Period finding for Quantum Computers Shor's algorithm shows how a quantum computer (with sufficiently many qubits) can solve the factorization problem efficiently. Also the discrete logarithm problem can be solved efficiently with such a quantum computer.
Both boil down into finding the period of a certain function.
Can quantum computer find the period of any given function efficiently? Are there any requirements towards the function?
Closely related, the discussed post-quantum schemes (e.g. the ones submitted to NIST) do not provide a function whose period could be exploited to break the scheme?
Can quantum computer find the period of any given function efficiently? Are there any requirements towards the function?

Well, the function has to be periodic; that is, we have, for some $c &gt; 0$, $f(x) = f(x+c)$, for all $x$ [1].
For both factoring and discrete log, we have such a function; and we know how finding the period of that function will give us the answer.
However, postquantum algorithms, we don't have such a way.  For code-based schemes, no periodicity is apparent.  For lattice-based schemes, there is (periodicity is inherent in the idea of a lattice), however we don't know how to convert finding such a periodicity into (say) an answer to the shortest vector problem.

[1]: Actually, I believe Shor's algorithm will work even if the above relation holds for most $x$; however for the functions of interest, true periodicity holds...