Under Quantum Lock And Key

China leads the world in quantum key distribution (QKD) technology, which promises to lay the foundations for an ‘unhackable’ quantum internet.

From the lab to the skies

The first pioneering efforts at QKD worldwide were made outside China. In the US, the first field test of QKD networking was carried out nearly 20 years ago in 2003, by the US Defense Advanced Research Projects Agency (DARPA). By the following year, this network connected Harvard University, Boston University and the offices of the research firm Bolt, Beranek and Newman.

Researchers in other countries followed suit, and by 2010, the European FP6 project was operating a QKD backbone network in Vienna, while researchers in Tokyo had demonstrated QKD operating at high enough rates for video encryption.

At the same time, Chinese researchers were also paving the way for their own innovations in QKD research. From 2005 to 2013, a series of experiments demonstrated that quantum entanglement and quantum teleportation could be achieved over kilometer-long distances in open air and run from experimental hot-air balloon platforms, demonstrating the feasibility of satellite-based QKD which could penetrate the thickness of the Earth’s atmosphere with quantum communications.

By August 2016, the quantum science satellite Micius was launched from Jiuquan in China’s Gansu province. One of its missions was to test satellite-based QKD, and in 2017, a group of researchers announced that Micius was indeed capable of sending secret keys over QKD at a rate of thousands of bits per second, over a distance exceeding 1,200 km.

This enabled a team of Chinese and Austrian researchers to hold the world’s first-ever intercontinental quantum-encrypted video call between Bai Chunli, former president of the Chinese Academy of Sciences in Beijing, and Anton Zeilinger, president of the Austria Academy of Sciences in Vienna.

Meanwhile, the University of Science and Technology of China (USTC), together with the Chinese Cable TV Corp, started to build a land-based quantum network to link up major cities along the east coast of China in 2013. The Beijing-Shanghai Quantum Secure Communication Backbone, completed in 2018, spans a distance of over 2,000 km with connections extended by 32 trusted nodes, providing QKD services to more than 150 industrial users including banks, power grids and government data centers.

In January 2021, a team of Chinese researchers led by USTC’s Professor Pan Jianwei published an update in Nature describing the current state of the Chinese QKD network. By integrating the Micius satellite link with the Beijing-Shanghai quantum backbone, the team announced that they had successfully integrated quantum communication capabilities over a distance of about 4,600 km—almost the entire width of China from east to west—with average QKD key rates of between 10 to 70 kilobits per second along each link.

The vast reach of this network shows that China has cemented a formidable lead in QKD research over the past decade. In contrast, the next largest QKD network under construction is planned to cover a 643 km distance between Columbus, Ohio, and Washington, D.C. in the US.

However, the same technology which enables this unprecedented long-range communication also demonstrates some of the lingering weaknesses in QKD technology and the path forward for research.

Shern Ren is studying towards a PhD degree in physics at the National University of Singapore. When he isn't working on the statistical mechanics of nanomachines and single-molecule systems, you may find him scratching his head over politics, education and the mathematics of Threes.

Related Stories from Asian Scientist