First of all, never say never. You may have heard that the blockchain is unbreakable but then wonder how can someone lose all their bitcoins from a hack if the blockchain is so secure? What’s the story? Which one is it? Safe or not safe? Secure or not secure?
Will quantum computers eventually be able to break it?
To my understanding, a quantum computer will not be able re-write the existing bitcoin blockchain and probably can’t beat a decentralized network such as bitcoin. The reason for this is that a copy of the blockchain is copied onto all the computers running the network and each copy is a verified copy, so there’s only one copy repeated everywhere. Put simply, the blockchain is a fancy name for a text file that is ‘permanent’ you can add information to it but you can’t erase what’s already written down in it. Second, if you wish to add to the chain you have to follow protocol and play by the rules. Even a quantum computer probably can’t beat the whole network, it’s way too fast to beat. I’m not saying this will always be the case. Just to be clear, as quantum computers become bigger and become better understood they might someday be able to solve the crypto-race faster than the entire bitcoin network of computers can and then, have complete control over the blockchain which means it could add to it however it wants. I don’t think it could alter the existing distributed copies [but it can add* to all of those blockchain copies] and certainly it would not be able to alter past copies that are offline. It is also possible that a blockchain network could someday use quantum cryptography to further secure the blockchain which might prevent other quantum computers from gaining control. It’s a technology race in the true sense of the word.
Ok, now that we’ve discussed security of the blockchain, let’s discuss the security of your wallet. Well, it’s really really secure and it’s really unlikely that someone with a quantum computer will break into your wallet because first they need your wallet address and then they have to use their quantum computer to crack your private key (unfortunately, that’s the easy part). Has this ever happened? Not that I’m aware, but as far as I know, it is possible.
Please keep in mind that it’s a lot easier to lose your bitcoin by other means like forgetting your password and/or deleting your wallet. You can also risk losing your bitcoin if you put them in the trust of a bitcoin exchange. If you use an exchange to trade some bitcoin you might have no problem, but if you leave your bitcoin on the exchange then there’s a risk that the exchange itself will be hacked and some or all of the bitcoin could be stolen or deleted. There are fail-safe measures against this and improvements are being made all the time at exchanges, but there are real risks to this. Also, you need never use an exchange to be able to use bitcoin, but if you want fiat money in exchange for your bitcoin, a bitcoin exchange might be your only option unless you know someone with cash that wants some bitcoin.
As to whether bitcoin is a safe investment? Well, it might be safe or it might not but that’s for another post.
*I know I’m being a bit liberal with terminology, the blockchain isn’t just a text file, it’s really a data file. Adding to the blockchain means that you reassign the holdings from one wallet to another but the past records of what has happened on the blockchain never gets deleted. It’s a log that grows with new entries but you can’t change what has already been written in the log.