  Blockchain technology is facing critical issues of scalability, efficiency
and sustainability. These problems are necessary to solve if blockchain is to
become a technology that can be used responsibly. Useful quantum computers
could potentially be developed by the time that blockchain will be widely
implemented for mission-critical work at financial and other institutions.
Quantum computing will not only cause challenges for blockchain, but can also
be harnessed to better implement parts of blockchain technologies including
cryptocurrencies. We review the work that has been done in the area of quantum
blockchain and hybrid quantum-classical blockchain technology and discuss open
questions that remain.
