Chinese scientists have achieved a milestone in quantum communication. They were able to send a pair of entangled photons over a distance of 1,200 kilometers (745 miles) using the satellite they launched last August. The previous record for quantum communication was about 100 kilometers (62 miles).
The Chinese team sent a transmission from the quantum satellite Micius to three Earth stations, each about 1,200 kilometers away from the other. The satellite has a laser system and a beam splitter that created two distinct polarizations of light. The system made photons oscillate in only two possible directions – one used for transmission and the other for receipt of the transmission.
The satellite is operated by the Chinese Academy of Sciences, but it’s an international project at heart. The University of Vienna and the Austrian Academy of Sciences are running the European receiving station. The success of this mission will hopefully lead to a European-Asian quantum encrypted network in the near future. The landmark study is published in Science.
Quantum communication works using the principle of entanglement, which Einstein famously described as “spooky action at a distance.” The general idea is that you can create a quantum system made of multiple particles whose properties are related to each other, so when one particle is manipulated, the other particles are affected instantaneously too, even if they are far apart.
This state is clearly delicate and that’s been the biggest obstacle to long-distance communication. This issue notwithstanding, fiber optics has successfully transmitted information long distances using relays. By having stations placed along segments of the route, researchers can increase the distance that transmissions cover, although the quantum information has to be “re-issued” like new from every relay. This is not a sustainable method for a global quantum network.
Researchers also used wireless systems, which worked moderately well in both moving and stationary transmitters. The signal losses scaled quadratically with distance, so a target twice as far would get four times less signal. And a wireless quantum transmitter needs to have a clear line of sight between the targets. Due to these issues and atmospheric turbulence, the wireless method is also limited to just a few hundred kilometers.
The satellite wasn’t just a technological test, it was also a sophisticated fundamental physics experiment. The satellite tested a quantum entanglement condition, known as Bell’s inequality, further than any experiment to date, and proved that entanglement might be “spooky”, but it’s a real phenomenon of the universe without hidden tricks.
With this demonstration and Canada's recent commitment to quantum satellites, the era of global quantum communication is suddenly a lot closer.