"A Key Role for Quantum Entanglement"
Quantum Key Distribution (QKD) promises communication security that conventional cryptography cannot provide. An international team of scientists, including ETH physicists, has now experimentally demonstrated for the first time a method of QKD that uses high-quality quantum entanglement to provide broader security guarantees than previous schemes. The art of cryptography is to transform messages so that they are meaningless to all but the intended recipients. Modern cryptographic schemes, such as those that underpin digital commerce prevent adversaries from illegitimately deciphering messages by requiring them to perform mathematical operations that consume a significantly large amount of computational power. However, since the 1980s, innovative theoretical concepts have been introduced in which security is not dependent on the eavesdropper's finite number-crunching capabilities. Instead, fundamental quantum physics laws limit how much information, if any, an adversary can intercept. Security can be guaranteed in one such concept with only a few general assumptions about the physical apparatus used. Implementing such "device-independent" schemes has long been desired but remained unattainable. An international team of researchers from the University of Oxford, EPFL, ETH Zurich, the University of Geneva, and CEA (France) reports the first demonstration of this type of protocol, paving the way toward practical devices with such high security. They developed a fundamental understanding as well as practical methods for proving the security of quantum cryptographic schemes under realistic conditions such as those involving noise and imperfections. With these tools, the team was able to determine how much information might be leaked to any adversary. Once the leakage is below a certain threshold, the bound can be made arbitrarily low through appropriate post-processing. It was thus critical to successfully prove that this threshold was indeed reached in the experiment. This article continues to discuss the team's demonstration of an approach to QKD involving high-quality quantum entanglement for better security.