Quantum cryptography: Hacking futile

• Its coding protocol is device-independent, making it even more secure than previous quantum cryptographic methods.
• The research lays the foundation for future quantum networks, in which absolutely secure communication is possible between far distant locations.

The Internet is teeming with highly sensitive information. Sophisticated encryption techniques generally ensure that such content cannot be intercepted and read. But in the future high-performance quantum computers could crack these keys in a matter of seconds.

Quantum key distribution (QKD) – as the jargon has it – is secure against attacks on the communication channel. This renders QKD immune against outside attacks from quantum computers, but not against attacks from or manipulations of the devices themselves. The devices could output a key which the manufacturer had previously saved and might conceivably have forwarded to a hacker. But device-independent QKD (abbreviated to DIQKD) is capable of testing the security of the devices. Theoretically known since the 1990s, this method has now been experimentally realized for the first time, by an international research group led by LMU physicist Harald Weinfurter and Charles Lim from the National University of Singapore (NUS).

In the present experiment, the physicists used two entangled rubidium atoms, situated in two laboratories located 400 meters from each other on the LMU campus, for the key distribution. The two locations are connected via a fiber optic cable 700 meters in length, which runs beneath Geschwister Scholl Square in front of the main building.

To exchange a key, the two parties measure the quantum states of their atoms. In each case, this is done randomly in two or four directions. If the directions correspond, the measurement results are identical on account of entanglement and can be used to generate a secret key. With the other measurement results, a so-called Bell inequality can be evaluated. John Bell originally developed these inequalities to test whether nature can be described with hidden variables. In DIQKD, the test is now being used “to ensure that there are no manipulations at the devices – that is to say, that hidden measurement results have not been saved in the devices beforehand,” explains Weinfurter.

The NUS protocol now uses two measurement settings. “This makes it much more difficult to intercept information. And so more noise can be tolerated and secret keys can be generated even when there is more noise,” says Charles Lim.

“With our method, we can securely generate secret keys even with uncharacterized and potentially untrustworthy devices,” explains Weinfurter. “Our work lays the foundation for future quantum networks, in which absolutely secure communication is possible between far distant locations,” says Charles Lim.

Paper: Zhang W., van Leent, T. Redeker, K. et al.: A device-independent quantum key distribution system for distant users, Nature, 2022.

Contact:
Prof. Harald Weinfurter
Experimental Quantum Physics
Faculty of Physics / LMU
Tel: +49 89 2180-2044
Email: h.w@lmu.de