An interesting post on quantum encryption is found at arXiv. The post explains why quantum encryption is not bullet-proof:
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, [the eavesdropper] 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.
He then points to an outline of a quantum eavesdropper.
