resolving time is important character for coincidence counter, it decides the maximum time window, in which two pulsed could be coincidence. Since different signal pulse has different coincidence time window, the coincidence window should be calibrated before experiments. Finally, to test the performance of multi-channel coincidence counter, two single photon counting modules are used. Their coincidence under different background light is detected by multi-channel coincidence counter.Multi-channel coincidence counter based on FPGA is designed for quantum optics experiments. It is small-sized and low cost with more channels than conventional coincidence counters, which is used for time correlation experiments. Its compact architecture allows user to modify it any time they want to meet different experiments demand.The characterized work is described as followed:Using FPGA instead of discrete chips or TAC for multi-channel coincidence counter. This brings flexibility and low cost to coincidence counter, which is helpful for quantum optics experiments.