  We propose a conceptual design for a quantum blockchain. Our method involves
encoding the blockchain into a temporal GHZ (Greenberger-Horne-Zeilinger) state
of photons that do not simultaneously coexist. It is shown that the
entanglement in time, as opposed to an entanglement in space, provides the
crucial quantum advantage. All the subcomponents of this system have already
been shown to be experimentally realized. Furthermore, our encoding procedure
can be interpreted as nonclassically influencing the past.
