Abstract:
Automated and instantaneous stabilizing control of the quantum key decoder is the mandatory procedure, which guarantees real-time and high-speed differential-phase-shift (DPS) quantum key distribution (QKD) at low erroneous rate, and ensures the delay-line interferometric (DLI) decoding of the weak-coherent-pulse quantum bit-stream at a qualified and constant visibility. As the QKD system often suffers from environmental temperature/acoustic/vibrational perturbations, employing in-situ software controlled active-feedback-loop circuits is the alternative solution, which automatically preserves the constant visibility of a DLI decoder by dynamically tuning the wavelength of the weak-coherent-pulse carrier. To compensate for wavelength or phase inaccuracy, this work proposes the active-visibility-control (AVC) of the DLI by in-situ monitoring its interferometric visibility and fine controls it with combined proportion integral-loop and hill-climbing-stepper (PI-loop and HC-stepper) feedback algorithms, thus providing the 40 km-long SMF transmission of the weak-coherent-pulse bit-stream at 192-Mbit/s clock rate and 1-Mbit/s receiving rate. By adopting the PI-loop+HC-stepper combined algorithm to stabilize the long-term visibility of the AVC-DLI, such an AVC-DLI loop effectively overcomes environmental disturbances to confine the overall spectral linewidth and wavelength drift of the DPSQKD carrier within 266 kHz, enabling the predicted lowest QBER of 0.4% when decoding the weak-coherent-pulse bit at the first second. With the AVC-DLI, the back-to-back weak-coherent-pulse transmission guarantees a secure key rate of 91.6 kbit/s with a QBER stabilized to 1.8% over 10 minutes, and the long-reach capability is extended to 40-km transmission with a secure key rate of 2.6 kbit/s and QBER of 3.7% for 25-60 minutes. The technical improvement of AVC-DLI unveils the dynamic long-distance weak-coherent-pulse DPS-QKD transmission.