  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 identical-looking 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.
