Quantum computers will break a few things, but not break as much as reduce. Right now around 1000 qubits could break secp256k1 which is the curve group that is chosen for Bitcoin. This is a kolbitz curve over a 256 bit prime field. The strength of ECC is the discrete logarithm problem, the strength of RSA is prime factoring.

Now to know there are a few "theoretical algorithms" for quantum computers that can be used to "reduce the security of" said things. One of them is grover's algorithm.
https://en.wikipedia.org/wiki/Grover's_algorithm

This reduces hashing security by a factor of about 3, which weakens SHA256 to 85 bit security.
ECC gets hit pretty hard in this because quantum computers have the ability to "reduce" the discrete logarithmic problem with Shor's algorithm.
https://en.wikipedia.org/wiki/Shor's_algorithm

I built Nexus with 1024 bit SHA3 and sect571r1 ECC for the greatest quantum resistance. Higher bit length is exponential increase in security.

Thank You,
Viz.