Researchers at the University of Science and Technology of China (USTC) demonstrated a quantum repeater module that addresses a key scalability bottleneck in fiber-based quantum networks.

Transmission efficiency in optical fiber decreases exponentially with distance, making direct entanglement distribution impractical beyond short spans. Quantum repeater architectures overcome this limitation by dividing long links into shorter segments and extending entanglement through entanglement swapping.

However, a persistent challenge has been memory coherence time. In prior demonstrations, entangled states decohered before adjacent segments could be connected, preventing deterministic scaling across multiple nodes.

The USTC team achieved remote memory-memory entanglement over six miles (10 kilometers) of fiber with a reported lifetime of approximately 550 milliseconds, enabled by a long-lived trapped-ion memory and high-efficiency ion–photon interface. The lifetime exceeded the average entanglement establishment time of 450 milliseconds, allowing neighboring segments to be connected reliably, which is a core requirement for scalable repeater operation.

The repeater result appeared in Nature.

In a related study published in Science, the researchers demonstrated device-independent quantum key distribution (DI-QKD) over fiber links of up to 62 miles (100 km) using single trapped atoms. At 7 miles (11 km), the system generated verifiably secure encryption keys using real experimental data. At 62 miles, the researchers showed that the protocol can produce secure keys in principle at metropolitan-scale distances.

DI-QKD bases its security on measurable quantum correlations rather than assumptions about trusted hardware, making it one of the most technically demanding approaches to quantum-secure communication. Previous experimental demonstrations were generally limited to distances of meters to hundreds of meters, making metropolitan-scale fiber deployment a significant extension.

Together, the results suggest incremental but concrete progress toward scalable quantum networks capable of supporting secure communications and, eventually, distributed quantum computing over metropolitan and regional fiber infrastructure.