Breaking and Making Quantum Money: Toward a New Quantum Cryptographic Protocol

Public-key quantum money is a cryptographic protocol in which a bank can create quantum states which anyone can verify but no one except possibly the bank can clone or forge. There are no secure public-key quantum money schemes in the literature; as we show in this paper, the only previously published scheme [1] is insecure. We introduce a category of quantum money protocols which we call collision-free. For these protocols, even the bank cannot prepare multiple identicallooking pieces of quantum money. We present a blueprint for how such a protocol might work as well as a concrete example which we believe may be insecure.