How do we protect the privacy of a communication if we cannot trust the devices used to communicate?

Researchers from IPhT, the University of Basel and ETH Zurich have provided innovative answers to this question, which is at the heart of research in quantum cryptography.

Hackers in possession of quantum computers pose a serious threat to some of today's crypto-systems. An interesting solution uses encryption methods based on keys produced by quantum principles. However, current quantum encryption protocols assume that the devices used to communicate are known and trustworthy. Otherwise, a door is opened for eavesdropping.

A team of physicists around Nicolas Sangouard from IPhT and the University of Basel, as well as Professor Renato Renner from ETH Zurich, have developed the theoretical bases of a communication protocol that offers ultimate protection of the privacy and can be implemented experimentally. This protocol guarantees security against hackers having a quantum computer with communication devices related to "black boxes", the reliability of which is unknown.

The researchers published their results in the journal Physical Review Letters and filed for a patent.

Article in Physical Review Letters

Physical Review Letters Abstract:

Device-independent quantum key distribution provides security even when the equipment used to communicate over the quantum channel is largely uncharacterized. An experimental demonstration of device-independent quantum key distribution is however challenging. A central obstacle in photonic implementations is that the global detection efficiency, i.e., the probability that the signals sent over the quantum channel are successfully received, must be above a certain threshold.

We here propose a method to significantly relax this threshold, while maintaining provable device-independent security. This is achieved with a protocol that adds artificial noise, which cannot be known or controlled by an adversary, to the initial measurement data (the raw key). Focusing on a realistic photonic setup using a source based on spontaneous parametric down conversion, we give explicit bounds on the minimal required global detection efficiency.

Link to the article by University of Basel and University of Zurich