&Cartridge; physics 14, p94
Bell non-locality is not enough to ensure the security of a secure quantum communication protocol called DI-QKD.
One type of secure quantum communication protocol known as device independent quantum key distribution (DI-QKD) uses peculiarities of quantum mechanics to generate its message encryption keys. These keys can be shared by end users over public channels, while remaining insensitive to eavesdroppers even over an untrusted network. Now analysis shows that a pre-established requirement for DI-QKD – known as “Bell non-locality” – does not keep some DI-QKD protocols secure  . The result could help researchers identify weaknesses in quantum encryption methods, the team says.
In DI-QKD, a device creates pairs of entangled particles and then separates them, sending one particle to one party and the other particle to another. Both parties then use different devices to take independent measurements on the received particles. The parties have no knowledge of the devices involved in this process, all of which in principle could be imperfect or even malicious. In order for the devices to be suitable for DI-QKD, the measurement results must have correlations that cannot be described in a classic way; that is, they do not have to be “local”.
To test whether this requirement is sufficient for unbreakable encryption, Máté Farkas from the Institute of Photonic Sciences, Spain, and colleagues subjected the most frequently examined DI-QKD protocols to a simple attack. An unsavory actor sets up all devices in such a way that a small fraction of the particles behaves deterministically during the measurement. The observed correlations will still not be local for honest users, but the eavesdropper knows the results of some measurements and enables him to crack the encryption key. The team shows that such an attack can compromise the protocols they are investigating.
Christopher Crockett is a freelance writer based in Arlington, Virginia.
- M. Farkas et al., “Bell non-locality is not sufficient for the security of standard device-independent quantum key distribution protocols”, Phys. Rev. Lett.127, 050503 (2021).