Quantum-Secure Internet Achieved Over City-Scale Distance

Marking a significant step toward a quantum-secure internet, researchers have demonstrated device-independent quantum key distribution over optical fibers spanning 100 kilometers (km). The findings show that cryptographic security can be guaranteed with this method, at the metropolitan scale – which represents a much greater distance than previous efforts – and help to close the gap between proof-of-principle quantum network experiments and real-world applications. Quantum key distribution (QKD) is a leading application of quantum technologies, enabling ultra-secure digital communications. Early forms of QKD derive security using trusted devices yet suffer from technical limitations and vulnerabilities. A more advanced approach – device-independent QKD (DI-QKD) – derives its security directly from fundamental quantum phenomena, specifically the violation of Bell inequalities, without requiring trust in quantum devices' internal workings. However, DI-QKA is extremely demanding and requires the creation of high-quality entanglement and efficient detection over long distances. To date, DI-QKD has only been demonstrated over short distances and in laboratory-based proof-of-principle experiments. Here, Bo-Wei Lu and colleagues report the realization of DI-QKD between two entangled atoms linked by 100-kilometer (km) optical fibers. By combining advanced techniques such as single-photon interference, quantum frequency conversion to low-loss telecom wavelengths, and noise-suppressed photon emission, Lu et al. successfully distributed high-fidelity entanglement over long distances, achieving provably secure quantum key generation over 11 km with finite data, and showed that positive key rates are possible even at 100 km. According to the authors, the achievement extends DI-QKD distances by more than two orders of magnitude compared to previous demonstrations.