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APL Photonics

Asynchronous-bit-rate differential phase-shift-keying

You-Cheng Lin, Kuan-Wei Hu, Hsing-Yi Huang, Yuanjie Yang, You-Xin Wang, Shih-Chang Hsu, Chih‐Hsien Cheng, Atsushi Matsumoto, Kouichi Akahane, Yuh‐Renn Wu, Hao‐Chung Kuo, Gong‐Ru Lin

Published Dec 1, 2024• 0 citations
Publisher

Abstract

An adaptive differential-phase-shifting (DPS) quantum key distribution (QKD) protocol improved by encoding asynchronous-bit-rate with harmonic clock tunability, decoded by shortened delay-line interferometer (DLI) with enhancing stability, and received by single-photon detection with long hold-off is demonstrated to realize low-erroneous transmission. To achieve long-term stabilized visibility with a maintained quantum bit-error-ratio (QBER) and secure key rate, the shortened and polarized DLI enlarges its free-spectral range (FSR) and reduces its power-to-wavelength slope (δP/δλ) to suppress its sensitivity to thermal gradient, channel leakage, and wavelength disturbance. Extremely low power and wavelength drifts of ΔP/P < ±0.02% and Δλ/λ = ±6.45 × 10−8 can be achieved, even when using a single-mode QKD carrier with a relatively broadened 300-kHz linewidth. Flexibly expanding the 1-bit-delay FSR of the polarized DLI from 0.04 to 1.00 GHz improves its immunity to environmental disturbances, enabling the DPS-QKD decoding with 210–213 patterns, 3.2% QBER, and 17.46-kbit/s under 0.22 photon/pulse in average. The asynchronous-bit-rate DPS-QKD protocol enables harmonic expansion of DLI’s FSR at limited single-photon avalanche detector’s bit rate, which lengthens the duration of stable visibility to support long-pattern DPS-QKD.

DOI
10.1063/5.0231047

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An adaptive differential-phase-shifting (DPS) quantum key distribution (QKD) protocol improved by encoding asynchronous-bit-rate with harmonic clock tunability, decoded by shortened delay-line interferometer (DLI) with enhancing stability, and received by single-photon detection with long hold-off is demonstrated to realize low-erroneous transmission. To achieve long-term stabilized visibility with a maintained quantum bit-error-ratio (QBER) and secure key rate, the shortened and polarized DLI enlarges its free-spectral range (FSR) and reduces its power-to-wavelength slope (δP/δλ) to suppress its sensitivity to thermal gradient, channel leakage, and wavelength disturbance. Extremely low power and wavelength drifts of ΔP/P < ±0.02% and Δλ/λ = ±6.45 × 10−8 can be achieved, even when using a single-mode QKD carrier with a relatively broadened 300-kHz linewidth. Flexibly expanding the 1-bit-delay FSR of the polarized DLI from 0.04 to 1.00 GHz improves its immunity to environmental disturbances, enabling the DPS-QKD decoding with 210–213 patterns, 3.2% QBER, and 17.46-kbit/s under 0.22 photon/pulse in average. The asynchronous-bit-rate DPS-QKD protocol enables harmonic expansion of DLI’s FSR at limited single-photon avalanche detector’s bit rate, which lengthens the duration of stable visibility to support long-pattern DPS-QKD.
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.
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.
A novel single-photon differential phase-shift keying (DPSK) scheme with asymmetric high-bit-rate coding (at 192 Mbit/s) and low-bit-rate gating (1 Mbit/s) coding data format is proposed to stabilize the long-term and long-distant point-to-point secure communication over 40-km standard single-mode-fiber (SSMF). The single-photon carrier is served by an attenuated distributed-feedback laser diode with ultralow phase and power fluctuations of Δθ/θ<0.005% and ΔP/P<0.01%. The single-photon carrier successively encoded with pulsed return-to-zero (RZ) clock and DPSK bits at 192-Mbit/s is decoded by a synchronized one-bit-delay interferometer at 192-Mbit/s with enhanced immunity to spectral and thermal noise. The single-photon DPSK data is detected within 1-ns transistor-transistor logic (TTL) gated window at 0.1% duty-cycle of the 1-Mbit/s receiver. Such an asymmetric DPSK encoding and receiving coding data format further stabilize the single-photon DPSK codes with a quantum bit error ratio (QBER) of 0.038% when the interfered free spectral range of 4.8 MHz expands broader than the linewidth of the single-photon carrier of 293 kHz to ensure the interferometric visibility of 99.2% with the operating time beyond 1065 s. Decoding the single-photon DPSK bits with an average photon number of 0.2 #/bit guarantees the BER at 1.5% with a secure key rate of 113.4 kbit/s under the back-to-back transmission, and remains the BER at 3.2% with a secure key rate of 10.1 kbit/s over 40-km-SSMF transmission. The low-bit-rate-gated receiving of high-bit-rate transmitted DPSK coding data format has unveiled a novel stabilization scheme for single-photon secure communications.
A novel type of quantum key distribution (QKD) protocol, called DPS (differential-phase-shift) QKD, was proposed several years ago. A sender transmits a highly-attenuated coherent pulse train with {0, π} phase, and a receiver receives it with a one-bit delay Mach-Zehnder interferometer followed by photon detectors. A secret key is created from photon detection events, whose security is based on the fact that an eavesdropper cannot perfectly measure the phase information of a highly-attenuated coherent pulse train. This protocol has some features of simple setup, potential for a high key creation rate, and robustness against photon-number-splitting attack. This paper overviews DPS-QKD. The operation mechanism is described, and then some experimental efforts are introduced, featuring use of a glass waveguide Mach-Zehnder interferometer and advanced single-photon detectors. The highest key rate and the longest distance have been achieved with the DPS-QKD protocol. Some modified schemes are also presented, including that utilizing quantum entanglement, that using decoy pulses, and that using macroscopic coherent light.
We demonstrate the first proof of principle differential phase shift (DPS) quantum key distribution (QKD) using narrow-band heralded single photons with amplitude-phase modulations. In the 3-pulse case, we obtain a quantum bit error rate (QBER) as low as 3.06% which meets the unconditional security requirement. As we increase the pulse number up to 15, the key creation efficiency approaches 93.4%, but with a cost of increasing the QBER. Our result suggests that narrow-band single photons maybe a promising source for the DPS-QKD protocol.
We have implemented a differential-phase-shift (DPS) quantum key distribution (QKD) system at 1310 nm with superconducting single-photon detectors (SSPDs). The timing jitter of the SSPDs is very small (∼60 ps) and their dark counts rate is low (<200 s −1 ). 1310 nm is an ideal quantum signal wavelength for a QKD system, where quantum signals coexist with classical communication signals at 1550 nm in one fiber. As the key element in the DPS QKD, a Michelson interferometer was designed and built using Faraday mirrors that can automatically compensate for the polarization evolution in the fiber. As the result, our DPS QKD system can be steadily operated at 2.5 GHz clock rate with a low quantum error rate of less than 4%.