Quantum entanglement is one of the hottest scientific subjects of the 21st century. For the first time, it has been accomplished in space.
A team of Chinese scientists carried out a satellite-based distribution of a pair of entangled photons over a distance of more than 1,200 km. The pair of photons remained entangled after crossing long distances.
This satellite-based technology opens up promising prospects for practical quantum communication and experiments on fundamental quantum optics over previously impossible ground distances, said Pan Jianwei, Academician of the Chinese Academy of Sciences.
This result was achieved by the first quantum satellite in the world, the Quantum Experiments at Space Scale (QUESS), also named Micius, which was launched by China on August 2016.
The experiment was carried out by two satellite descending links with a total length of between 1,600 km and 2,400 km. The efficiency of the links obtained was much higher than that of the direct two-way transmission of two photons used by telecommunication fibers, said Pan, chief scientist of QUESS.
Quantum entanglement is a phenomenon of quantum physics so disconcerting that Albert Einstein described it as “a ghostly action at a distance” in 1948.
Scientists have discovered that when two entangled particles are separated, one particle can, for unknown reasons, instantly affect the action of another very distant particle.
Quantum physicists have great interests in the distribution of entangled particles over increasing distances as well as in the study of the behavior of entanglement under extreme conditions.
Previously, the distribution of entangled particles was achieved only over a distance of 100 km because of the loss of photons in the optical fibers or in Earth’s free space.
One way to improve the distribution lies in the protocol of quantum repeaters, whose practical use is however hampered by the challenges of quantum storage and the reading efficiency, Pan said.
Another way is to use technologies based on satellites and space, as a satellite can easily cover two remote locations on Earth. The main advantage of this method is that most of the photon transmission path is almost in a vacuum, with absorption and decoherence close to zero.
After feasibility studies, Chinese scientists developed and launched QUESS with the goal to study entanglement in depth. Three earth stations cooperate with QUESS: the Delingha Observatory in Qinghai, the Nanshan Observatory in Xinjiang and the Gaomeigu Observatory in Yunnan.
One photon of an entangled pair was sent to Delingha, and the other to Gaomeigu. The distance between these two ground stations is 1,203 kilometers. The distance between the satellite in orbit and the two stations varies from 500 to 2,000 kilometers, Pan said.
Because entangled photons can not be amplified as conventional signals, new methods are needed to reduce the attenuation of the link in the entanglement distribution between the satellite and the ground. In order to optimize the link’s efficiency, Chinese scientists have combined a narrow beam divergence with the high-bandwidth, high-precision acquisition, pointing and tracking technique.
By developing a spatial source of a ultra-bright two-photon entanglement with the APS high precision technique, the team established an entanglement between two photons with a distance of 1,230 kilometers.
Compared to the previous methods on the distribution of entanglement by the direct transmission from the same source to two photons – using respectively the most common commercial and best performing telecommunications fibers – the Chinese Satellite approach is up to 17 orders of magnitude higher, he explained.
Distributed entangled photons have direct utility for the distribution of entangled quantum keys, which for the moment is the only way to establish secure keys between two remote locations on Earth without depending on a trusted relay, he added.