IBM just announced "a new 53-qubit quantum computer".
How does it compare to classical computers, performance-wise, for cryptanalytic tasks? E.g. finding a 48- or 64-bit value whose SHA-256 has a certain value (edit: or factoring the product of two distinct primes, or computing some discrete logarithm).
How does it compare to classical computers, performance-wise, for cryptanalytic tasks?
Not at all - IBM's quantum computer cannot perform any nontrivial cryptanalytic task.
For one, 53 physical qubits far too few to do anything interesting; for example, implementing SHA-256 would take thousands of logical qubits.
For another, the qubits are not even close to be reliable enough. The IBM quantum computer cannot do any quantum error correction - this means that, as it performs operations on the qubits, the errors pile up. Any interesting cryptanalytic task requires us to perform millions (or more) of quantum operations; even a slight amount of error accumulation would overwhelm any result.
