In the cat and mouse game of preparing and eavesdropping on secret messages, quantum encryption trumps all. At least, that’s what we’ve been told.

The truth is a little more complex. Quantum key distribution, the quantum technique by which a classical encryption key can be transferred, is perfectly secure in theory. In practice, there are a number of loopholes that can give an eavesdropper a grandstand view of the conversation.

Here’s one loophole. The security of quantum encryption schemes depends on our inability to make a copy of a quantum state. If that were possible, Eve could make a copy of the message and pass on the original without anybody being the wiser. But in the quantum world, copying anything destroys the original, so the sender and receiver can always tell if they’ve been overheard by examining the error rates in their message. If it rises above a certain limit, the line is not secure.

That would be pretty convincing were it not for our ability to make imperfect copies of quantum states without destroying the original. That’s a loophole that an eavesdropper can exploit to extract information from a quantum message without the sender or receiver knowing. It should work as long as Eve is careful to keep the error rate below the critical limit.

Today, Yuta Okubo from the University of Tskuba in Japan and a few mates outline the design of a quantum eavesdropper that works on just this principle. They’ve yet to build their device but the publication of its plans should raise the blood pressure in a few government agencies and more than one hi-tech start up that has been selling quantum encryption as a new generation of perfectly secure communication.

Ref: arxiv.org/abs/0806.1778: Proposal of an Eavesdropping Experiment for BB84 QKD Protocol with 1→3 Phase-covariant Quantum Cloner