For some public key systems, the search becomes polynomial. Luckily, we know of ones for which that is not the case (as far as we know), and so if a practical

For some public key systems, the search becomes polynomial. Luckily, we know of ones for which that is not the case (as far as we know), and so if a practical quantum computer could be built we would just deploy new cryptosystems (pricey, but not end-of-the-world pricey).