Why haven’t quantum computers factored 21 yet?
Algorithmic Assertions
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30/08/2025
I follow the advances of quantum computers with great interest, mainly because I'm curious when, or if, they will ever be able to break current cryptography algorithms. The holy grail of the algorithms is called 'Shor's algorithm', which can factorize numbers quickly. Already in 2001, a quantum computer factorized 15! Yet since then, no quantum computer has managed to factorize larger numbers, like 21. This article explains why 15 was easy, and why 21 is much harder. Instead of 21 operations, 2,405 operations are needed, or 240'500 operations if error correction is included.