Hackers in possession of quantum computers pose a serious threat to some of today's "cryptosystems". One interesting solution is to use encryption methods based on keys produced by quantum principles. However, current quantum encryption protocols assume that the communication devices are known and trustworthy. And if they are not, then the door is wide open to eavesdropping!

A team of physicists working in collaboration with Nicolas Sangouard from the IPhT and the University of Basel, and Renato Renner from the ETH Zürich, has developed the theoretical basis for a communication protocol that offers absolute confidentiality, and which can be implemented experimentally. This protocol guarantees security against hackers having a quantum computer with communication devices related to "black boxes" of unknown reliability.

Although some theoretical protocols for communication using black boxes have already been proposed, there is still an obstacle preventing their experimental use. The devices must be extremely efficient in detecting information on the encryption key. If too many units of information – i.e., pairs of entangled photons – go undetected, it is impossible to know if they could be intercepted by a third party.

The new protocol overcomes this obstacle with a trick: the researchers deliberately add noise to the information on the encryption key. Even if many units of information go undetected, a "spy" will receive so little real information about the encryption key that the security of the protocol remains guaranteed. The researchers can thereby reduce the demand on the detection efficiency of the devices.

"Given recent advances in the development of quantum computers, we urgently need new protection solutions," explains Nicolas Sangouard. "Our work represents a step towards achieving ultra-secure communications". 

The researchers have filed a patent application for this work.