Olomouc and Danish scientists increase security of quantum communication in optical networks

Vladyslav Usenko and Radim Filip
thursday 15. november 2018, 14:20 – Text: Šárka Chovancová

Significantly higher security of quantum communication in optical networks has been ensured by a new method, conceived by a team of scientists from the UP Department of Optics at Palacký University Olomouc and the Technical University of Denmark (DTU) in Lyngby. The new protocol is based on the use of nonclassical states of light which makes the information transmission more resistant to potential eavesdropping. The scientists have published their results in the prestigious journal npj Quantum Information.

At the same time, the joint team is also investigating the secure transmission of an encrypted key over thousand-mile distances by means of telecommunication satellites.

Secure quantum communication in optical networks uses the changes within the detection bandwidth of white noise, basically produced by eavesdropping during the encrypted key transmission. Whoever eavesdrops on an optical line with weak optical signal is thus always detectable. If the eavesdropping is not too strong, it is possible to use complex computer algorithms in order to sufficiently reduce the influence of eavesdropping on the encrypted key under the necessary threshold. However, in order for this computing method to be effective, it will always be complicated and slow and it will reduce the speed of the key transmission, which is the major setback of this method.

“Our team has proposed a radically different communication protocol. We used nonclassical, ‘squeezed’ states of light with reduced noise and optimal modulation so that eavesdropping cannot acquire practically any information or only the minimum. Whatever the eavesdropper hears thus becomes useless. This allows a speedup in secret key generation without demanding and slow algorithms, however at the cost of a rather shorter secret key transmitted per second. The key per second can also be extended by using many frequency channels in optical fibres, as it is done in existing modern communications,” explained Radim Filip from the UP Department of Optics.

The future use of this method in many frequency channels of optical fibres simultaneously may achieve a large capacity, dense transmission network even within large cities, and stable security with less efficient, yet fast data processing. The new protocol for quantum communication in optical networks has been tested for the first time at the Technical University of Denmark which not only has necessary sources of squeezed light and detectors, but above all, quantum key distribution of this type is its principal research programme.

“This experiment has concluded a long-term programme leading to our participation in a large CiViQ project within the starting FET Flagship on Quantum Technologies programme. We’re responsible for the key theory pack in this project. In addition to this, the Danish team is an important partner of CiViQ, responsible for the most related experimental pack. Together with the first realisation of quantum key distribution with squeezed states, this experiment has become the pillar of our research into this area,” said Vladyslav Usenko, the investigator of this prestigious project for UP.

Olomouc scientists are also aiming their attention at long-distance secure encrypted key transmission using telecommunication satellites. The first step in this direction has been made by China, where in 2017 a group led by Prof Jian-Wei Pan tested the first key distribution between a satellite and land station at a distance of 1,200 kilometres. Due to huge losses on such distances, another quantum protocol with highly sensitive detectors registering individual photons has to be used. At the same time, it is necessary to carefully analyse the trusted and untrusted parties in the protocol in order to allow secure key distribution. This challenge was immediately met with interest among German researchers and industries.

“Our activity in this direction is tied to our partner workplace in the Max Planck Institute for the Science of Light in Erlangen, Germany. Our goal is an analysis of feasibility and security of suitable protocols for satellite communication. This is important knowledge that could help another generation of young scientists to progress further towards applications and national as well as transnational telecommunication companies, as in developed countries,” said Usenko, the investigator of this project as well, to describe the strategy.

The Max Planck Institute, together with German companies Tesat-Spacecom and SES Satellites, has prepared a project for the ScyLight Programme by the European Space Agency, which involves other key teams from Europe. The participation of Czech scientists was funded by the Czech Ministry of Transport. “Without the ministry, our participation and plans in the consortium would be impossible,” noted Vladyslav Usenko.

Quantum communication technology is now at the centre of attention of the EU and its companies. Transnational support in this area of research has been provided by the latest and largest EU programme – the FET Flagship on Quantum Technologies, whose budget is one billion euros. In the first call of this programme, the Department of Optics team around Vladyslav Usenko and Radim Filip won a big key project in the field of secure quantum communications.

The new three-year project Continuous Variable Quantum Communications (CiViQ) uniting 21 international partners (including companies such as German Telekom and Huawei, French Orange, Spanish Telefonica, and laboratories such as Nokia Bell Labs in France and ICFO in Spain) will develop a platform for secure quantum communication with coherent states of light towards commercial use. Quantum key distribution has been implemented in several optical fibre networks in Austria, Switzerland, China, Japan, and the United States. Its growth has already become attractive for the industries in many developed countries.