In theory quantum key distribution systems (AKA quantum cryptography) are unbreakable. In practice they aren't. See http://www.qudev.ethz.ch/content/QSIT15/Q

In theory quantum key distribution systems (AKA quantum cryptography) are unbreakable. In practice they aren't. See http://www.qudev.ethz.ch/content/QSIT15/Q_Cryptography_Q_Hacking.pdf http://www.qudev.ethz.ch/content/QSIT15/Q_Cryptography_Q_Hac... https://phys.org/news/2015-12-quantum-cryptography-vulnerable-hacking.html https://phys.org/news/2015-12-quantum-cryptography-vulnerabl... P.S. They describe a property where the money can't be cloned which seems to be further explored in this paper which the author has commented on this thread (see amluto). https://arxiv.org/pdf/1004.5127.pdf https://arxiv.org/pdf/1004.5127.pdf (Quantum Money from Knots) .