ENCRYPTIONA Key Role for Quantum Entanglement

Published 1 August 2022

A method known as quantum key distribution has long held the promise of communication security unattainable in conventional cryptography. An international team of scientists, including ETH physicists, has now demonstrated experimentally, for the first time, an approach to quantum key distribution that uses high-quality quantum entanglement to provide much broader security guarantees than previous schemes.

The art of cryptography is to skillfully transform messages so that they become meaningless to everyone but the intended recipients. Modern cryptographic schemes, such as those underpinning digital commerce, prevent adversaries from illegitimately deciphering messages — say, credit-​card information —by requiring them to perform mathematical operations that consume a prohibitively large amount of computational power. Starting from the 1980s, however, ingenious theoretical concepts have been introduced in which security does not depend on the eavesdropper’s finite number-crunching capabilities. Instead, basic laws of quantum physics limit how much information, if any, an adversary can ultimately intercept. In one such concept, security can be guaranteed with only a few general assumptions about the physical apparatus used. Implementations of such ‘device-independent’schemes have long been sought after, but remained out of reach. Until now, that is. Writing in Nature, an international team of researchers from the University of Oxford, EPFL, ETH Zurich, the University of Geneva and CEA (France) report the first demonstration of this sort of protocol — taking a decisive step towards practical devices offering such exquisite security (also see this Nature article).

The Key Is a Secret
Secure communication is all about keeping information private. It might be surprising, therefore, that in real-world applications large parts of the transactions between legitimate users are played out in public. The key is that sender and receiver do not have to keep their entire communication hidden. In essence, they only have to share one ‘secret’; in practice, this secret is string of bits, known as a cryptographic key, that enables everyone in its possession to turn coded messages into meaningful information. Once the legitimate parties have ensured for a given round of communication that they, and only they, share such a key, pretty much all the other communication can happen in plain view, for everyone to see. The question, then, is how to ensure that only the legitimate parties share a secret key. The process of accomplishing this is known as ‘key distribution’.