Quantum cryptography allows communication that is guaranteed to be secure, thanks to the laws of physics. And it is becoming increasingly important.
Physicists have long known that quantum computers will be able to break almost all other types of cryptography. Since these devices are becoming more capable, the writing is on the wall for conventional encryption. So commercial businesses, governments, and the military are all waiting with bated breath for practical quantum cryptography systems to be developed.
But there is a problem. The quantum cryptography relies on individual photons to carry quantum information. But even the best optical fibers can carry these photons only so far—around 200 kilometers—before light absorption makes the process impossible. So quantum cryptography has never worked over much longer distances.
Today that changes, thanks to an extraordinary Chinese satellite launched in in 2016. The Micius satellite has racked up a number of milestones in the year or so since it started operating. Last summer, it teleported the first object from Earth to orbit—a single photon.
Now the satellite has set up the first intercontinental quantum cryptography service. Researchers have tested the system by setting up a secure videoconference between Europe and China. For the first time, the security of this videoconference was guaranteed by the laws of physics.
The method is straightforward. Quantum cryptography relies on what’s called a one-time pad to guarantee privacy. This is a set of random numbers—a key—that can be used by two parties to encode and decode a message.
Traditionally, the problem with one-time pads is in ensuring that only the transmitter and the receiver have them. How can both parties be sure that no eavesdropper has copied the key while it is distributed?
This problem is neatly solved by sending the key using quantum particles such as photons, since it is always possible to tell whether a quantum particle has been previously observed. If it has, the key is abandoned and another sent until both parties are sure they are in possession of an unobserved one-time pad.
That’s quantum key distribution—the crucial process at the heart of quantum cryptography. After both parties have the key—the one-time pad—they can communicate over ordinary classic channels with perfect security.
The Micius satellite simply distributes this key from orbit. Because it is in a sun-synchronous orbit over the poles, the satellite passes over every part of the Earth’s surface at roughly the same local time each day.
So when the satellite is over the Chinese ground station at Xinglong in China’s northern Hebei province, it sends the one-time pad to the ground, encoded in single photons using a well-established protocol. As the Earth rotates beneath the satellite and as the ground station at Graz in Austria comes into view, Micius sends the same one-time pad to the receiver there.
The two locations then both possess the same key that allows them to initiate completely secure communication over a classic link.
However, the experiment goes one step further. The goal was to set up a videoconference between the Chinese Academy of Sciences in Beijing and the Austrian Academy of Sciences in Vienna, so the key has to be distributed securely to both these locations. And for that the teams use ground-based quantum communication over optical fibers.
Finally, they set up a video link secured by the Advanced Encryption Standard (AES) that is refreshed every second by 128-bit seed codes. In September, they held a pioneering videoconference that lasted for 75 minutes with a total data transmission of roughly two gigabytes.
“We have demonstrated intercontinental quantum communication among multiple locations on Earth with a maximal separation of 7,600 kilometers,” say the teams, which are led by Anton Zeilinger at the University of Vienna and by Jian-Wei Pan at the University of Science and Technology of China in Hefei, China.
There are some potential weaknesses in the system to work on for the future. Perhaps the most significant is that the satellite is considered secure during the time it takes to connect the two ground stations. That may well be true—who could hack an orbiting satellite?—but this security is not guaranteed by the laws of physics. However, the teams say that this can be addressed in future designs with an end-to-end quantum relay.
Whatever the shortcomings, this is impressive work. It is a proof-of-principle demonstration of secure communication on a global scale. “Our work points towards an efficient solution for an ultralong-distance global quantum network, laying the groundwork for a future quantum Internet,” say Zeilinger, Jian-Wei, and their colleagues.
Plenty of governments, military operators, and commercial businesses are eager for a similar capability. So it surely won’t be long before commercial versions of the Micius satellite are selling this kind of secure communication around the world. With China leading the way.
Ref: arxiv.org/abs/1801.04418 : Satellite-Relayed Intercontinental Quantum Network
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