Entanglement distribution over 155 km metropolitan fiber using a CMOS-compatible silicon chip

Transmitting entangled states over long distances is crucial for developing quantum networks. Previous demonstrations using satellites or fibers relied on photon pairs generated from bulk crystal arrangements. Polarization entanglement distribution based on CMOS-compatible silicon chips has long been restricted to lab-scale demonstrations spanning only a few meters, due to the difficulty of achieving sufficient off-chip brightness. We report a silicon chip platform that provides an off-chip entangled photon pair brightness ranging from 8,000 to 460,000 pairs per second, exceeding previous reports by three orders of magnitude. The entanglement fidelity reaches 99.85(6)% and 97.90(3)%, respectively. After addressing key challenges in long distance entanglement distribution over deployed fiber, including phase drift and chromatic dispersion, entangled photons were successfully distributed over 155 km (66 dB loss). These results demonstrate that CMOS-compatible silicon chips can perform competitively with bulk crystal sources and represent an important step toward scalable, chip-based quantum networks.

💡 Research Summary

The authors present a comprehensive demonstration of long‑distance polarization‑entangled photon distribution using a CMOS‑compatible silicon nanophotonic chip. By integrating an 8 mm silicon waveguide with low‑loss edge couplers (0.64 dB each) into a Sagnac interferometer, they generate photon pairs via spontaneous four‑wave mixing (SFWM) when a continuous‑wave pump at 1550.12 nm is injected from both directions. The Sagnac configuration ensures that horizontally (H) and vertically (V) polarized photon pairs acquire no relative phase, directly yielding the maximally entangled Bell state |Φ⁺⟩ = (|HH⟩ + |VV⟩)/√2 at the output PBS.

Key performance metrics of the source are:

• Off‑chip pair brightness tunable from 8 k pairs s⁻¹ to 460 k pairs s⁻¹, a three‑order‑of‑magnitude improvement over previous silicon‑based sources.
• Entanglement fidelity of 99.85 % at low brightness and 97.90 % at the highest brightness, with a CHSH S‑parameter up to 2.82, clearly violating the classical bound.
• Raw coincidence rate of 230 kcps (corresponding to 460 k pairs s⁻¹) when the source is operated at high pump power.

The paper addresses the two principal obstacles for fiber‑based distribution of polarization entanglement: polarization mode dispersion (PMD) and chromatic dispersion. For chromatic dispersion, a non‑local dispersion compensation module (DCM) is inserted in the idler arm, improving the coincidence‑to‑accidental ratio (CAR) and raising the fidelity upper limit by 32 %. PMD effects are mitigated by (i) using a Sagnac loop that forces both polarizations to travel the same physical fiber, and (ii) employing a near‑symmetric transmission scheme where both signal and idler photons traverse comparable lengths of deployed metropolitan fiber, thereby reducing temperature‑induced phase drift.

Two transmission experiments are reported:

1. Asymmetric 93 km distribution – The signal photon travels 93 km of deployed fiber (35.4 dB loss) between the National University of Singapore (NUS) and Singapore University of Technology and Design (SUTD), while the idler photon is detected locally. After DCM insertion, the measured raw coincidence rate is 132 cps with a fidelity of 93.3 %.

2. Near‑symmetric 155 km distribution – Both photons are sent over separate deployed fibers (NUS–NTU and NUS–SUTD loops), totaling >155 km and 66 dB loss (56 dB fiber loss + 10 dB from the DCM). By increasing pump power, the post‑transmission pair rate reaches 0.7 cps with a fidelity of 87.6 %. Accounting for the 66 dB loss, the inferred off‑chip brightness is 2.8 Mcps and the intrinsic fidelity before loss is estimated at 93.9 %.

These results are directly compared with state‑of‑the‑art bulk crystal sources (PPLN, PPKTP) that have demonstrated entanglement distribution over up to 404 km in low‑loss spools. Despite the higher loss of deployed metropolitan fiber, the silicon chip source achieves comparable brightness and fidelity, demonstrating that integrated photonics can compete with bulk nonlinear optics for quantum networking applications.

The discussion highlights several avenues for further improvement:

• Reducing the insertion loss of the DCM or replacing it with on‑chip dispersion engineering.
• Employing micro‑ring resonators to generate narrow‑band photon pairs less susceptible to PMD.
• Exploiting dense wavelength‑division multiplexing (DWDM) to create multiple parallel entangled channels, dramatically increasing the quantum channel capacity.
• Integrating additional degrees of freedom such as time‑bin encoding to realize high‑dimensional entanglement.
• Porting the system‑level optimization strategies to other platforms (AlGaAs, thin‑film lithium niobate, SiC, etc.) and to satellite‑to‑ground links, where the demonstrated 66 dB loss exceeds the 62 dB loss of the Micius double‑downlink experiment.

In summary, the work establishes a bright, CMOS‑compatible silicon entangled photon source capable of distributing polarization entanglement over 155 km of real‑world metropolitan fiber with acceptable fidelity. The combination of high on‑chip brightness, robust dispersion management, and system‑level engineering positions silicon nanophotonics as a viable, scalable technology for future quantum communication networks, quantum key distribution, and hybrid fiber‑free‑space quantum links.