Paulo Mateus

Device-independence (DI) is the golden standard for quantum key distribution (QKD) security: it allows unconditional security based on the laws of physics even for untrusted or maliciously designed devices. Nevertheless, DI-QKD, for now, remains extremely challenging. The first proof-of-principle experiments having been performed only very recently, almost 40 years after the invention of BB84. It then becomes naturally interesting to study scenarios which may reach a compromise between experimental challenge and security: for example, by assuming than the users have a partial description of the devices. This intermediate scenario is called semi-device-independent (SDI) QKD and aims for a reasonable trade-off between the highest level of security, device-independence, and experimental feasibility.

The use of quantum systems allows for new insights which promise to revolutionize information processing. Quantum cryptography, especially key distribution, has become one of the most prominent applications of quantum technology. However, this task still requires users to be capable of performing quantum operations, such as state preparation or measurements in multiple bases. A natural question, therefore, is can users' technological requirements be reduced? In this work, we experimentally demonstrate a novel quantum key distribution protocol where users are fully classical and quantum operations are only performed by an untrusted third party acting as a server. We derive an information theoretic proof of security for our protocol along with an experimental demonstration.