  A variable-delay optical interferometer with 128 actively selectable delays
and a technique of active phase stabilization are innovatively designed and
applied for the first time in the experiment of round-robin differential phase
shift quantum key distribution (RRDPS-QKD). According to the RRDPS protocol,
larger number of delay channels in interferometer can ensure higher tolerance
of bit errors, eliminating the fundamental threshold of bit error rate of 11%
of traditional BB84 protocol. Considering that the implementation of RRDPS
protocol relys on the realizaiton of the interferometer, therefore, an
interferometer with 128 selectable delay paths is constructed and demands the
ability of fast switching at the rate of 10 kHz, which requires dynamic
stability of multiple paths. Thus, a specific designed phase stabilization
technique with closed real time feedback loop is introduced to guarantee the
high visibility of interferometer selections dynamically. The active phase
stabilization technique employs a phase modulator (PM) driven by a DAC to
adjust the relative phase between the two arms of the interferometer. By
monitoring photon counting rates of two Up-Conversion Detectors (UCD) at two
output ports of the interferometer, a Field Programmable Gate Array (FPGA)
calculates and finds the optimal code value for the DAC, maintaining a high
visibility of the interferometer every time a new light path is selected. The
visibility of most of the 128 interferometer selections can simultaneously be
maintained over 96% during the QKD, which lays the foundation for the RRDPS
experiment.
