A major breakthrough in the fight against quantum hacking
The rise of quantum computers raises fears for the confidentiality of global communications: these machines could soon break current encryption with alarming ease. To counter this possibility, research is accelerating around quantum key distribution (QKD). This security method is based on an absolute physical principle: any attempt at espionage disrupts the quantum state of the data, making the intrusion immediately detectable. Until now limited by technical constraints, this digital shield has just taken a decisive step forward. In a study published in the journal Science, a team of Chinese researchers demonstrated the viability of « device-independent » quantum encryption over metropolitan distances, reaching 100 kilometers via fiber optics for the first time.
Overcoming hardware vulnerabilities
Quantum key distribution (QKD) requires a physical medium, usually fiber optic cables. The problem lies in the exponential weakening of the signal over distance. To compensate, amplifiers are traditionally used, but these devices introduce a vulnerability: they require extremely precise calibration to guarantee security. This hardware complexity significantly slows down the large-scale deployment of the technology.
The alternative being studied by scientists is « device-independent » QKD (DI-QKD). This system uses entangled particles: if a hacker attempts to intercept the message, the entanglement state is broken. Security no longer depends on the reliability of the hardware, but on the laws of physics. However, this method has so far encountered technological barriers. Early attempts, using trapped ions or photons, were only able to generate positive keys over a few hundred meters. Subsequent advances in frequency conversion were not enough to make DI-QKD viable, with researchers constantly struggling with difficulties related to entanglement fidelity and detection efficiency.
A technological leap on a metropolitan scale
To push these limits, the research team implemented a new strategy aimed at reducing signal loss in the fiber. They used single-photon interference, where entangled pairs are produced on demand using a detector that signals the successful creation of the quantum state. At the same time, they used quantum frequency conversion to reach telecommunications wavelengths, which are less prone to loss. The results are convincing. The researchers achieved high-fidelity atom-atom entanglement and positive secure key rates over tested distances of 11, 20, 50, 70, and 100 km. As the authors of the study point out: « The use of the single-photon interference scheme for remote entanglement announcement allowed us to achieve a metropolitan entanglement speed several orders of magnitude higher than the two-photon-based schemes used in previous DI-QKD experiments. »
Crucially for security, violations of the CHSH Bell inequality, which prove the presence of quantum entanglement, were maintained at all distances, ensuring secure key generation up to the 100-kilometer threshold.
Towards a practical application despite obstacles
Despite this technical success, the widespread deployment of DI-QKD will still take time. The experiment has certain limitations: all the nodes in the network were located in the same laboratory, which means that the locality gap has not yet been completely closed. In addition, the event rate continues to decrease with distance due to losses inherent in optical fiber. The future of this technology will likely involve the use of lower-loss fibers and continuous improvement in frequency conversion. The authors of the study remain optimistic about the scope of their work: « The demonstration of device-independent QKD on a metropolitan scale helps bridge the gap between quantum network experiments in principle and real-world applications. » Beyond simple encryption, this architecture offers a versatile platform for quantum random number generation (DI-QRNG), self-testing of quantum devices, and provides a fundamental test bed for quantum mechanics itself. This high-fidelity entanglement could become the building block essential for scaling future quantum networks.
According to the source: phys.org
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