There's no such thing as quantum computing. The current prototypes can just about add two two-bit numbers together.

Still not

According to the NISTIR 8105 Report on Post-Quantum Cryptography, a large-scale quantum computer, if ever built, would be able to compromise many of the current public-key cryptosystems in use. In practice, there are known algorithms for exploring the quantum mechanical phenomena that would efficiently solve problems such as integer factorization and the discrete logarithm over various groups. Thus, any cryptographic scheme with security associated with these problems would be automatically compromised. Yet more specifically, NIST provides a table of the impact of practical large-scale quantum computers over well-known cryptographic solutions in which we see that the symmetric key cryptosystem AES would need larger keys, the SHA-2, SHA-3 hashing algorithms would need larger outputs while asymmetric key cryptosystems such as RSA, ECDSA, ECDH, and DSA would no longer be secure.

Blockchain makes use of asymmetric key cryptosystems and hashing algorithms (for digital signature routines and other verification purposes) and, in theory, would be affected if practical large-scale quantum computers were available.

However, as of right now, this is a big if.