(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.
I'm very familiar with the Schrodinger cat thought experiment and I've seen it mentioned several times here on this forum. I think the conclusion of this discussion is that right now in its current state is that quantum computers are a few years off from becoming a threat to traditional algorithms and even then Bitcoin already has options readily available to combat the issue when it does become a realistic threat.