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IEEE Transactions on Quantum Engineering

Automated Active-Visibility-Control Loop for Stabilized DPS-QKD Decoding Under Dynamic Wavelength Drift

Kuan-Wei Hu, Chih‐Hsien Cheng, Atsushi Matsumoto, Kouichi Akahane, Gong‐Ru Lin

Published Jan 1, 2026• 0 citations
Publisher

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, and enabling the predicted lowest quantum bit-error-ratio (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 min, 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 min. The technical improvement of AVC-DLI unveils the dynamic long-distance weak-coherent-pulse DPS-QKD transmission.

DOI
10.1109/tqe.2026.3712194

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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, and enabling the predicted lowest quantum bit-error-ratio (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 min, 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 min. The technical improvement of AVC-DLI unveils the dynamic long-distance weak-coherent-pulse DPS-QKD transmission.
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Under the quantum key distribution with differential phase-shift-keying protocol, the ultimate stability of the system is limited by the residual wavelength drift of the quantum-key carrier and the polarization/refractive-index/physical-length fluctuations induced by environmental thermal disturbance. To permanently overcome such a shortcoming, a 1-bit-delayed interferometric fiberized decoder with ultrastable visibility maintenance and long-term immunity to environmental perturbations is demonstrated by employing all polarization maintaining-fiber to construct an asymmetric-arm Mach-Zehnder interferometer, and surrounding such a delay-line interferometric decoder with thermal-isolated sponge and styrofoam container. Receiving the DPS-QKD bit-stream with a high sifting key rate and low bit-error ratio is approached by maintaining the high interfered visibility and eliminating the wavelength drift with a highly adiabatic package. Such an ultrastable long-term 1-bit delay interferometric decoder is particularly suitable for the persistent DPS-QKD operation towards future commercialization.
In comparison with the rapidly developing progress on the optical C-band (1525-1565 nm) master-to-slave injection-locked transmitter to perform the coherent single-photon quantum key distribution (QKD), the development on the O-band (1250-1350 nm) QKD transmitter is somewhat delayed as the commercially available wavelength-matched narrow-linewidth distributed feedback laser diode (DFBLD) pair is hardly accessible up to now. By using the DFBLDs with only sub-MHz linewidth and relatively deviated wavelength for the first time, this work demonstrates the optically differential-phase-shift-keying (DPS) QKD by an O-band master-to-slave injection-locked DFBLD pair. The master and slave DFBLDs with wavelength fluctuations of ±0.05% and ±0.2 pm are controlled by a thermo-electric cooler with a feedback gain of 100. The 1-bit delay interferometer (DI) under thermo-insulation maintains its visibility at >96% with dP o /P o and dP o /dt measured below ±0.1% and ±1 × 10 −3 mW/s. By RZ-OOK modulating the master DFBLD with step-like power coding at 150 µW to induce π phase shift in the injection-locked slave DFBLD, the rising-/falling-edge DPS envelope distortion of the slave DFBLD diminishes by decreasing the bias current of the master DFBLD from 7I th (35 mA) to 2I th (10 mA). This phenomenon enables the 128-bit DPS-QKD transmission with a quantum bit-error rate (QBER) of 3.57% and a secure key rate of 3.524 kbit/s in the 6-km SMF link. The O-band injection-locked single-photon DPS-QKD bit-stream with a mean photon number of 0.2 #/bit minimizes its decoding QBER to 3.88% and 4.84% for 512-bit and 1024-bit, respectively.
Differential phase shift quantum key distribution(DPSQKD) experiment based on weak coherent pulses have been performed in the laboratory. We encoded the bit information between the consecutive pulses. The bit information was decoded at Bob's side by Michelson-Faraday interferometer and single photon detector. This kind of quantum key distribution system can provide stable key distribution process, high efficiency of key creation, and high security key. So it is easily implemented in optical fibers using readily available optical telecommunication tools. We experimented DPSQKD over 76 km on fiber, the error rate of the sifted key is 5.3%. The quantum key distribution process is controlled by micro-computer based on ARM (advanced RISC(reduced instruction set computing) machines) processor.
Phase drift is an inevitable problem in the practical implementation of phase-coding quantum key distribution (QKD) systems. Conventional active phase tracking and compensation solutions cannot be implemented during the key transmission process, resulting in reduced efficiency of the system. In this paper, we propose a single-photon level real-time phase tracking scheme using only the estimated quantum bit error rate (QBER) and mismatched-basis data to acquire the phase drift parameter instead of a phase scanning process or reference light pulses, without extra hardware or interrupting the transmission of quantum signals. This scheme is applied into a phase-coding QKD system with active phase disturbance and obtains an average QBER of 1.01%1.01\%1.01% with a standard derivation of 0.37%0.37\%0.37% over 50 h of continuous operation. Experimental results show that this scheme enables QKD systems to operate continuously with long-term stability and keep a low level of QBER even with rapid phase drift, suggesting its suitability for practical applications.
We present a design for a quantum key distribution (QKD) system in a Sagnac loop configuration, employing a novel phase modulation scheme based on frequency shift, and demonstrate stable BB84 QKD operation with high interference visibility and low quantum bit error rate (QBER). The phase modulation is achieved by sending two light pulses with a fixed time delay (or a fixed optical path delay) through a frequency shift element and by modulating the amount of frequency shift. The relative phase between two light pulses upon leaving the frequency-shift element is determined by both the time delay (or the optical path delay) and the frequency shift, and can therefore be controlled by varying the amount of frequency shift. To demonstrate its operation, we used an acousto-optic modulator (AOM) as the frequency-shift element, and vary the driving frequency of the AOM to encode phase information. The interference visibility for a 40 km and a 10 km fiber loop is 96% and 99%, respectively, at single photon level. We ran BB84 protocol in a 40-km Sagnac loop setup continuously for one hour and the measured QBER remained within the 2%-5% range. A further advantage of our scheme is that both phase and amplitude modulation can be achieved simultaneously by frequency and amplitude modulation of the AOM's driving signal, allowing our QKD system the capability of implementing other protocols, such as the decoy-state QKD and the continuous-variable QKD. We also briefly discuss a new type of Eavesdropping strategy ("phase-remapping" attack) in bidirectional QKD system