Why it matters: This test represents an additional advancement in the practical use of quantum communication – a highly secure method of transmitting information that cannot be hacked. But the test is only the first step in a long road to solving technical difficulties.

Scientists from Russia and China have successfully demonstrated quantum communication over satellite. The test is significant as it portends the development of advanced encrypted communication networks that cannot be hacked by other nations and the possible establishment of a secure means of communication between BRIC (Brazil, Russia, India, China) countries. From a geopolitical perspective, it is further proof that Beijing and Moscow are deepening high-tech cooperation for military purposes.

The test used China's quantum satellite, Mozi, which was launched into orbit in 2016 and is managed mainly by the Chinese Academy of Sciences. It took place over a distance of 2,300 miles between a ground station in Zvenigorod, near Moscow, and another near Urumqi in China's northwestern Xinjiang region.

The encrypted transmission contained two images secured by quantum keys that were distributed from the Zvenigorod ground station, to the Mozi satellite in Earth orbit, and then transmitted to the station in China.

This latest test is an advancement of the research the two countries are conducting in quantum communications. Alexey Fedorov, from Russia's National University of Science and Technology and the Russian Quantum Centre, reported that Russia and China conducted their first full cycle test last year. Key to this endeavor is Mozi, which is expected to be central to developing both national and international quantum communication networks.

There are practical limitations to such a feat though. Scalability is still a problem as much of the advanced infrastructure needs to be built out.

Additionally, maintaining quantum signals over long distances remains problematic. That is because quantum communications use particles of light to transmit data and are in a highly fragile state. This process protects the data from being stolen, as the particles collapse if interfered with; however, it also limits the distance they can travel, explains Professor Marco Lucamarini from the University of York's Institute for Safe Autonomy and School of Physics, Engineering and Technology, who led a research team that successfully tested last year a fiber-optic cable to pass quantum communications under the Irish Sea between Ireland and England.

It was the longest stretch of fiber-optic cable ever used to enable quantum communications underwater. The longer the distance, however, the more likely it is that photons are lost, absorbed or scattered in the channel, which reduces the chances of the information reaching its target, Lucamarini said.