New and Hardened Quantum Crypto System Notches "Milestone" Open-Air Test

Quantum cryptography remains the ostensibly impervious technology for which new hacks and potential vulnerabilities continue to be uncovered—perhaps ultimately calling into question its alleged unhackability. 

Yet now comes word of a successful Chinese open-air test of a new generation quantum crypto system that allows for untrustworthy relay stations. 

The previous iteration of quantum cryptography did not have this feature. Then again, the previous iteration did have successful ground and satellite experiments, establishing quantum links to transmit secret messages over hundreds of kilometers.

The present new and more secure quantum crypto system is, perhaps not surprisingly, much more challenging to implement.  

It creates its secret cryptographic keys based on the quantum interference of single photons—light particles—that are made to be indistinguishable despite being generated by independent lasers. As long as sender and receiver use trusted laser sources, they can securely communicate with each other regardless of whether they trust the detectors performing the measurements of the photons’ quantum interference.

Because the cryptographic key can be securely transmitted even in the case of a potentially hacked detector at the relay station, the new method is called measurement-device independent quantum key distribution (MDI-QKD).

“It is not far-fetched to say that MDI-QKD could be the de-facto [quantum cryptography] protocol in future quantum networks, be it in terrestrial networks or across satellite communications.” says Charles Ci Wen Lim, an assistant professor in electrical and computer engineering and principal investigator at the Centre for Quantum Technologies at the National University of Singapore; he was not involved in the recent experiment.

In their unprecedented demonstration, Chinese researchers figured out how to overcome many of the experimental challenges of implementing MDI-QKD in the open atmosphere. Their paper detailing the experiment was published last month in the journal Physical Review Letters.

Such an experimental system bodes well for future demonstrations involving quantum links between ground stations and experimental quantum communication satellites such as China’s Micius, says Qiang Zhang, a professor of physics at the University of Science and Technology of China and an author of the recent paper.

The experiment demonstrated quantum interference between photons even in the face of atmospheric turbulence. Such turbulence is typically stronger across horizontal rather than vertical distances. The fact that the present experiment traverses horizontal distances bodes well for future ground-to-satellite systems. And the 19.2-kilometer distance involved in the demonstration already exceeds that of the thickest part of the Earth’s atmosphere.

To cross so much open air, the Chinese researchers developed an adaptive optics system similar to the technology that helps prevent atmospheric disturbances from interfering with astronomers’ telescope observations.

Even MDI-QKD is not 100 percent secure—it remains vulnerable to hacking based on attackers compromising the quantum key-generating lasers. Still, the MDI-QKD security scheme offers, Zhang claims, “near perfect information theoretical security.” It’s entirely secure, in other words, in theory. 

The remaining security vulnerabilities on the photon source side can be “pretty well taken care of by solid countermeasures,” Lim says. He and his colleagues at the National University of Singapore described one possible countermeasure in the form of a “quantum optical fuse” that can limit the input power of untrusted photon sources. Their paper was recently accepted for presentation during the QCRYPT 2020 conference.

All in, Lim says, the Chinese team’s “experiment demonstrated that adaptive optics will be essential in ensuring that MDI-QKD works properly over urban free-space channels, and it represents an important step towards deploying MDI-QKD over satellite channels.” From his outside perspective, he described the Chinese team’s work as a “milestone experiment for MDI-QKD.”