It has been explained to me, albeit, in layman's terms, that one of the reasons our modern cryptography works so well on classical computers is that they rely on prime factorization which classical computers don't do so well. This has been key to maintaining our computers and networks secured. One of the things Quantum computers do better than classical computers is prime factorization. How will the advent of Quantum computing impact cryptography? Will technologies like blockchains and bitcoin be affected?
Quantum computers are a reality and not a myth anymore! Companies like IBM, Google have invested billions in their quantum computing project and has seen a preliminary success. It's definitely a threat to cryptography but not an immediate one!
Quantum computers are exceptionally costly to build and only a couple of nations have shown interest in it! If you think hackers will use such computers to launch an attack on cryptocurrencies like bitcoin, then you are daydreaming. Quantum computing is not yet a threat to cryptography, at least not now and not in immediate future!