Real-time measurement and adjustment of random phase in frequency-nondegenerate entanglement swapping experiment

In this paper, a circuit module developed for the experiment of the frequency-nondegenerate photons entanglement swapping is reported. The module transfers the random time interval into level from +1 to - 1 voltage to drive an electro-optic modulator (EOM). This experiment, aiming to entangle the frequency-nondegerate photons that never interacted, is an advanced strategy for quantum communication. The key point of the experiment is to realize the time interval real-time measurement, computation and feedback control. A precision TDC is built in a field programmable gate array (FPGA) with 40ps per LSB (a RMS resolution about 20ps). A programmable look-up table is used as computing unit and the delay time of the module is less than 110ns.