Have scientists killed off quantum cryptography?

Firstly, you can relax; this bit of research probably isn't going to totally spoil the potential of quantum computing to provide totally secure cryptography systems. But the scientists behind is say it could lead to better protocols for quantum cryptography in the future.

The underlying principle is fairly well known. Briefly: users of a quantum key distribution (QKD) system would be able to use entangled quantum bits to exchange secure keys to a cipher. Any attempt to eavesdrop on the exchange would disrupt the system in a measurable way, alerting the users that their key exchange had been snooped, allowing them to abort the now-insecure communication.

But a group of scientists now suggest that theoretically at least, this might not be the case after all. According to PhysOrg, researchers have shown that a highly entangled quantum state they describe as 'antisymmetric' can fail to offer the total secrecy needed for secure key exchange.

One of the researchers, Matthias Christandl, says that in practice creating an entanglement of the kind his research identifies would be very difficult, so it offers very little threat to the possibility of secure key exchange in the real world.

However, he and his colleagues are determined to demonstrate that some entangled states offer no secrecy at all. This, they say, would debunk the notion that quantum cryptography ought to be synonymous with total secrecy, and could help determine protocols for quantum cryptography by determining which types of entanglement would be secure.