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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).

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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.

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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Ella Rose
    Commented Sep 20, 2019 at 18:04

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