Quantum lightQuantum light: a 2000::km fibre optic network enables quantum key cryptography between four cities in China.
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 photonsIn 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. Technical Information , Technology,Inventory,Technical Mechzone
Vol.3
5am, the Beach
As long as I was on the path I walked hard, but when I came to the black beach I had to run. For the tide was now nearly flowed; and to get through with my powder dry between the surf and the steep hill, took all the quickness I possessed. As it was, even, the wash caught me to the knees, and I came near falling on a stone. All this time the hurry I was in, and the free air and smell of the sea, kept my spirits lively; but when I was once in the bush and began to climb the path I took it easier.
The fearsomeness of the wood had been a good bit rubbed off for me by Master Case’s banjo-strings and graven images, yet I thought it was a dreary walk, and guessed, when the disciples went up there, they must be badly scared. The light of the lantern, striking among all these trunks and forked branches and twisted rope-ends of lianas, made the whole place, or all that you could see of it, a kind of a puzzle of turning shadows. They came to meet you, solid and quick like giants, and then span off and vanished; they hove up over your head like clubs, and flew away into the night like birds. The floor of the bush glimmered with dead wood, the way the match-box used to shine after you had struck a lucifer. Big, cold drops fell on me from the branches overhead like sweat. There was no wind to mention; only a little icy breath of a land-breeze that stirred nothing; and the harps were silent.
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