Entanglement distribution over deployed fiber links is essential for implementing quantum communication protocols. However, environmental noise in deployed fibers, such as classical signals, stray light, and Raman scattering, can significantly degrade entanglement, ultimately destroying the nonlocal correlations required for quantum communication. In this work, we demonstrate noise-resilient quantum communication over 40 km of intra-city deployed fiber, with a total transmission loss of 23.04 dB, by utilizing hyperentanglement in the time-bin and frequency degrees of freedom. By exploiting the frequency anticorrelation of the photon pairs and implementing frequency-to-time mapping using dense wavelength-division multiplexing combined with calibrated delay lines, we realize frequency-resolved detection that discriminates frequency-anticorrelated entangled photon pairs from uncorrelated noise. This enables reliable entanglement distribution with over an order-of-magnitude improvement in signal-to-noise ratio and a substantial increase in Bell-state fidelity, even in high-noise and high-loss conditions of deployed fibers. Moreover, entanglement—initially lost due to entanglement sudden death—is successfully restored, enabling a clear violation of the Bell inequality. These results establish a practical and scalable method for entanglement distribution in noisy and lossy environments, advancing the feasibility of real-world quantum communication over existing fiber infrastructure.
Entanglement distribution over deployed fiber links is essential for implementing quantum communication protocols. However, environmental noise in deployed fibers, such as classical signals, stray light, and Raman scattering, can significantly degrade entanglement, ultimately destroying the nonlocal correlations required for quantum communication. In this work, we demonstrate noise-resilient quantum communication over 40 km of intra-city deployed fiber, with a total transmission loss of 23.04 dB, by utilizing hyperentanglement in the time-bin and frequency degrees of freedom. By exploiting the frequency anticorrelation of the photon pairs and implementing frequency-to-time mapping using dense wavelength-division multiplexing combined with calibrated delay lines, we realize frequency-resolved detection that discriminates frequency-anticorrelated entangled photon pairs from uncorrelated noise. This enables reliable entanglement distribution with over an order-of-magnitude improvement in signal-to-noise ratio and a substantial increase in Bell-state fidelity, even in high-noise and high-loss conditions of deployed fibers. Moreover, entanglement—initially lost due to entanglement sudden death—is successfully restored, enabling a clear violation of the Bell inequality. These results establish a practical and scalable method for entanglement distribution in noisy and lossy environments, advancing the feasibility of real-world quantum communication over existing fiber infrastructure.
Quantum information protocols are being deployed in increasingly practical scenarios, via optical fibers or free space, alongside classical communications channels. However, entanglement, the most critical resource to deploy to the communicating parties, is also the most fragile to the noise-induced degradations. Here we show that polarization-frequency hyperentanglement of photons can be effectively employed to enable noise-resistant distribution of polarization entanglement through noisy quantum channels. In particular, we demonstrate that our hyperentanglement-based scheme results in an orders-of-magnitude increase in the SNR for distribution of polarization-entangled qubit pairs, enabling quantum communications even in the presence of strong noise that would otherwise preclude quantum operations due to noise-induced entanglement sudden death. While recent years have witnessed tremendous interest and progress in long-distance quantum communications, previous attempts to deal with the noise have mostly been focused on passive noise suppression in quantum channels. Here, via the use of hyperentangled degrees of freedom, we pave the way toward a universally adoptable strategy to enable entanglement-based quantum communications via strongly noisy quantum channels.
The coexistence of quantum and classical signals in optical fiber infrastructures represents a major challenge for large-scale quantum networks, as noise sources such as Raman scattering can significantly impact entanglement distribution, and the quantum protocols based on it. In this work, we analyze Raman scattering in the C-band, used for entanglement distribution, generated by a classical the O-band signal. The main contribution of this study lays in investigating these effects in a real metropolitan-fiber network, moving beyond controlled laboratory experiments to deployed telecommunication environments. Measurements are performed over a 7 km metropolitan fiber link using commercial sources and narrowband lasers. The experimental results shows good agreement between the measurements taken under laboratory conditions, although, within the metropolitan-scale loop, localized spectral anomalies are observed in the deployed fibers. Therefore, our results show that, whenever a quantum signal propagates in the C-band alongside an O-band classical channel within the same fiber, careful selection of the operating frequency is required, as Raman scattering and other real-world noise sources can significantly affect the quality and stability of the quantum transmission. In particular, we identify spectral regions that are less affected by Raman noise, thereby providing practical guidelines for optimal quantum channel allocation. We demonstrate that Raman-induced noise constitutes a dominant contribution to the quantum signal-to-noise ratio (SNR) in realistic deployments, beyond background and detector noise. Overall, our findings offer practical insights for deploying quantum communication systems over existing fiber networks, supporting the development of robust and scalable quantum infrastructures.
Integrating quantum communication into existing fiber infrastructures requires coexisting with high-power classical traffic, which generates noise in the quantum channel. Spontaneous Raman Scattering (SpRS) is the dominant noise source, generating a broadband photon background that degrades quantum states and, in Quantum Key Distribution (QKD), can entirely suppress secure key generation. In this work, we implement single-copy entanglement distillation utilizing energy-time and frequency hyperentanglement, and we exploit it to perform Quantum Privacy Amplification (QPA) directly at a physical layer. Strict frequency anti-correlations of spontaneous parametric down-conversion are used to selectively discard uncorrelated Raman photons while preserving genuine entangled pairs, suppressing accidental coincidences by a factor 1/N, where N is the number of frequency channel pairs. Using passive borosilicate-glass photonic integrated circuits and standard DWDM demultiplexers over a 20~km single-mode fiber testbed, we demonstrate a 4~dB improvement in tolerance to classical laser launch power PC when QPA is performed, extending the secure key threshold from PC=−13.3 dBm to PC=−9.3 dBm. The result shows that off-the-shelf telecom components can be repurposed as physical-layer quantum privacy amplifiers.
Hyperentanglement, as a high-dimensional quantum entanglement phenomenon with multiple degrees of freedom, plays a critical role in quantum communication, quantum computing, and high-dimensional quantum state manipulation. Unlike entangled states in a single degree of freedom, hyperentangled states establish entanglement relationships simultaneously in multiple degrees of freedom, such as polarization, path, and orbital angular momentum. Through entanglement-based distribution techniques, high-dimensional quantum information networks can be constructed. On this basis, a fully connected quantum network with hyperentanglement is constructed in this work, and the polarization and time-bin degree-of-freedom hyperentanglement is realized through the process of second-harmonic generation and spontaneous parametric down-conversion in periodically poled lithium niobate (PPLN) waveguide cascades. The hyperentangled state is then multiplexed into a single-mode fiber by using dense wavelength division multiplexing (DWDM) technology for transmission to terminal users. The quality of the entangled states in the two degrees of freedom is characterized using Franson-type interference and photon-pair coincidence measurement techniques. Polarization entangled states are subjected to quantum state tomography, and entanglement distribution technology is employed to achieve long-distance distribution and quantum key transmission within the network. Experimental results show that the two-photon interference visibility of both polarization and time-bin entanglement is greater than 95%, demonstrating the high quality of the hyperentanglement in the network. After 100-km-entanglement distribution, the fidelity of the quantum states in both degrees of freedom remains above 88%, indicating the effectiveness of long-distance entanglement distribution in this network. Additionally, it is verified that this network supports the distribution of quantum keys over a distance of more than 50 km between users. These results confirm the feasibility of a fully connected quantum network with hyperentanglement and demonstrate the potential for constructing large-scale metropolitan networks by using hyperentanglement. As a higher-dimensional entanglement, hyperentangled states can significantly enhance the capacity and efficiency of quantum information processing. Although the quantum communication is still in its early stages of development, achieving stable storage and transmission of entangled states in large-scale metropolitan networks remains a great challenge. By utilizing the frequency conversion properties and high integration characteristics of the periodically poled lithium niobate waveguides, the three-user hyperentangled quantum network constructed in this work provides a new solution for developing the large-scale metropolitan networks with high-dimensional quantum information networks. It is expected to provide a new platform for quantum tasks such as superdense coding and quantum teleportation.
The distribution of quantum entanglement between distant parties is a key challenge in the pursuit of a worldwide quantum network. Quantum repeaters and optical satellite links have both been proposed to overcome the distance limitations of fiber-based quantum networks. In order to test the viability of a world spanning quantum satellite network, several proof-of-concept studies have already demonstrated high-fidelity transmission of photonic entanglement via terrestrial long-distance free-space links. With the recent launch of the Micius quantum satellite by the Chinese Academy of Sciences, the technological challenge of implementing this scheme in situ was overcome. All of these free-space experiments utilized entanglement in a two-dimensional state space, which in most cases was encoded in the polarization of photons. However, high-dimensional entanglement can yield significant benefits in quantum key distribution by increasing the secure key rate and enhancing the resilience to eavesdropping attacks. Energy-time entanglement is an established photonic degree of freedom (DOF); it is routinely used for the distribution of entanglement in fiber-based quantum cryptography networks but has only recently been considered as a viable option for atmospheric free-space quantum communications. The dimensionality can be further increased by exploiting simultaneous entanglement in several DOF. This so-called hyperentanglement has previously been used in various quantum protocols, but not yet been demonstrated outside a protected laboratory environment. For our proof-of-concept experiment (doi:10.1038/ncomms15971), we use energy-time and polarization hyperentanglement to distribute, for the first time, 4-dimensional entanglement via a 1.2-km-long intra-city free-space link. A source of hyperentangled photons and a detection module (Alice) were located in our lab and a receiver station (Bob) was located at a different university building. The source produced polarization entangled photon pairs via type-0 down-conversion in a Sagnac loop configuration. The emission time of a photon pair is uncertain within the significantly longer coherence time of the pump laser, thus resulting in an energy-time and polarization hyperentangled state. One photon was guided to a local measurement module and the partner photon was guided to a transmitter telescope on the roof of the institute. After transmission over the 1.2-km-long free-space link, the photons were received by a telescope and detected in Bob’s measurement module. The measurement modules for Alice and Bob each featured a polarization and an energy-time analyzer. We observe high-visibility two-photon interference in both polarization and energy-time subspaces. The measured visibilities certify entanglement in both subspaces individually. Additionally, they establish a lower bound on the Bell-state fidelity of the hyperentangled state of 0.94, thus certifying genuine 4-dimensional entanglement and 1.47 ebits of entanglement of formation. We have thus successfully distributed hyperentangled photon pairs via an intra-city free-space link under conditions of strong atmospheric turbulence. The transmission of quantum information embedded in a genuine high-dimensional state space under real-world link conditions is a first important step towards real-world implementations of advanced quantum information processing protocols in the future. In particular, it enables the implementation of high-dimensional QKD protocols over long-distance free-space links, and, ultimately, over satellite links with only minor changes to existing mission proposals.