Can quantum computers kill Bitcoin?

[achow101]

It means that the PoW will change from being practically optimization free,
as currently the case with near-optimal ASICs, to becoming extremely optimization prone,
with huge advantages available only to the most advanced and well-funded organizations
(like your favorite 3-letter agency).

That is, mining power will go from fairly decentralized to absolutely centralized.

A post-quantum bitcoin will need to move away from Hashcash to some asymmetric PoW.

Simply increasing the key space of the hash to say, 512 bits, would provide us the same security as 256 bits does today. Hashcash would not be dead, we would just need to have larger key spaces.

I don't think that the QC mining will increase the difficulty and let the blocks stay the same size. Because QC for now are reaaaaaaally rare and that's means that only few can afford to use them and few + bitcoin node are already a problem, so I don't think it will be accepted the worsening of the situation.

The difficulty and the block size have nothing to do with each other. Do not conflate the issues.

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[Searinox]

1. Bitcoins are safe from QCs as long as every address has been used only once, because the public key is only out once funds get spent from it and if they were all flushed it's too late.
2. Q-resistant digital signature algorithms exist, and could be enforced with a core update that forces all signatures to use Q-resistant algorithms after a certain block.
3. QCs will make headways in slowly, and their effects will be evident and cause shifts in the security and software community to start adapting, naturally bitcoin will move within reasonable time.

Your coins will likely be safe. Now the question is whether your operating system will.

[vino.gcs]

Let's say, quantum computers are little affordable. It won't kill bitcoins. Instead, it'll kill miners who can't afford quantum computers.

Bitcoin had been designed that not to get 21 million bitcoins at all, theoretically. But we might achieve that so soon which in turn might start the fall of bitcoins.

[Karartma1]

For those of you who are interested in the matter I would like to point out the following discussion I had with some others users in a different section.
https://bitcointalk.org/index.php?topic=1645557.msg16600877#new