Low-nonlinearity advantage of hollow-core fiber for wavelength multiplexing of quantum entanglement and classical DWDM signals

AI summary

Overview

• Research area: Quantum communication / quantum networks, specifically the coexistence of quantum entanglement distribution with classical dense wavelength-division multiplexed (DWDM) optical communications, and the use of hollow-core fiber (HCF) as a low-nonlinearity transmission medium.
• Technical level: Intermediate. Readers will benefit from familiarity with polarization entanglement, spontaneous parametric down-conversion, quantum state tomography, and DWDM terminology, but the core comparison (noise in HCF versus standard single-mode fiber) is explained in accessible terms.
• Scope: An experimental field trial comparing equal-length deployed HCF and SMF links (approximately 500 m, Keio University Shin-Kawasaki campus) carrying polarization-entangled photons alongside four classical QPSK DWDM channels at a total launch power of 11.2 dBm.

What This Paper Is About

Future quantum networks are expected to share optical fiber infrastructure with ordinary classical communications, but intense classical light generates nonlinear optical noise (four-wave mixing and Raman scattering) that can drown out fragile quantum signals in conventional silica single-mode fiber. The authors ask whether hollow-core fiber, which guides light mostly in air and therefore has intrinsically low optical nonlinearity, protects entangled photons better than standard fiber. They answer this by transmitting the same entangled states and the same classical DWDM signals through equal-length HCF and SMF links installed in the same campus cable, then directly comparing noise and entanglement fidelity.

Key Contributions

1. First direct experimental, side-by-side comparison of deployed HCF and SMF links of equal length (approximately 500 m) under identical launch conditions for coexistence of polarization entanglement and classical DWDM signals, addressing the gap left by earlier work in which HCF-versus-SMF comparison was limited to simulations.
2. Quantitative measurement of nonlinear noise suppression: background counts at the quantum channel are approximately two orders of magnitude lower in HCF than in SMF, and this difference survives correction for channel transmittance, ruling out simple leakage or loss differences as the explanation.
3. Full quantum state tomography of the transmitted states, showing entanglement survives in HCF but is severely degraded in SMF under the same classical launch power, with fidelities of 0.912 ± 0.003 (HCF) versus 0.517 ± 0.004 (SMF) to the target Bell state.
4. A scalability argument for dense multiplexing: the authors note that a 100-channel DWDM system at the same total power of 13.2 mW (11.2 dBm) would correspond to 132 µW per channel, a reasonable per-channel power for practical optical communication.

Main Findings

• Noise suppression: Across the entire scanned classical input power range (5.2 to 14.2 dBm), background count rates at the quantum channel in HCF were significantly lower than in SMF; at 11.2 dBm the noise was approximately two orders of magnitude lower.
• Noise spectrum structure: In SMF, distinct peaks attributed to four-wave mixing appear above a broadband background suggestive of Raman scattering; a few peaks and broadband background also appear in HCF but are substantially weaker.
• Loss correction confirms the mechanism: The HCF link had 4.7 dB loss versus 0.6 dB for the SMF link, so the authors re-plotted background counts corrected for channel transmittance; the substantial difference remained, indicating the improvement stems from intrinsically reduced nonlinearity rather than attenuation or classical signal leakage.
• No classical signals present: High-fidelity entanglement was observed for both fiber types, with fidelities of 0.937 ± 0.006 (HCF) and 0.939 ± 0.002 (SMF), confirming fiber transmission alone does not significantly degrade the state.
• With classical signals at 11.2 dBm total: Fidelity fell to 0.517 ± 0.004 for SMF (substantial entanglement degradation) but stayed at 0.912 ± 0.003 for HCF (robust preservation).
• Residual HCF noise attributed to external factors: The small remaining background in HCF is likely dominated by weak classical-signal leakage and nonlinear effects in short SMF segments preceding the HCF, rather than nonlinearity inside the HCF itself.
• The test configuration favored SMF: The quantum channel at 192.15 THz sits on the rising tail of the Stokes Raman spectrum rather than near its approximately 13 THz downshift maximum, so the SMF result understates the difficulty in a denser, less favorable allocation.

Methodology in Plain English

The team set up a loop of fiber in a campus cable at Keio University containing both a standard single-mode fiber and a hollow-core fiber of the same length (about 500 m), so signals launched into either link return to the same room for measurement.

• Quantum source: Polarization-entangled photon pairs in the state |Ψ⁺⟩ = (|HV⟩ + |VH⟩)/√2, generated by spontaneous parametric down-conversion in a type-2 periodically poled lithium niobate crystal inside a Sagnac interferometer, pumped by second-harmonic generation of a telecom pulsed laser. A dual-band polarizing beam splitter routes pump and photons, and a half-wave plate designed to act only at 780 nm leaves telecom-band polarization untouched.
• Detection: One photon of each pair is measured immediately in a local tomography setup (quarter-wave plate, half-wave plate, polarizing beam splitter, superconducting nanowire single-photon detector). The partner photon is sent through the deployed fiber and measured with an identical tomography setup after propagation.
• Wavelength plan: The quantum channel is filtered to a spectral mode near 192.15 THz. Four classical channels at 192.4, 192.5, 192.6, and 192.7 THz carry QPSK at 25 Gbaud; the nearest classical channel is 250 GHz from the quantum channel. The quantum channel was placed as close as feasible to the classical channels, at the lowest frequency where the detector still operates even with SMF transmission.
• Isolating fiber nonlinearity: Two bandpass filters before multiplexing ensure that measured noise reflects processes inside the deployed fiber, not the amplifiers or source. An extra DWDM filter before the detector suppresses leaked classical light and lets the team isolate nonlinear noise.
• Two measurement modes: (1) Background-noise-only runs, with zero quantum input, sweeping total classical power from 5.2 to 14.2 dBm and also scanning a tunable filter to obtain noise spectra at 11.2 dBm; (2) full coexistence runs at 11.2 dBm where density matrices were reconstructed for the state before transmission and after transmission through each fiber type.

Why This Matters

The work converts a widely assumed but largely simulated advantage of hollow-core fiber into a measured, quantitative one under realistic field conditions, using a campus-deployed cable rather than a lab spool. It shows that the constraint on quantum channels in shared fiber is not only about filter isolation but about the fiber medium itself, and that changing the medium relaxes wavelength-allocation rules that would otherwise crowd quantum channels out of busy DWDM bands.

Real-world applications:

• Quantum-secured telecom infrastructure: Co-propagating entanglement or QKD channels alongside conventional DWDM traffic without dedicating scarce, expensively filtered spectrum.
• Entanglement distribution in metropolitan networks: Delivering entangled photon pairs between nodes of a city-scale quantum network that shares ducts and cables with classical traffic.
• Distributed quantum computing and quantum sensor networks: Interconnecting quantum devices over installed fiber where classical synchronization and data traffic also flow.
• High-capacity links with quantum overlays: Using HCF's other advertised properties (low latency, high power handling, low dispersion) while simultaneously carrying quantum signals.

Industry relevance: Telecom operators and equipment vendors planning quantum-safe or quantum-enabled networks can read this as evidence that fiber-medium choice, not just filter engineering, is a design lever. The authors' 100-channel arithmetic (132 µW per channel at 13.2 mW total) is aimed directly at the per-channel power levels used in real dense DWDM systems.

Future Directions

1. Move beyond photonic-bandgap HCF: The authors explicitly note that their HCF is a photonic-bandgap design chosen for low nonlinearity despite relatively high loss; anti-resonant HCFs with reported low transmission loss are an obvious next medium to test, since the 4.7 dB loss over 500 m is a major budget item.
2. Test denser and less favorable wavelength allocations: Repeating the experiment with many more classical channels, and with the quantum channel placed near the Raman gain maximum (approximately 13 THz downshift) rather than on its rising tail, would establish how much headroom HCF actually provides.
3. Extend to longer spans and higher launch powers: The reported HCF advantage spans 3 to 30 mW of total input power over 500 m; scaling to km-class links and to the higher powers used in real amplifier chains remains open, as does the question of whether residual noise from SMF pigtails and connectors can be engineered away.
4. Field deployment with full classical payloads: The demonstration used four 25 Gbaud QPSK channels; integrating into a live DWDM system with the full channel count, and evaluating practical metrics such as secret-key rates from the transmitted states, would connect this physical-layer result to network-level performance.

Target Audience

Quantum communication and quantum network researchers, particularly those working on entanglement distribution in shared fiber and on coexistence with classical traffic. It is also valuable for optical fiber and transmission-system engineers evaluating hollow-core fiber, for DWDM network planners reasoning about spectrum allocation for quantum channels, and for graduate students seeking a clear experimental template for field-deployed quantum-versus-classical coexistence measurements.

Authors’ abstract

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.