In quantum cryptography, the laws of quantum mechanics are exploited to send messages with higher security than is possible in conventional cryptographic schemes based on classical physical phenomena. In principle, quantum communication enables absolute security — that is, no adversary can intercept messages or tinker with them. But in practice such unconditional security is not realizable. One main route for an unauthorized person to ‘listen in’ is to manipulate in advance the communication devices that will be used later. Writing in *Nature Communications*, Rotem Arnon-Friedman and colleagues prove that there exist quantum-cryptographic protocols that ensure nearly optimal security even if the devices are manipulated, and that such device-independent quantum cryptography should be possible with current quantum technology.

Device-independent quantum cryptography is the ‘gold standard’ of quantum communication, as the advantages of quantum cryptography over its classical counterpart can be realized without having to worry whether the device used can be trusted or not. This is an appealing prospect, but so far device-independent quantum cryptography has been mostly a theoretical construct, with experimental requirements that are not achievable under realistic conditions.

Therefore the appeal of the new work of Arnon-Friedman, a PhD student in the group of Prof. Renato Renner in the Institute of Theoretical Physics at ETH Zurich, and co-workers in the US, France and the Czech Republic. The team developed a new theoretical concept, dubbed ‘entropy accumulation’, and applied it to quantum cryptography. They find that any attack strategy, no matter how complex, can be decomposed into a sequence of simple steps. This is very helpful for security proofs, which are notoriously hard because every possible attack strategy that an adversary may conceive has to be taken into account. With their new approach, Arnon-Friedman and her colleagues now prove, for the first time, the security of device-independent quantum cryptography in a regime that is attainable with state-of-the-art quantum technology, thus paving the way to practical realization of such schemes.

**Provided by:**

ETH Zurich Department of Physics

**More information: **Rotem Arnon-Friedman, Frédéric Dupuis, Omar Fawzi, Renato Renner, Thomas Vidick.

**Practical device-independent quantum cryptography via entropy accumulation**.

*Nature Communications*, 2018; 9 (1) DOI: 10.1038/s41467-017-02307-4

**Image: **The key to proving the security of device-independent quantum cryptography in a regime that is attainable with state-of-the-art quantum technology is the realization that any attack strategy, no matter how complex (symbolized in the lower row), can be decomposed into a sequence of simple steps (upper row).

*Credit:*Image from Arnon-Friedman et al. Nature Comms 9, 459; 2018