Entanglement Swapping with Integrated Narrowband Photon Sources for Quantum Repeaters

As quantum networks scale, they require quantum repeaters to overcome signal loss in optical fibers. A primary challenge is that the most promising quantum memories—based on solid-state atomic systems—operate with very narrow bandwidths, whereas standard integrated photonic sources are typically broadband. This study investigates whether integrated silicon nitride micro-ring resonators can generate narrowband entangled photon pairs suitable for entanglement swapping, a critical process for linking distant quantum nodes.

The researchers utilized two independent, state-of-the-art silicon nitride micro-ring resonators to generate energy-time entangled photon pairs. To mimic the conditions of a real-world quantum network, the sources were operated at different frequencies (separated by 1.6 THz) and were not synchronized in time. The team implemented a robust stabilization scheme using off-the-shelf fiber components to lock the frequency and phase of the two independent sources. They performed a Bell state measurement (BSM) on one photon from each source, which projected the remaining two photons into an entangled state. The quality of this entanglement was verified using a folded Franson interferometer.

Findings

The study achieved a high degree of photon indistinguishability, with a background-subtracted Hong-Ou-Mandel (HOM) visibility of 0.99. Following the entanglement swapping process, the researchers measured a net swapped state visibility of 0.88 ± 0.06. This result confirms that the generated entanglement is strong enough to violate a Bell inequality, proving that integrated photonic sources can be successfully interfaced with atomic-based quantum memories for future field-deployed quantum repeaters.

This work bridges a significant gap in quantum communication technology. By demonstrating that integrated photonics can produce narrowband photons compatible with atomic memories, the authors provide a scalable, compact path toward building practical quantum repeaters. The use of commercially available components for system-level stabilization further suggests that these methods are viable for integration into existing fiber-based network infrastructures.