A team of physicists of the University of Basel and ETH Zurich have developed the theoretical foundations for a QKD communication protocol that offers ultimate privacy protection and can be implemented experimentally. This protocol guarantees security not only against hackers with quantum computers, but also in cases where the devices used for communication are “black boxes” whose trustworthiness is a completely unknown quality.
Quantum computing represents a serious future threat to today’s crypto-systems. Researchers are therefore working on new encryption methods. However, current encryption protocols assume that the communicating devices are known, trustworthy entities. But what if this is not the case and the devices leave a backdoor open for eavesdropping attacks?
Device-independent Quantum Key Distribution (QKD) 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.
While there are already some theoretical proposals for communication protocols with black boxes, there was one obstacle to their experimental implementation: the devices used had to be highly efficient in detecting information about the crypto key. If too many of the information units (in the form of entangled pairs of light particles) remained undetected, it was impossible to know whether they had been intercepted by a third party.
The team proposes 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, they give explicit bounds on the minimal required global detection efficiency.
Even if many of the information units are undetected, an “eavesdropper” receives so little real information about the crypto key that the security of the protocol remains guaranteed. In this way, the researchers lowered the requirement on the detection efficiency of the devices. (ScienceDaily)
The team published their results in the journal Physical Review Letters and have applied for a patent.