A Metropolitan-scale Multiplexed Quantum Repeater with Bell Nonlocality
Abstract
Quantum repeaters can overcome exponential photon loss in optical fibers, enabling heralded entanglement between distant quantum memories. The definitive benchmark for this entanglement is Bell nonlocality; however, recent metropolitan-scale demonstrations based on single-photon interference (SPI) schemes have been limited to generating low-quality entanglement, falling short of Bell nonlocality certification. Here, we introduce a multiplexed quantum repeater protocol based on time measurements (MQR-TM), successfully combining the high heralding rate of SPI schemes with the phase robustness of two-photon interference (TPI) schemes. This approach achieves heralded entanglement distribution between two solid-state quantum memories over a record 14.5~km separation, generating a Bell state with a fidelity of 78.6 2.0\%. We observe a CHSH-Bell inequality violation by 3.7 standard deviations, marking the first certification of Bell nonlocality in metropolitan-scale quantum repeaters. Our architecture supports autonomous quantum node operation without fiber channel phase stabilization, offering a practical framework for scalable quantum-repeater networks.
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