Lov K. Grover. “A fast quantum mechanical algorithm for database search.” In Proceedings of the 28th Annual ACM Symposium on Theory of Computing (STOC 1996), pp. 212–219. ACM, 1996.

Presented the quantum algorithm for unstructured search that finds a marked element among $N$ items in $O(\sqrt{N})$ quantum queries, compared to $\Omega (N)$ classically. This is optimal for quantum query complexity. In cryptographic terms, Grover’s algorithm gives a generic quadratic speedup against symmetric-key primitives, implying that $n$-bit security requires $2n$-bit keys in the post-quantum setting (e.g., AES-256 for 128-bit post-quantum security).