Current encryption methods protect information sent over the Internet relatively well, but the data is not completely safe from unauthorized access. In the future, a quantum internet could solve exactly this problem.
Physicists have already demonstrated several times that by exchanging entangled – i.e. quantum mechanically coupled – photons, an unwanted eavesdropping attack never goes undetected and a data transfer cannot be eavesdropped by a third party.
However, almost all of these pilot tests involved point-to-point connections between the transmitter and receiver. For a complex network, each transmitter would have to be linked to each receiver, which would correspond to an exorbitant number of data lines for many participants. But now Chinese researchers have succeeded the construction and operation of a more complex quantum network with a total of 46 nodes in the city of Hefei without numerous point-to-point connections. What is currently the world’s largest quantum network can be seen as a small but important step towards a global quantum internet.
Three years of testing in Hefei
Teng-Yun Chen’s team from the University of Science and Technology of China in the eight million city of Hefei linked a total of 40 computers with its quantum network. These were located in the buildings of banks, universities and the city administration. The researchers connected around a third of each computer in three subnetworks, each between eleven and 18 kilometers apart. To transmit the entangled photon pairs, they used commercially available fiber optic cables with a transmission wavelength of 1,550 nanometers.
For almost three years, Chen and colleagues tested the transmission of quantum physically encrypted codes between the three subnetworks. Three switching modules or alternatively relay stations (relays) were used. The switches served as a relay station for the photons and made direct point-to-point connections between all 40 computers superfluous. The disadvantage: if only one switch fails, the connections to all computers in the connected subnetwork are also interrupted.
Alternatively, several relay stations could guarantee the exchange of quantum keys between all senders and receivers. Due to possible diversions, these relay stations are more robust than the links via switches. But they have another disadvantage: the quantum key of a transmitter is read out at a relay station and a new key is generated and forwarded. There is a potential security gap here, as the transmitted data could be intercepted here. A relay station would therefore have to be specially protected against unauthorized access in order to be considered trustworthy.
The quantum researchers tested both variants in their network with switches or relay stations. The transmission of the quantum keys is still relatively slow. It took up to five minutes before a reliable connection between two computers could be established. In addition, the network only allowed a modest data rate of 49.5 kilobits per second – comparable to the speed of telephone modems in the early 1990s.
This Hefei quantum network shows how individual computer clusters can be connected to one another via an exchange – the switches or relay stations. Building on this principle, quantum cryptographic keys could now also be exchanged via clusters that are significantly further apart – for example via satellite channels. The relay stations designated as “trustworthy”, however, still represent a vulnerable point. In the future, quantum networks that could do without such switching centers would be ideal.
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