A network for quantum key distribution (QKD) spanning thousands of kilometres has been built in China. It links four quantum metropolitan area networks (QMANs) in cities in eastern China with a remote location in the far west of the country. The system comprises a 2000 km fibre optic link between the cities of Shanghai, Hefei, Jinan and Beijing and a satellite link spanning 2600 km between two observatories – one east of Beijing and the other just a few hundred kilometres from China’s border with Kazakhstan.
The network was built by Jian-Wei Pan at the University of Science and Technology of China in Hefei along with colleagues in academia and industry.
QKD uses the principles of quantum mechanics to allow two parties to share a secret cryptography key. A crucial feature of QKD is that the two parties can tell if an eavesdropper has intercepted the key while it is being shared. Once the secrecy of the key is established it can be used to exchange encrypted messages using a conventional telecoms network.
Quantum states of photons
In standard QKD implementations, information is encoded in the quantum states of photons – which are exchanged between the two parties. Photons are used because they can travel several hundred kilometres in optical fibres before their quantum information is lost. Photons can also carry quantum information between ground stations and satellites, allowing QKD to be performed between locations thousands of kilometres apart.
The Chinese network serves about 150 users and comprises more than 700 fibre links and two high-speed satellite-to-ground free-space links – all of which support QKD transmission. The fibre links are supported by 32 “trusted relay nodes” that are capable of forwarding quantum information. The individual QMANS contain trusted relay nodes as well as user nodes and optical switches. The Jinan QMAN is the largest, containing 50 user nodes supporting 95 users.
The satellite portion of the network makes use of the Micius quantum communications satellite, which was launched by China in 2016. Just one year later, Micius was used to make a QKD connection between Beijing and Vienna, which are separated by 7400 km.
The quantum space race
To ensure that large numbers of users can access the network, its architecture involves five different layers. These are a quantum physical layer; quantum logical layer; classical physical layer; classical logic layer; and an application layer.
According to the University of Science and Technology of China, Pan and colleagues will further expand the network by working with partners in Austria, Italy, Russia and Canada. The team is also developing low-cost satellites and ground stations for QKD.
The network is described on Nature.