Active phase stabilization for the interferometer with 128 actively selectable paths

A variable-delay optical interferometer with 128 actively selectable delays and a system of active phase stabilization with closed-loop feedback are innovatively designed and applied for the first time in the experiment of round-robin differential phase shift quantum key distribution (RRDPS-QKD). The active phase stabilization technique employs a phase modulator (PM) driven by a digital–analog converter (DAC) to adjust the relative phase between the two arms of the interferometer. By monitoring photon counting rates of two up-conversion detectors (UCDs) at two output ports of the interferometer, a field-programmable gate array (FPGA) calculates and finds the optimal code value for the DAC, maintaining high visibility of the interferometer every time a new light path is selected. A high proportion up to 100% of the visibility of interferometer selections can simultaneously be maintained over 96% during the QKD, which lays the foundation for the RRDPS experiment and yields possibilities for reprogrammable optical circuits.