The first quantum-cryptographic satellite network will be Chinese

IN THE never-ending arms race between encryptors and eavesdroppers, many of those on the side that is trying to keep messages secret are betting on quantum mechanics, a description of how subatomic particles behave, to come to their aid. In particular, they think a phenomenon called quantum entanglement may provide an unsubvertable way of determining whether or not a message has been intercepted by a third party. Such interception, quantum theory suggests, will necessarily alter the intercepted message in a recognisable way, meaning that the receiver will know it is insecure. This phenomenon depends on the fact, surprising but true, that particles with identical properties which are created simultaneously are entangled in a way that means one cannot have its properties altered without also altering the other, no matter how far apart they are.

Researchers in several countries have experimented with the idea of quantum encryption, with some success. They have sent quantum-entangled messages through optical fibres, and also through the air, as packets of light. This approach, though, suffers from the fact that the signal is absorbed by the medium through which it is passing. The farthest that a quantum signal can be sent through an optical fibre, for example, is about 100km. Sending one farther than that would require the invention of quantum repeaters, devices that could receive, store and re-transmit quantum information securely. Such repeaters are theoretically possible, but so technologically complex that they remain impossible in practice.

An alternative is to beam entangled photons through the vacuum of space, where there is nothing to absorb them. This would mean transmitting them via satellite. Whether that can be done while preserving entanglement was, for a long time, unclear. But it is clear now. Experiments conducted recently, by Pan Jianwei, a physicist at the University of Science and Technology of China, in Hefei, have shown that it can.

The keys to the high castle

Such tests have been made possible by the launch, in August 2016, of Micius, the world’s first quantum-communication satellite. Micius (named after a Chinese philosopher of the 5th century BC, who studied optics) now orbits Earth at an altitude of 500km. Using it, Dr Pan and his colleagues have been testing the protocols that a global quantum-communications network will need to work.

Their first study, published in June, showed that entangled photons sent by the satellite to pairs of ground stations remain entangled, even when those stations are as much as 1,200km apart. Following that success, they attempted to use entanglement to “teleport” information from the ground to orbit. Information teleporting, so called because it happens without anything physical passing from one place to another, involves the sender changing a quantum aspect of one photon of an entangled pair that he has control over, and the receiver observing the same change in the other member of the pair, over which he has control. A series of such changes on successively transmitted photons can carry information, provided a code has been agreed on in advance.

To minimise the amount of atmosphere in the way, and thus the risk of signal disruption, Dr Pan and his team put their ground station for this experiment in Ngari, a region of south-western Tibet that has an altitude of 5,100 metres. They beamed one of an entangled pair of photons to Micius and kept the other on the ground. They then entangled the grounded photon with a third photon, and measured how this altered its polarisation and the polarisation of the photon on the satellite. The result, reported in July, was that the two do, indeed, change in lockstep. The team had thus succeeded in teleporting information from the ground to the satellite.

In a third study, also published in July, Dr Pan showed that Micius is able to transmit useful information, in the form of quantum-encryption keys, to a ground station in Xinglong, near Beijing. The transmission of such keys is crucial to quantum cryptography. Quantum-encryption keys are the quantum states of long strings of photons. Using one, a receiver can decrypt a message which has been encrypted with the key in question.

The security of quantum cryptography relies on the fact that eavesdropping breaks the entanglement by observing what is going on. It is a real-life example of the thought experiment known as Schrödinger’s cat, in which a cat in a box remains both dead and alive until someone opens the box to look—at which point it becomes one or the other. Though entanglement-breaking will not be noticed by the receiver of a single photon, doing it to a series of photons will be statistically detectable, alerting him that the line is insecure.

This third demonstration of Micius’s capabilities paved the way for a subsequent, successful, attempt to share a secure key between Xinglong and a station 2,500km away in Nanshan, a town in Xinjiang, China’s westernmost province. To do so, Micius sent one half of a stream of entangled photon pairs to Xinglong when it passed over the place, and held the other half on board for two hours until it passed over Nanshan on its succeeding orbit.

The next stage, scheduled to happen in about five years’ time, will be to launch a quantum-communications satellite in a higher orbit than Micius’s. The altitude Dr Pan has in mind is 20,000km, which will permit the satellite to communicate simultaneously with a much bigger part of Earth’s surface and allow him to test the feasibility of building a practical quantum-communications network. He is also hoping to put an experimental quantum-communications payload on board China’s space station, which is scheduled for completion by 2022. Having this device on board the station will mean it can be maintained and upgraded by human operators—a rare example of space-station crew doing something that could not easily be accomplished by robots. If all this goes well, the ultimate goal is a world-spanning ring of satellites in geostationary orbits.

One question Dr Pan and his colleagues particularly want to answer with their next experiments is whether entanglement is affected by a changing gravitational field. They could do this by comparing photons that stay in the weaker gravitational environment of orbit with their entangled partners sent to Earth. He also has other questions about the basic physics underlying entanglement—in particular, how it is that an entangled particle “knows” the result of changes made to its far-distant partner? That would be Nobel-prizeworthy stuff. Albert Einstein, famously, called the phenomenon of quantum entanglement “spooky actions at a distance”. Dr Pan’s work is helping to exorcise those particular ghosts.