Although spontaneous Raman scattering is often regarded as the dominant impairment in hybrid quantum– classical fiber links, practical deployment also requires clear limits on linear inter-channel crosstalk between entangled photons and neighboring dense wavelength-division multiplexing (DWDM) channels. In this work, we experimentally determine such limits using a controlled bench-top testbed in which a high-fidelity (F > 0.97) polarization-entangled photon source shares a DWDM demultiplexer with a single classical optical channel separated by 100 GHz. Residual classical leakage into the quantum receiver is controlled through spectral filtering and systematically varied by sweeping the classical power incident on the detector from −120 dBm to −40 dBm. A thin-film bandpass filter with 0.9 nm bandwidth, providing approximately 10 dB of additional suppression, is employed in the quantum path. Entanglement quality is assessed through polarization-interference visibility measured in multiple polarization bases and through the coincidence-to-accidental ratio (CAR). We observe that high visibility and CAR are maintained over a broad range of crosstalk levels, with degradation governed by the relative rate of accidental coincidences rather than by absolute optical power alone. Acceptable performance, defined here as visibility ≥ 90% and CAR ≥ 20, is preserved provided that the classical leakage power at the detector does not exceed the quantum-signal power by more than 12 dB for a continuous-wave carrier or for a 100 GBd dual-polarization QPSK signal. These experimental results establish filtering and isolation requirements for entanglement distribution in hybrid quantum–classical DWDM optical systems.
Although spontaneous Raman scattering is often regarded as the dominant impairment in hybrid quantum– classical fiber links, practical deployment also requires clear limits on linear inter-channel crosstalk between entangled photons and neighboring dense wavelength-division multiplexing (DWDM) channels. In this work, we experimentally determine such limits using a controlled bench-top testbed in which a high-fidelity (F > 0.97) polarization-entangled photon source shares a DWDM demultiplexer with a single classical optical channel separated by 100 GHz. Residual classical leakage into the quantum receiver is controlled through spectral filtering and systematically varied by sweeping the classical power incident on the detector from −120 dBm to −40 dBm. A thin-film bandpass filter with 0.9 nm bandwidth, providing approximately 10 dB of additional suppression, is employed in the quantum path. Entanglement quality is assessed through polarization-interference visibility measured in multiple polarization bases and through the coincidence-to-accidental ratio (CAR). We observe that high visibility and CAR are maintained over a broad range of crosstalk levels, with degradation governed by the relative rate of accidental coincidences rather than by absolute optical power alone. Acceptable performance, defined here as visibility ≥ 90% and CAR ≥ 20, is preserved provided that the classical leakage power at the detector does not exceed the quantum-signal power by more than 12 dB for a continuous-wave carrier or for a 100 GBd dual-polarization QPSK signal. These experimental results establish filtering and isolation requirements for entanglement distribution in hybrid quantum–classical DWDM optical systems.
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.
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.
Two-photon interference is a cornerstone of photonic quantum technologies. However, its practical use in hybrid architectures is constrained by the need for photon indistinguishability, particularly in linewidth and the associated time scale. While narrowband filtering can improve interference visibility, it introduces significant photon loss - a critical limitation for applications. Here, we experimentally demonstrate an efficient approach to enable non-classical two-photon interference between spectral-bandwidth mismatched photons using an electro-optic time lens. We increase the visibility of Hong-Ou-Mandel (HOM) interference between spectral bandwidth mismatched photons from (31.0±1.7)%, to (91.7±2.6)%, with correction for accidental coincidences, achieving highly non-classical two-photon interference visibility without spectral filtering. Moreover, we demonstrate a simultaneous correction of the central wavelength mismatch of ±1.1 nm, while maintaining the non-classical interference visibility. This result opens the possibility to efficiently integrate quantum systems operating at different time scales for hybrid quantum communication, teleportation, entanglement swapping, distributed sensing, and hybrid quantum computing.
Filtering is commonly used in quantum optics to reject noise photons, and also to enable interference between independent photons. However, filtering the joint spectrum of photon pairs can reduce the inherent coincidence probability or loss-independent heralding efficiency. Here we investigate filtering for multiphoton applications based on entanglement and interference (e.g., quantum teleportation). We multiplex C-band entangled photons and C-band classical communications into the same long-distance fibers, which enables scalable low-loss quantum networking but requires filtering of spontaneous Raman scattering noise from classical light. Using tunable-bandwidth filters, low-jitter detectors, and polarization filters, we distribute wavelength-degenerate time-bin entangled photons in the C band (1536.5 nm) over 25 km/25 km standard fibers (50 km) each carrying 10 Gbit/s C-band classical data signals. Narrow filtering enables milliwatt-level C-band power, which exceeds the power achieved by comparable studies by roughly an order of magnitude and could feasibly support terabit per second classical rates. We evaluate how performance depends on pump and filter bandwidths, multipair emission, filter shapes, loss, phase matching, and how quantum information is measured. We find a trade-off between reducing noise impact and increasing single-mode purity, and discuss mitigation methods toward optimal multiphoton applications. Importantly, these results apply to noise in free space and in quantum devices (sources, frequency converters, switches, detectors, etc.) and provide insight into filter-induced degradation of single-photon purity and rates even in noise-free environments.
Integrating quantum key distribution (QKD) on the existing optical fiber network through dense wavelength-division multiplexing (DWDM) can greatly reduce the costs of quantum channels and improve the scalability and flexibility of the quantum communication network. In the scenarios of an access network or local area network with a large number of users, the connected optical fibers are relatively short and the quantum signal can be adjacent to the classical channels. In this case, the four-wave-mixing noise generated by the classical lights will have a severe impact on the performance of the QKD. We establish a theoretical model to characterize the excess noise in continuous-variable QKD induced by the four-wave mixing, and experimentally demonstrate the validity of the theoretical predictions. In addition, the influence of cross-phase-modulation noise between the carrier of the quantum signal and classical data channels on the QKD is investigated. To suppress the effect of the four-wave-mixing noise, an unequal channel spacing technique is employed. We realize coexistence of entangled state-based continuous-variable QKD and five adjacent DWDM classical channels with 10 dBm launch power and nonreturn-to-zero on-off-keying modulation at 2.5 Gb/s (10 Gb/s).
The fundamental properties of quantum mechanics, such as the no-cloning theorem, make management and control of quantum networks challenging. Quantum wrapper networking (QWN) offers a solution to this problem by wrapping quantum payloads with classical bits (as headers and/or tails) that can be used to assist with networking, performance monitoring, and probing the properties of the fiber interconnection. Since the classical header and quantum payload travel in the same fiber, one should carefully design the system to mitigate potentially deleterious effects such as noise and cross-talk. In this paper, we identify and characterize the noise induced by the classical headers by in-fiber scattering processes. We study the noise by using a tunable continuous-wave laser to emulate O-band (∼1310 nm) classical headers, which are time- and wavelength-multiplexed with O-band quantum signals. The noise is characterized as a function of the headers’ power, classical-quantum wavelength detuning, and optical fiber length. We demonstrate that the dominant noise contribution originates from spontaneous Raman scattering for wavelength detunings that are larger than 3 nm. We also observe that some noise photons can bleed into the quantum payload due to Rayleigh scattering of backward Raman scattered photons. We further investigate system level impacts of the noise created by classical headers that are produced by employing a small form-factor pluggable transceiver. We demonstrate that the associated noise level has negligible impact on quantum payloads for a 47.8 km deployed fiber link, the longest length used in QWN experiments to date, achieving a coincidences-to-accidentals ratio of 56 and a two-photon interference visibility of 88.8%.