Also, the blockchain is not encrypted, it is cryptographically signed. Any clients "decrypt" it all day every day, that is how you verify it. The security comes from making it hard to find a hash of a block that has the right structure. I don't think it is known whether any quantum algorithm could improve significantly on this problem (there might be a small improvement from using the quantum O(sqrt(n)) Grover's algorithm instead of the classical O(n) searches, but that's not hard to work around). Note that quantum computers are only known to be exponentially faster at a handful of very specific problems, not in the general case. While this is not established fully, it is considered highly likely by those in the field to be the case.

* there are some computers that implement specific quantum algorithms that are sometimes called quantum computers, but those can't do the more interesting algorithms that could break encryption.

- No timeline for working quantum computers of the necessary size yet.

- Blockchains can and will upgrade to newer quantum-resistant cryptographic primitives when needed.

- Bitcoin addresses are only part of the public key, the full public key is only published when a transaction is made. This increases the search space tremendously, as you don't even know the public key you are trying to crack. This is another reason why it is best practice to not reuse Bitcoin addresses, always move coins to fresh ones.

This page in the Bitcoin Wiki [0] has more details about quantum computing threats to Bitcoin.

[0] https://en.bitcoin.it/wiki/Quantum_computing_and_Bitcoin

Like airplanes falling from the sky and national power grid outages type of problems.