BME Researchers Send Photons Over The Danube

BME’s building I and tower 1 of the Millennium Tower office complex on the opposite bank of the Danube are no more than 700 metres apart – close enough for even relatively small details to be visible to the naked eye. Nevertheless, it is a significant achievement that researchers from BME’s Department of Networked Systems and Services, working in collaboration with colleagues from the Department of Atomic Physics, have established a communication link between the two buildings: they transmitted photons from one side of the Danube to the other using a quantum communication link.

“Our aim was to perform quantum key distribution through free space rather than via optical fibre. As a next step, we would like to carry out the same experiment between our ground station, which is currently under construction, and the first European quantum satellite,” Máté Galambos, a research fellow at BME, told bme.hu.

The transmitter

In simple terms, this means using photons to transmit a secret key from one location to another. According to the laws of quantum mechanics, every measurement – and therefore every attempt at eavesdropping – leaves a detectable trace, making provably unbreakable encryption possible.

This new approach is important because although the encryption methods currently in widespread use are adequate for the time being, this may not always be the case. Once quantum computers with far greater computational capacity than conventional computers can be built, our passwords and other encrypted information could become vulnerable. Such a situation could even arise without quantum computers, for example if a new mathematical formula were discovered that made it possible to break current encryption methods.

“Certain types of information, such as genetic data, require long-term protection, as they affect not only us but also our children. 

That is why we need to find a solution as soon as possible that cannot be circumvented even a hundred years from now, because the laws of physics will not allow it,” Máté Galambos added.

The essence of the experiment was that, in a custom-built optical system, one half of an entangled photon pair travelled between the transmitter and receiver telescopes, accompanied by a laser beam used for aiming and time synchronisation. The system was complemented by custom-built GPS clocks linked to an atomic clock, as well as data-processing software developed at BME.

The receiver

“We measured the polarisation of both photons in the pair. One was measured locally, on the transmitter side, while the other was measured remotely after being collected by the receiver telescope. By repeating the measurement many times, we performed a statistical test that demonstrated the high quality of the entanglement,” the researcher explained.

A functioning quantum key distribution system will, however, require significant investment in hardware development. The optical receiving station designed for satellite links is already under construction and will eventually be installed on the roof of building I. Meanwhile, researchers at the department are continuously exchanging experience with scientists at other universities working on similar systems currently under development. They recently visited the University of Innsbruck on a study trip, for example.

Click on the photo to see bigger version. The red dot visible in the window under the roof arch of Building I, in the center of the image, is the reflected light of the aiming laser.

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