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Journal of Lightwave Technology

Novel Single-Photon Differential Phase-Shift Keying With Asymmetric High-Bit-Rate Coding and Low-Bit-Rate Gated Receiving Coding Data Format

Kuan-Wei Hu, Chih‐Hsien Cheng, Atsushi Matsumoto, Kouichi Akahane, Jau-Yang Wu, Yuh‐Renn Wu, Gong‐Ru Lin

Published May 15, 2024• 1 citation
Publisher

Abstract

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.

DOI
10.1109/jlt.2024.3401147

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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.
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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.
For weak coherent single-photon secure data communication among short-reach metropolitan intra-/inter-city networks at the O-band (1250-1350 nm), the commercially available semiconductor laser sources are emerging but still suffering from high single-mode-fiber (SMF) loss, broad linewidth, and unstable wavelength. To overcome such disadvantages for enabling the efficient phase-coding link with sufficient secure key rate, a specifically designed adiabatic package with active temperature-/current-feedback control is proposed for the paired O-band MHz-linewidth master-to-slave injection-locked DFBLDs and a polarization-maintaining 1-bit-delay interferometer is stabilized with using a passively adiabatic cell to achieve accurate differential phase decoding. Even though, the phonon-induced phase fluctuation still occurs at rising and falling edges of the decoded long-pattern secure data bits delivered from the slave DFBLD, which is mainly attributed to the intra-cavity heating under excessive free-carrier generation via the master DFBLD injection. To stabilize the differential-phase-shift (DPS) keying protocol, the phase-code distortion caused by over-injection-induced Auger heating is effectively suppressed by reducing the overly biased injection with precise master-injection-level control. The rising-/falling-edge damping distortion of the phase-shift-encoded secure bit-stream envelope is suppressed by appropriately decreasing the DC bias current and adjusting the AC encoding amplitude of the master DFBLD. Such operation reduces the incorrect π phase shift in the injection-locked slave DFBLD biased at optimized below-threshold DC offset, thus allowing single-photon DPS-keying data transmission over 15-km SMF with slightly increasing the single-photon bit-error ratio from <3% (0-km) to 6.2% (15-km).
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.
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.