Abstract The twin-field quantum key distribution (TF-QKD) requires that two lasers hundreds of kilometers apart must have the same frequency to achieve the high single-photon interference visibility. This means that both lasers need to be stabilized to the same frequency reference. A simple and robust system is presented for simultaneously stabilizing two 1550-nm lasers at the single-photon level. By utilizing the single-photon multi-frequency modulation technology, both the fiber laser and distributed feedback (DFB) laser are stabilized to the π -phase shifted fiber Bragg grating at the same time. The frequency fluctuations of fiber laser and DFB laser are bounded within 1.39 MHz and 1.53 MHz over 2000 s, respectively. These two frequency-stabilized 1550-nm lasers could be used for TF-QKD.
Abstract The twin-field quantum key distribution (TF-QKD) requires that two lasers hundreds of kilometers apart must have the same frequency to achieve the high single-photon interference visibility. This means that both lasers need to be stabilized to the same frequency reference. A simple and robust system is presented for simultaneously stabilizing two 1550-nm lasers at the single-photon level. By utilizing the single-photon multi-frequency modulation technology, both the fiber laser and distributed feedback (DFB) laser are stabilized to the π -phase shifted fiber Bragg grating at the same time. The frequency fluctuations of fiber laser and DFB laser are bounded within 1.39 MHz and 1.53 MHz over 2000 s, respectively. These two frequency-stabilized 1550-nm lasers could be used for TF-QKD.
Abstract Twin-field quantum key distribution (TF-QKD) has attracted considerable attention in quantum communication for the extension of secure communication distance. In TF-QKD, precise photon phase control is required and has been realized using dual-band stabilization (DBS). In this study, we performed residual phase noise reduction in DBS using laser frequency stabilization. The achieved frequency stability of the light sources was better than 1 × 10 −12 when the averaging time is larger than 1 s, indicating that the laser frequency variation was less than 200 Hz at 1.5 μm. The integrated phase noise at 1 s was less than 200 mrad, and the corresponding quantum bit error rate at 1 s was less than 1% after applying laser frequency stabilization. This result demonstrates that the stringent phase stability requirements in TF-QKD can be achieved without relying on complex optical frequency combs, enabling a simpler and more cost-effective experimental setup.
Twin-field (TF) quantum key distribution (QKD) has emerged as a promising method for repeater-free long-distance secure communication, with the longest demonstration covering 1000 km [1]. It is a measurement-device-independent QKD protocol, where the two users send weak coherent states, to a central node for single-photon interference followed by single-photon detection. Due to this, TF-QKD has a square root dependence of the key rate on channel transmission, making it possible to cover longer distances than conventional or commercial devices[2]. In previous implementations, a narrow linewidth laser has been used as the master or reference laser, frequency stabilised by locking the laser to a frequency reference, such as the atomic transition of acetylene or an ultra-low expansion (ULE) cavity using RF-locking [1], to achieve high coherence throughout the entire system. The RF-locking method requires a large and complex infrastructure that is sensitive to misalignment and is relatively expensive, bulky, and fragile [3]. Here, we demonstrate an all-fibre frequency reference as an alternative to ULE cavities for TF-QKD, offering lower complexity and intrinsic alignment, a simplified setup built with telecom compatible components, and a smaller footprint, which makes it easier to deploy in existing fibre telecom infrastructures.
Twin-field quantum key distribution (TFQKD) overcomes the linear rate-loss limit, which promises a boost of secure key rate over long distance. However, the complexity of eliminating the frequency differences between the independent laser sources hinders its practical application. We analyzed and determined the frequency stability requirements for implementing TFQKD using frequency-stabilized lasers. Based on this analysis, we proposed and demonstrated a simple and practical approach that utilizes the saturated absorption spectroscopy of acetylene as an absolute reference, eliminating the need for fast frequency locking to achieve TFQKD. Adopting the 4-intensity sending-or-not-sending TFQKD protocol, we experimentally demonstrated the TFQKD over 502, 301, and 201 km ultralow-loss optical fiber, respectively. We expect this high-performance scheme will find widespread usage in future intercity and free-space quantum communication networks.
Quantum key distribution (QKD) enables information-theoretically secure communication against eavesdropping. However, phase instability remains a challenge across many QKD applications, particularly in schemes such as twin-field QKD and measurement-device-independent QKD. The most dominant source of phase fluctuation arises from the frequency offset between independent lasers. Here we propose a method to address this issue by employing a classical photodiode to compensate for the laser frequency difference. As an application of this method, we implement this technique in a mode-pairing QKD system, achieving an error rate approaching the theoretical limit and surpassing the linear key-rate bound over a fiber distance of 296.8 km. This approach provides a practical solution for frequency matching between independent lasers and can be extended to other fields requiring precise phase stabilization.
Phase coherent fibers (PCF) are essential to distribute nearly monochromatic photons, ultra-stable in their frequency and phases, which have demanding requirements for state-of-the-art networked experiments, quantum as well as very high-speed communications. We report the development of a system that produces PCF links, also actively corrects the unavoidable slow frequency drift of the source laser. The PCF follows white phase noise limited σo × τ−1 stability behavior having σo values 1.9(2) × 10−16 and 2.6(1) × 10−16 for a 3.3 km field-deployed and 71 km spool fibers, respectively, with up to 47.5 dB suppression of the phase noise compared to a normal fiber. Additionally, the system is featured to correct the source laser’s 33.8 mHz/s frequency drift to as low as ≃ 0.05 mHz/s. Therefore, this all-in-one solution producing a quantum link can potentially enhance the effectiveness of the twin field quantum key distribution (TF-QKD) by nearly a 73-fold reduction of the QBER that arises from using unstabilized fiber links, as well as relaxes the laser frequency drift correction constraints by severalfold. The authors develop a technique to generate Phase Coherent optical Fibers (PCF) with additional capability of active correction of the source laser’s frequency drift. PCF is capable in distributing nearly monochromatic and low phase noise photons, which has demanding requirements in quantum communication, distributed sensing, time & frequency metrology.
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.