It can only pose an imminent threat to bitcoin security it can’t destroy bitcoin and moreover, bitcoin has overcome so many threats in the past.
Although, Quantum Computer will surpass the processing power of today’s classical computers, and if it does then it could break RSA (Rivest–Shamir–Adleman) encryption, a tool used to secure data transmission on the Internet. In a similar vein could also break the digital signatures used in Bitcoin and other cryptocurrencies. And the upshot of that is not good.
I believe that bitcoin will overcome this threat.