Chinese scientists make remarkable achievement in long-haul fiber quantum network

HEFEI, June 27 (Xinhua) -- Chinese scientists have achieved twin-field quantum key distribution (TF-QKD) through a 511-km optical fiber linking two distant metropolitan areas, proving the technical feasibility in a practical environment and paving the way towards a long-haul fiber quantum network.

Led by Pan Jianwei from the University of Science and Technology of China, the field test was conducted through a channel between Qingdao and Jinan in east China's Shandong Province. The experiment results have been published in Nature Photonics, a peer-reviewed scientific journal under the Nature Publishing Group.

The basic principle of quantum mechanics guarantees the unconditional security of quantum key distribution (QKD) at the cost of forbidding the amplification of a quantum state. Despite remarkable progress in worldwide metropolitan QKD networks over the past decades, a long-haul fiber QKD network without a trusted relay has not yet been achieved.

Similar to optical-fiber communication (OFC), the quantum state is also attenuated exponentially with the transmission distance. For OFC, an optical amplifier is used to relay the optical signal every 80 km to build a long-haul fiber network.

The amplifier for a single unknown quantum state, however, does not exist because the quantum signal either collapses or additional noises are induced once it is amplified due to quantum non-cloning theorem. Therefore, a simple optical amplifier cannot be used for long-haul fiber QKD. So far, the longest reported field test of QKD is around 90 km.

Twin-field QKD is a new technology that may realize long-haul fiber QKD theoretically. The research team had already realized TF-QKD of 500 km in their lab, but human voice or other activities cannot be forbidden in a real environment, which can interfere with quantum stability. Therefore, putting the experiment to a field test is not only necessary to prove its feasibility, but also an important step for exploring the possibilities of future global QKD networks.

The work proves the feasibility of TF-QKD in a practical environment and paves the way towards long-haul fiber quantum networks. We expect that the techniques developed in this work will find immediate use in more general applications such as quantum repeaters and phase-based architecture for the quantum internet, said the research paper.