Feedback Circuit Module in Frequency-Nondegenerate Entanglement Swapping Experiment

In this paper, a dedicated feedback circuit module is designed for an experiment of entanglement swapping with frequency-nondegenerate photons. This experiment, aiming to entangle two frequency-nondegerate photons without prior interaction, is an advanced strategy for quantum communication. The key point of the experiment is to actively compensate a random phase shift originating from the frequency difference. The feedback circuit module has functions of measuring the time intervals, converting the time information to control level codes, and outputting control levels to an electro-optic modulator (EOM). A precision TDC is implemented in a field programmable gate array (FPGA) with 40 ps per LSB ( σRMS is less than 30 ps). A programmable look-up table is used as computing unit. The propagation delay time of the whole circuit module is less than 110 ns.