At the same time, quantum computers pose a major threat to the asymmetric encryption system used to secure most electronic data, including blockchain. This system relies on math problems that take too long for a classical computer to solve. The only way to crack this encryption is to reverse factor a large semi-prime number to its original primes. Such a calculation takes eons for a classical computer, but will be instantaneous for a large universal quantum computer—even against blockchain. Charles Harvey Jr., senior adviser for American Defense International, has said, “I call the day quantum computers are able to break classic computer encryption methods ‘Q-Day.’ Q-Day is coming.”

https://www.nationalreview.com/2018/02/quantum-computing-blockchain-technology-threat/