Perfect eavesdropping on a quantum cryptography system

From arXiv: Perfect eavesdropping on a quantum cryptography system, by Gerhardt, Liu, Lamas-Linares, Skaar, Kurtsiefer, Makarov. The abstract:

The stated goal of quantum key distribution (QKD) is to grow a secret key securely between two parties with a minimum of additional assumptions. The number of assumptions has been continuously reduced, from requiring the validity of quantum mechanics in early QKD, to more general constraints on the laws of physics in device-independent QKD. Despite steady theoretical progress in dealing with known limitations of current technology, in practice the security of QKD relies not only on the quantum protocol but on the physical implementation. A variety of attacks have been conceived to exploit weaknesses of current systems. Here we demonstrate the first full field implementation of an eavesdropper attacking an established QKD connection. The eavesdropper obtains the complete 'secret' key, while none of the results measured by the legitimate parties indicate a breach in security. This confirms that non-idealities in physical implementations of QKD can be fully exploitable.

Position-Based Quantum Cryptography

Here's an interesting paper: Position-Based Quantum Cryptography by Nishanth Chandran, Serge Fehr, Ran Gelles, Vipul Goyal, and Rafail Ostrovsky.You can get the paper here on arXiv and there is a good popular science summary here on Technology Review. Quantum cryptography has some great applications, adding in location makes it even more so.

A quantum crypto network in China

This sounds impressive: Field experiment on a robust hierarchical metropolitan quantum cryptography network. Like a lot of things in quantum information, the leap between the lab and practical real world use is a big one. What I found most interesting about this is the use of quantum routers.

As the article states, routers are needed because of the no-cloning theorem. The no-cloning theorem states that you cannot copy the state of an arbitrary quantum system. In other words you cannot do a copy and paste. You can transfer it, essentially a cut and paste, via teleportation. You cannot do the copy and paste because to copy the system you basically end up doing a measurement, which alters the state of the system.