1-Mbps Twin-Field Quantum Key Distribution over 200 km Using Independent Dissipative Kerr Solitons

Abstract

Twin-field quantum key distribution (TF-QKD) dramatically enhances the secure key rate (SKR) over inter-city distances through its square-root scaling. Further improvements in aggregate SKR can be achieved by wavelength-division multiplexing (WDM) of parallel QKD channels. However, direct implementation in TF-QKD poses significant challenges, as each wavelength channel requires an independent ultra-stable seed laser, narrow-linewidth transmitters, and optical phase-locked loops (OPLLs), which are not easily scalable. Here, we circumvent these limitations by employing two independent, integrated dissipative Kerr soliton (DKS) microcombs at Alice and Bob as multi-wavelength sources. High-visibility single-photon interference across all wavelength channels is achieved by stabilizing the frequencies of every comb line - requiring only the stabilization of the pump wavelength and repetition rates of the two microcombs. Based on this architecture, we perform a full TF-QKD experiment using the sending-or-not-sending protocol, achieving a total SKR of 1.57 Mbps over 201.1 km of fiber using 16 DWDM channels. This result represents more than an order-of-magnitude enhancement compared with single-wavelength TF-QKD at the same distance. Given that a single DKS comb can support over 100 coherent lines across the C-band, this approach offers a scalable pathway toward high-rate quantum key distribution over inter-city distances.

Figures (11)

• Figure 1: Principle of WDM-based TF-QKD architectures.(a) TF-QKD using an array of independent narrow-linewidth lasers for WDM-parallelization. Each laser is phase-locked via an OPLL to a remote USL. The quantum signals are combined and transmitted via a fiber channel for interference and detection at the central untrusted node. (b) TF-QKD using DKS microcombs as multi-wavelength sources. A single laser is phase-locked to a remote USL via a single OPLL. This pump laser drives a chip-integrated microresonator to generate a frequency comb. In both configurations, the sender setup at Bob's side is identical to that at Alice's.
• Figure 2: Soliton microcomb generation.(a) The experimental setup for DKS microcomb generation. A fiber laser serves as the pump for the Si$_3$N$_4$ microresonator, with its wavelength controlled by an arbitrary function generator (AFG). The pump light is amplified by an erbium-doped fiber amplifier (EDFA), and the amplified spontaneous emission (ASE) noise is suppressed using a bandpass filter (BPF). A phase modulator (PM) controled by the signal generator (SG) is inserted to extend the soliton steps and facilitate multi-soliton generation; an acousto-optic modulator (AOM) is used to lock the pump frequency and the DKS microcomb repetition rate. The resulting soliton spectrum is recorded by an optical spectrum analyzer (OSA). At the output of each DKS microcomb source, a wavelength-selective switch (WSS) isolates the designated QKD channels, and an additional AOM fine-tunes the optical frequency of each comb line. (b) Photograph of one leveraged Si$_3$N$_4$ microresonators. (c) The optical spectra of the generated single solitons. The line spacings are 50.070 GHz and 50.076 GHz, respectively. The wavelength channels used for TF-QKD are indicated by the dashed box, spanning C26 to C41 (1544.53 nm to 1556.55 nm).
• Figure 3: (a) The frequency difference between the corresponding comb lines (C26 to C41) from Alice and Bob. (b) The standard deviation of the relative frequency difference across the 16 microcomb line pairs. (c) The standard deviation of the phase drift rate between the corresponding comb lines.
• Figure 4: Performance of the WDM-based TF-QKD system. The bars in (a) and (b) represent the X-basis QBERs and SKRs of TF-QKD using 16 comb line pairs from two independent microcombs. The red dashed line in (a) and (b) represents the QBER and SKR of TF-QKD using two independent lasers as the source. (c) The blue stars and the red hexagon are the experimentally obtained SKRs of 16 comb-line pairs from independent microcombs and the aggregate SKR. The solid lines represent the theoretical simulation of SKR using a single wavelength (red) and 16 wavelength channels (blue). The black dashed line denotes the repeaterless secret key capacity bound ($\mathrm{SKC_0}$). The remaining symbols represent state-of-the-art SKRs from recent QKD experiments for comparison.
• Figure 5: Fabrication and characterization of silicon nitride microresonators. (a). The DUV subtractive process flow of 6-inch-wafer Si$_3$N$_4$ foundry fabrication. WOX, thermal wet oxide (SiO$_2$). (b) A typical transmission of the microresonators. The intrinsic/external linewidth is 17.3/36.5 MHz. (c) Statistical distribution of the intrinsic linewidths of the resonances of two Si$_3$N$_4$ microresonators. The maximum-likelihood intrinsic linewidths are $\kappa^\mathrm{A}_0/2\pi = 17~\mathrm{MHz}$, $\kappa^\mathrm{B}_0/2\pi = 17~\mathrm{MHz}$. (d) The integrated dispersions of the two Si$_3$N$_4$ microresonators. The FSRs ($D_1/2\pi$) are 50.073 GHz and 50.072 GHz, which are quite close to each other.