The researchers of the Faculty of Electrical Engineering and Informatics at Budapest University of Technology and Economics (BME) are investigating satellite-based quantum communication. It is still an open question whether the Chinese satellite using 850 nm wavelength or the planned European satellite operating at 1500 nm will perform better. According to BME’s simulations based on realistic technological parameters, there is no clear winner. If the weather is hazy, losses are smaller at 1500 nm, but in clear weather 850 nm performs better. The reason behind this is that aerosols attenuate 850 nm photons more, however, the longer 1500 nm wavelength is more sensitive to atmospheric turbulence such as the air shimmering above a hot surface. At 1050 nm (which is currently not used by any large-scale planned mission) performs between the other two wavelengths and can potentially exhibit the strengths of both.

Meanwhile, researchers of BME are working on other questions related to satellite quantum communication. For example, they are actively examining the consequences of multiple independent quantum key distribution service providers being present in a network. All quantum key distribution is based on the fact that observing a particle changes its properties. If photons are used to transmit a password (a so-called secret key) an eavesdropping attempt can be detected based on these changes. However, this does not mean, that keys can be transmitted arbitrarily far. With current technology, a large number of trusted nodes are needed between the two endpoints of communication. Unfortunately, it is enough for the weakest link among these trusted nodes to be vulnerable, and that means the secret keys are no longer confidential. Researchers at BME came up with a solution that combines multiple keys from multiple service providers to produce a more secure key. This key remains secure if just one of the service providers remains reliable. Satellite quantum communication is ideal for this system since the communication endpoints can connect to a large number of independent providers, each of which operates only a handful of repeater nodes between the two endpoints.

Máté Galambos, a doctoral student at the BME Department of Network Systems and Services talked about the actual results in two different events in May. He participated in the Hungarian Space Forum held between May 3-5 in Sopron. He also gave a presentation at the ESA ScyLight Workshop on Optical and Quantum Communication held in Athens by the European Space Agency between May 15-17.