Let me be clear - there is no functional quantum computers on this planet.
And there will be no such technology in at least next 50 years, dont bother thinking about that.
When there will be a quantum computer, it will be able to hack btc.
That is certainly true, mate. While it may take quite some time before quantum computers become practical in the mainstream world, they could threaten the security of Bitcoin if the pioneer cryptocurrency doesn't implement a Quantum-resistant algorithm. Of course, given that Bitcoin has a wealth of talented developers, such issues might not happen in the future, as upgrades are implemented into the Bitcoin protocol.
I believe that as Bitcoin grows stronger each day, it will become resilient in a way, where no external forces can penetrate its immutable ledger. In the moment, a few cryptocurrencies have already implemented Quantum-resistance such as Nexus and QRL. While they're quite ahead of Bitcoin in protecting their ledgers against quantum attacks, I'm sure that Bitcoin will not delay in implementing a new algorithm which serves such purpose.
In the end, as mining hashrate grows, and the number of full nodes becomes greater, Bitcoin will become a truly unstoppable cryptocurrency against Quantum attacks or any other attack we could think of. Just my opinion