I did study quantum mechanics, but it is going back a few years and this is a fast-moving field.

With my admittedly limited expertise, I would agree with the point quoted above. Simplistically, outcomes in quantum mechanics occur when the quantum wave function collapses, and the act of interfering with ("measuring") a quantum system triggers this collapse. So whilst a true quantum computer would find cracking any classical encryption to be quite straightforward, using its immense power to simply brute-force its way through, a quantum encryption system is another matter entirely. Quantum encryption methods are theoretically tamper-proof and theoretically 100% secure, as any attempt to break the encryption collapses the wave function and destroys the ability to read the data. (You know the Schrödinger's Cat thought experiment? The cat is neither alive nor dead until the check is made - it exists in a superposed combination of states, and it is the act of checking that collapses the probability function into a definite alive or dead outcome.)

I say "theoretically" tamper-proof and "theoretically" 100% secure because as I say it's a fast-moving field and who knows what advances tomorrow may bring? But certainly quantum cryptography as currently understood should provide a very robust security mechanism.