Quantum computing is able to short-circuit asymmetric encryption, aka public key infrastructure (PKI). This is paired-key two-way encryption. It is the digital certificate based infrastructure that secures HTTPS communications, so all of the secure communications on the internet (credit card transactions, SSH, IPSec, etc.) is secured this way. This technique is very new, it's only been around since the 1970s, all of it is ultimately based on the RSA algorithm for two-way encryption. For this algorithm, encryption keys come in pairs. What one key encrypts, only the other key can decrypt.
Quantum can theoretically defeat PKI thousands of times faster than normal computers can. PKI provides the "keys" that hodlers are always talking about. Every hodler owns his "private" key that is revealed to no one else. The "key" is proof of ownership of a datablock on the chain. Quantum can steal the ownership keys for blockchain elements.
Quantum computing has no special advantage over symmetric encryption. This is single-key two-way encryption. It is a centuries-old standard that has many variants, but uses the same key to both encrypt and decrypt. One-time pads remain an essentially unbreakable version of symmetric encryption. Quantum computing cannot do symmetric decryption any faster than a normal computer.
Hashing is one-way encryption. It creates a signature string from the original data, but the string cannot be converted back to the original text. However, if the original text is changed, the hash changes as well, so hashing algorithms are used to verify the integrity of a message. Blockchain "chains" its database blocks together using hashes. Quantum computing cannot do anything at all to hashing algorithms. SHA-256 is a hash algorithm.
