For symmetric encryption, as your parent explained, yes, simply double the key length and you're fine. (In practice you might get away with less than double because it seems unlikely such quantum computers, even if eventually constructed, would be similarly cheap to conventional computers)

But for asymmetric (public key) encryption the larger key lengths barely help, so you'd need infeasibly enormous keys. e.g. terabyte RSA keys have been "proposed" facetiously for this purpose.

Thus, Post Quantum Encryption research is mostly interested in finding solutions for two asymmetric problems: Key Agreement (a replacement for today's Elliptic Curve Diffie Hellman) and Signatures (a replacement for things like ECDSA) that would survive an attack by a larger quantum computer able to execute Shor's algorithm.