Coexistence of classical and quantum light in optical fiber is a key requirement for scalable quantum networks. However, it is fundamentally limited by nonlinear crosstalk from intense classical signals. Hollow-core fiber (HCF), with its intrinsically low optical nonlinearity, offers a promising solution. Here, we experimentally demonstrate the coexistence of polarization-entangled photons with dense wavelength-division multiplexed (DWDM) classical signals with a total launch power of 11.2 dBm, and perform a direct comparison between equal-length single-mode fiber (SMF) and photonic-bandgap-based HCF deployed in a campus network. We observe that HCF suppresses background noise by approximately two orders of magnitude compared to SMF, while preserving high-fidelity entanglement distribution. Our results provide clear experimental evidence of the advantage of HCF and establish its potential as a transmission medium.
Coexistence of classical and quantum light in optical fiber is a key requirement for scalable quantum networks. However, it is fundamentally limited by nonlinear crosstalk from intense classical signals. Hollow-core fiber (HCF), with its intrinsically low optical nonlinearity, offers a promising solution. Here, we experimentally demonstrate the coexistence of polarization-entangled photons with dense wavelength-division multiplexed (DWDM) classical signals with a total launch power of 11.2 dBm, and perform a direct comparison between equal-length single-mode fiber (SMF) and photonic-bandgap-based HCF deployed in a campus network. We observe that HCF suppresses background noise by approximately two orders of magnitude compared to SMF, while preserving high-fidelity entanglement distribution. Our results provide clear experimental evidence of the advantage of HCF and establish its potential as a transmission medium.
The coexistence of quantum information and classical signals in a single fiber is essential for future quantum networks that leverage the well-established optical fiber infrastructure. Although multiplexing technologies can separate quantum and classical signals, pure silica core fibers (PSCFs) remain fundamentally limited by the high nonlinearity, which generates substantial Raman scattering and four-wave mixing noise. Hollow-core fibers (HCFs), guiding light predominantly in air, offer an attractive solution with intrinsically ultra-low nonlinearity and strongly suppressed nonlinear noise. In this work, we demonstrate the entanglement-based key coexisting with data over an 18-km HCF link. We achieve time-encoded high-dimensional quantum key distribution (HD-QKD) carrying 0 dBm of bidirectional received power, corresponding to a theoretical data capacity of up to 2.3 Tbps. During 24 hours of continuous operation, an average secret key rate (SKR) of 10.56 kbps is obtained. Theoretical analysis further predicts SKRs above 135 kbps over transmission distances exceeding 200 km using state-of-the-art low-loss HCFs. These results show significantly improved performance compared with PSCF-based systems and highlight the potential of HCFs for scalable quantum-classical coexistence compatible with the architectures of established fiber-optic networks.
The integration of quantum key distribution (QKD) with classical optical networks has emerged as a pivotal strategy for building secure communication infrastructures. However, achieving their coexistence in silica-core fibers faces inherent limitations due to nonlinear interference, particularly under high classical signal power. In this paper, we experimentally demonstrate the coexistence of classical optical transport networks and QKD over 101.6 km of hollow-core fiber (HCF) with classical power exceeding 20 dBm for the first time to the best of our knowledge. Through systematic theoretical analysis, we characterize HCF’s transmission loss and nonlinear noise generation mechanisms, revealing its unique compatibility with high-power classical–quantum coexistence compared to conventional fibers. To address spectral interference from HCF’s absorption peaks and nonlinear effects, we propose a spectrally optimized multi-stage allocation (SOMA) scheme that coordinates low-loss channel selection and noise suppression. Experimental validation shows that, under the SOMA scheme, coexisting classical signals achieve 19 dBm in single-wavelength configuration and 21 dBm in multi-wavelength configuration, while maintaining a secure key rate above 10 kbps. This breakthrough establishes HCF as a viable platform for simultaneous high-capacity classical communication and quantum-secured data transmission, overcoming critical power limitations of existing fiber-based coexistence systems.
Quantum key distribution (QKD) is approaching industrial maturity, but its integration with high-power classical signals in shared optical fiber is hindered by noise from nonlinear effects like spontaneous Raman scattering and four-wave mixing. Hollow-core fiber (HCF) offers a promising solution by inherently suppressing these nonlinearities. However, a comprehensive noise analysis for pure HCF or standard single-mode-fiber–HCF hybrid links, validated experimentally under realistic high-power conditions, remains unavailable. This work bridges that gap through a thorough theoretical and experimental investigation of an HCF-featured coexistence system. We demonstrate a silicon-photonics-based QKD system operating with an optical transport network (OTN). The system achieves multiplexed launch powers of 20 dBm for counter-propagation and 14 dBm for co-propagation, meeting the highest requirements of modern OTNs. Over a 10 km HCF link, it simultaneously supports 12×583 Gbps classical channels across the C-band and achieves secure key rates of 5.7 kbps (counter-propagation) and 2.5 kbps (co-propagation) over a 1550 nm quantum channel. Our results provide critical benchmarks and a practical framework for deploying HCF-based quantum-classical networks, paving the way for future inherently secure telecommunications.
In this paper, we investigate the impact of classical optical communications in quantum key distribution (QKD) over hollow-core fiber (HCF), multi-core fiber (MCF) and single-core fiber (SCF) and propose wavelength allocation schemes to enhance QKD performance. Firstly, we theoretically analyze noise interference in QKD over HCF, MCF and SCF, such as spontaneous Raman scattering (SpRS) and four-wave mixing (FWM). To mitigate these noise types and optimize QKD performance, we propose a joint noise suppression wavelength allocation (JSWA) scheme. FWM noise suppression wavelength allocation and Raman noise suppression wavelength allocation are also proposed for comparison. The JSWA scheme indicates a significant enhancement in extending the simultaneous transmission distance of classical signals and QKD, reaching approximately 100 km in HCF and 165 km in MCF under a classical power per channel of 10 dBm. Therefore, MCF offers a longer secure transmission distance compared with HCF when classical signals and QKD coexist in the C-band. However, when classical signals are in the C-band and QKD operates in the O-band, the performance of QKD in HCF surpasses that in MCF. This research establishes technical foundations for the design and deployment of QKD optical networks.
Integrating quantum key distribution (QKD) with classical optical networks is essential for the large-scale deployment of quantum-secure communication. However, the nonlinear impairments induced by classical signals pose significant challenges to co-propagation systems based on single-mode fiber. Although hollow-core fiber (HCF) offers low nonlinearity and low transmission loss, non-uniform attenuation and residual nonlinear effects still degrade the performance of the coexistence transmission system. To address these issues, we construct a co-propagation system based on HCF and propose a quantum-classical joint optimization (QCJO) scheme that effectively minimizes nonlinear noise and enhances the overall robustness of the system. In addition, we demonstrate the co-propagation of a commercial QKD system and an optical transmission network with 40 Tbps (50×800 Gbps ) classical communication capacity over a 101.6 km HCF link. The result represents the recorded classical capacity in fiber-based quantum-classical coexistence transmission and the secure key rate of 9.56 kbps satisfies the fundamental requirements for secure encryption. This work marks significant progress toward massive quantum network deployment and lays a foundation for inherent-security and high-capacity telecommunication infrastructures.
The co-propagation of quantum and classical signals through shared optical fibers is crucial for scalable quantum networks. However, this coexistence is fundamentally limited by spontaneous Raman scattering (SpRS) from the bright classical light, which generates overwhelming noise that disrupts the single-photon-level quantum signals. Here, we overcome this long-standing challenge by leveraging the inherently ultralow nonlinearity of hollow-core fiber (HCF) to suppress SpRS noise. By operating both the quantum time synchronization (QTS) and classical optical frequency transfer (OFT) signals within the telecom C-band, separated by only ~10 nm, we successfully demonstrate their simultaneous transmission over a 122-km HCF link. With a classical OFT power of 1 mW, the QTS performance shows negligible degradation, maintaining sub-picosecond time stability at 2000 s, while the OFT achieves a fractional frequency instability of 10^-20. Near-sub-picosecond QTS stability is preserved even when the classical power is increased to 3 mW. Furthermore, simulations based on our experimental data indicate that with next-generation low-loss HCF, the platform can tolerate classical powers beyond 10 mW and extend the QTS range to over 500 km. By realizing a unified quantum-classical time-frequency distribution framework, this work establishes HCF as a highly capable and practical platform for future scalable quantum networks.