The Chinese “Micius” satellite has successfully set up the world’s most secure video conference, using quantum cryptography to connect scientists in Europe and China for an unhackable, intercontinental chat.
The feat marks another milestone for the satellite, officially called Quantum Experiments at Space Scale (QESS), which only last year was making headlines for transmitting an “unbreakable” quantum code to the Earth’s surface.
What is quantum cryptography?
Quantum cryptography hinges on a technique called quantum key distribution (QKD). When a secure conversation happens normally, a set of random numbers called a one-time pad are used by the two parties to encode and decode the message. But it’s difficult to tell if this key has been intercepted by a third-party eavesdropper or not. With a quantum key, the code is sent using quantum particles such as photons, and it is always possible to tell if these have been observed.
This means that, no matter how powerful the computers of the potential hackers, the laws of physics state that any attempt to look at the key will be flagged by the system. If it is, a new key is sent out, until it can be verified that only the parties involved in the connection have seen it.
The problem with quantum cryptography is that photons can only travel a certain distance before light absorption disrupts their use. Unless you’re sending a code within a few hundred kilometres of each other, it’s going to be hard to have an unhackable conversation.
This is where Micius comes in. The satellite solves this problem by sending the quantum key from orbit to a ground station. In this latest experiment, Micius did this twice, over two locations. The first was a station at Xinglong in China’s Hebei province. Then, after the Earth had rotated, it did it again at a station in Graz in Austria – sending the one-time pad, encoded in photons.
The next part of the challenge was to get the key from these ground stations to their destinations in Beijing’s Chinese Academy of Sciences in and Vienna’s Austrian Academy of Sciences respectively. To do this, the scientists used ground-based quantum communication across optical fibers. Finally, the video conference itself was secured by the Advanced Encryption Standard (AES), refreshed every second by 128-bit seed codes.
China sends unhackable quantum code from space
“We have demonstrated intercontinental quantum communication among multiple locations on Earth with a maximal separation of 7600 km,” the researchers write. “Our work already constitutes a simple prototype for a global quantum communications network. To increase the time and area coverage for a more efficient QKD network, we plan to launch higher-orbit satellites and implement day-time operation using telecommunication wavelength photons and tighter spatial and spectral filtering.”
The video conference was held in September 2017 and lasted 75 minutes. While the work is pioneering, there are a number of clear limitations to the system. For one, the QKD method means having to wait for a satellite to orbit across the Earth to two locations, which not only takes time, but also leaves a window in which the satellite could be – at least in theory – hacked. This would be hard to do, but it is one part of the system not “unbreakable” by the laws of quantum physics.
Further refinements of the system, as well as more QKD-capable satellites will aim to iron out these kinks, but the experiment remains a success and will surely have governments across the globe pricking up their all-hearing ears.