ITEM: Researchers at the National University of Singapore (NUS) and Singtel say they have figured out a better way of enabling quantum key distribution (QKD) over a standard optical network by making sure quantum-entangled photons stay synchronized during transmission.
In case you didn’t understand a word of that lede, we’ll break it down for you.
QKD is an emerging form of cryptography – said to be unbreakable even once quantum computers become a commercial reality – that involves leveraging two key characteristics of a modern, state-of-the-art optical network: (1) superfast data transmission speeds, and (2) the actual photons flowing inside the network.
In essence, the photons themselves can be used as encryption keys by taking advantage of the principle of quantum entanglement (put simply, this is when particles display physical properties that interact to the point that changing one affects changes in the other, regardless of how far apart they are physically).
In the case of QKD, two parties attempting to communicate securely generate an encryption key using a pattern of entangled photons. Each party can ensure that the photon sequences match when they arrive at their destination. In order for this to work, you need to be able to track the photons as they travel through the network to ensure they remain synchronized and the sequence remains intact.
That’s tricky because when photons don’t take the exact same network path, they typically run into different physical obstacles (network splices, junction boxes, etc) that affect their arrival times, which in turn jumbles the sequence, which means the keys won’t match.
The researchers from the NUS-Singtel Cyber Security Lab say their technique ensures the photons in quantum keys stay in sync even when they take different paths on multiple segments. An article on Phys.org about the project from Dr James Grieve – senior research fellow at the Centre for Quantum Technologies at NUS – describes how this works:
The technique works by carefully designing the photon source to create pairs of light particles with colours either side of a known feature of optical fibre called the ‘zero-dispersion wavelength’. Normally, in optical fibres bluer light would arrive faster than redder light, spreading out the photons’ arrival times. Working around the zero-dispersion point makes it possible to match the speeds through the photons’ time-energy entanglement. Then the timing is preserved.
Singtel says the research team at the NUS-Singtel Cyber Security Lab is “now working on developing the findings for actual use cases where quantum-resistant secure communication is needed to provide long term security, such as government, military and bank services”.
Future applications could also include better security for online payment services, as well as high-precision clock synchronization suitable for time-critical operations such as real-time big data analytics and financial trading.
However, QKD still has several obstacles to overcome before it becomes a commercially viable option for anything. For a start, QKD demos so far have typically been point-to-point scenarios requiring highly specialized equipment (including the optical cables). Security expert Bruce Schneier expressed skepticism over the prospect of widespread QKD usage in a September 2018 essay:
… Does anyone expect a system that requires specialized communications hardware and cables to be useful for anything but niche applications? The future is mobile, always-on, embedded computing devices. Any security for those will necessarily be software only.
The point about specialized hardware is one reason why researchers are focusing on getting QKD to work on existing optical networks. Indeed, the NUS-Singtel test was partly conducted on two segments of Singtel’s live optical network, prompting associate professor and project principal investigator Alexander Ling to declare that the results “indicate that current commercial fibre networks are ready for quantum key distribution.”
However, this brings up the other challenge with QKD: sustaining quantum transmissions over really long distances on terrestrial optical networks. As a case in point, the NUS-Singtel team only managed a transmission distance of 10 km on Singtel’s network. However, the researchers did manage a transmission distance of up to 80 km in the lab, and the research paper says the technology would be able to support QKD in sufficiently-fibered metros.
Meanwhile, there’s a lot of R&D in progress elsewhere to overcome the distance limitation problem. For example, a paper from Toshiba researchers earlier this month claimed it has developed a QKD technique that can reach distances of 550 km without the use of quantum repeaters on standard optical fiber.