Technique of active phase stabilization for the interferometer with 128 actively selectable paths

TL;DR

This paper presents a novel active phase stabilization technique for a 128-path variable-delay interferometer used in quantum key distribution, ensuring high visibility and stability during rapid path switching.

Contribution

It introduces a dynamic phase stabilization method with real-time feedback for a complex interferometer in RRDPS-QKD, enabling fast, stable path selection.

Findings

Interferometer visibility maintained over 96% during operation

Successfully implemented 128 selectable delay paths with 10 kHz switching rate

Demonstrated high stability and phase control in quantum key distribution experiments

Abstract

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.