this post was submitted on 10 Feb 2026
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Why Haven’t Quantum Computers Factored 21 Yet? (hackaday.com)
submitted 3 days ago by technocrit@lemmy.dbzer0.com to c/technology@lemmy.world
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If you are to believe the glossy marketing campaigns about ‘quantum computing’, then we are on the cusp of a computing revolution, yet back in the real world things look a lot less dire. At least if you’re worried about quantum computers (QCs) breaking every single conventional encryption algorithm in use today, because at this point they cannot even factor 21 yet without cheating.

In the article by [Craig Gidney] the basic problem is explained, which comes down to simple exponentials. Specifically the number of quantum gates required to perform factoring increases exponentially, allowing QCs to factor 15 in 2001 with a total of 21 two-qubit entangling gates. Extrapolating from the used circuit, factoring 21 would require 2,405 gates, or 115 times more.

underlying article: https://algassert.com/post/2500

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[–] BreadstickNinja@lemmy.world 9 points 2 days ago

There's a good 3blue1brown video that walks through the actual math they can perform, which does a lot to temper expectations about what they can accomplish and how much of an improvement they would be over digital computers for certain types of problems.

Here's the YouTube link, or search for "But What Is Quantum Computing? (Grover's Algorithm)" on your preferred front-end: https://youtu.be/RQWpF2Gb-gU