I've heard quantum computing doesn't reduce time complexity from exponential to polynomial, it merely reduces the degree of the polynomial.

SpaceHodler

It's also worth noting that the records for factoring numbers with quantum are still very unimpressive: 

https://en.wikipedia.org/wiki/Integer_factorization_records#:~:text=the%20linear%20algebra.-,Records%20for%20efforts%20by%20quantum%20computers,been%20factored%20by%20several%20labs

bitcoin

Response to 1,000+ Qubit Quantum: The End of Cryptocurrency

antic

It's also worth noting that the records for factoring numbers with quantum are still very unimpressive: https://en.wikipedia.org/wiki/Integer_factorization_records#:~:text=the%20linear%20algebra.-,Records%20for%20efforts%20by%20quantum%20computers,been%20factored%20by%20several%20labs.