Under Quantum Lock And Key

China leads the world in quantum key distribution (QKD) technology, which promises to lay the foundations for an ‘unhackable’ quantum internet.



The road ahead

Lest you think the technology is mature enough for you to head out to the shops and buy a router to connect yourself to the quantum internet, there are many technical challenges to be overcome. A central limitation of QKD is the fragility of the quantum states used to carry information, or qubits. The same no-cloning theorem which forbids an attacker from copying a qubit also makes it exceptionally difficult to amplify a quantum signal.

These hard technical problems place fundamental limits on the distance over which a qubit can be successfully transmitted, even when using fiber optic cables with the highest grades of purity.

Researchers around the world are focused on developing quantum repeater technology, which could theoretically use quantum teleportation to transfer information from one qubit to the next without ever having to perform a classical measurement, but these devices have so far been too slow to be incorporated into a practical QKD network.

Instead, China’s land-based QKD links are patched together by so-called ‘trusted relays,’ which create secure links between two distant QKD stations in two steps. The relay first obtains two separate secret keys using QKD, one from each station.

It then encrypts the first secret key with the second, and sends the result back to both stations, allowing both stations to deduce the pair of secret keys while leaving an attacker with only an encrypted transmission. The flaw in this scheme, however, is that the relay itself will hold records of both keys. Therefore if it cannot be trusted, communications between the two stations become vulnerable.

Even with this vulnerability, an extended QKD network is still more secure than a classical network since the latter can be compromised by eavesdropping anywhere along a communication link. Nonetheless, closing the loophole of trusted relays is another key area of continuing research.

Chinese researchers have also been at the forefront of developing ‘twin-field QKD,’ in which two stations attempting to share a key will separately broadcast their qubits to a central detector, which then announces the results of its joint measurements. The lack of a direct link allows both stations to communicate over twice the distance, while advanced algorithms allow the stations to construct a secure key even if the central detector is unreliable.

Chinese researchers have already demonstrated a direct 511 km link using this technology, obtaining transmission rates ten times faster than what would be achievable with previous QKD techniques. On this front, European researchers are not far behind, having successfully tested this technology over 600 km-long fiber optic cables.

But while the European setup used long-spooled cable to simulate the long-distance link, the Chinese team managed to deploy their technology under actual field conditions between distant cities.

Shern Ren is studying towards a PhD degree in physics at the National University of Singapore. When he isn't working on the statistical mechanics of nanomachines and single-molecule systems, you may find him scratching his head over politics, education and the mathematics of Threes.

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