First of all, take a look on this thread, it's a nice discussion: I don't believe Quantum Computing will ever threaten Bitcoin.

As far as I know, if a quantum computer so strong as we imagine it gets created from some organization tomorrow, it will be a threat to bitcoin mainly for the public key part. There are already people that try to find a way to reverse a public key back to its private key and I can see that they've done some good work behind all these advanced maths that I don't understand. So, since someone makes a transaction and his public key is known, he may be in danger from such attack. Reversing a public key is "possible", but currently is infeasible. I guess that that's the big problem that bitcoin will face from a quantum computer. It can be faced really easily, though. Once you make a transaction send the rest of the money to a non-used address you own.

The other catastrophic theory is that a quantum computer can brute force a sha256 result. I don't know if that'll be true. I hope not. Otherwise, bitcoin is dead. As for the 51% attack, I wouldn't worry. Even if it happens, it will be something temporary. Sooner or later, the computer will spread to the rest of the world.

As far as I know, this is just a rumor, there is no proof that quantum computers will destroy cryptocurrencies.
Until now, cryptocurrencies are still very safe, hacker cases also often occur in centralized exchanges,
and private wallets are still very safe, only cases of phishing attack so far.

I'm not sure, but quantum computers, I'm imagining, are known to have an enormous number of calculations per second. Maybe it intervenes the 2²⁵⁶/sec. Again, I'm not sure, I haven't made my search. I'm just saying some words a friend of mine told me. It may be totally BS.

Yeah, about that. How exactly are we going to do this? Bitcoin developers can't just make many changes to the code. I will be considered as fork. Also, even if they can, the damages that will have been caused from quantum computers will already be huge.

Talking about possible threats to bitcoin, quantum computing is one of the top concerns though this is still of the future and nobody can be sure when this technology will be of use to anyone. So I am expecting that by the time quantum is operating and equipped to sow chaos against cryptocurrency, Bitcoin has already evolved a lot and its security is already tight and ready to withstand any kind of threats available. So maybe we should not worry much on this beast as it is still imaginary, to say the least.

Lol!! Quantum computers are still being built and has not yet became a reality. Probably we will be able to see a real quantum computer in next 3 - 5 years. But there's no need to panic because quantum computers will not be available in the market for anyone to buy. So no group will be able to use it for destroying bitcoin! Access to such computers will be highly restricted and the developers will be using them for pre-specified tasks! So I see no reason to worry here!

Try reading this to have idea what in quantum computer https://en.bitcoin.it/wiki/Quantum_computing_and_Bitcoin . And read why you should not be afraid about it, even we already have that or we don't I don't think there will be a big affect in crypto space unless someone can bought that computer cheaply which is most likely not.


Try to read his explaination here why they think it will not threatened Bitcoin I don't believe Quantum Computing will ever threaten Bitcoin

Quantum computers won't destroy Bitcoins
Some reasons for my arguments:
1. Government websites and their softwares are far too vulnerable when it comes to **being destroyed by quantum computers** , so they won't allow these devices to be handled inappropriately but rather their distribution will be highly regulated.
2. They are going to be hella expensive so not like everyone around the corner is going to buy them.
3. Companies and people alike are researching on post Quantum cryptography which means in few years we might have foolproof plan against them.

Therefore I do believe we might find some answers in the future and at the same time the government would for sure regulate it highly like they regulate everything :'))

That's where consensus or shared agreements comes to play:


https://bitcoin.org/en/bitcoin-core/

So it doesn't mean they will have to do fork, they could all agree and improve Bitcoin against the threat of quantum computers.

In theory and in practice, such a threat still exists.  When it becomes reality, it will be too late for many to panic.  They will simply be at a loss to consider their empty wallets.  However, the first thing to be wary of is the big whales.  In this case, they will be the first to be targeted by attackers.  Let's hope that by that time they will find how to secure the cryptocurrency from this problem.