Building out a supercomputer capable of breaking cryptography is exactly the kind of thing I expect governments to be working on now. It is referenced in the article, but the analogy to the Manhattan Project is clear.

Prior to 1940 it was known that clumping enough fissile material together could produce an explosion. There were engineering questions around how to purify uranium and how to actually construct the weapon etc. But the phenomenon was known.

I say this because there’s a meme that governments are cooking up exotic technologies behind closed doors which I personally tend to doubt.

This is almost perfect analogy to the MP though. We know exactly what could happen if we clumped enough qubits together. There are hard engineering challenges of actually doing so, and governments are pretty good at clumping dollars together when they want to.

> There were engineering questions around how to purify uranium and how to actually construct the weapon etc. But the phenomenon was known.

FWIW, constructing a weapon with highly enriched uranium is, relatively, simple. At the time, the choice was made to use a gun-type weapon that shot a projectile of highly enriched uranium into a a "target" of highly enriched uranium. The scientists were so sure it would work that the design didn't necessitate a live test. This was "little boy", which was eventually dropped on Hiroshima.

Fat Man utilized plutonium which required an implosion to compress the fissile material that would set off the chain reaction. This is a much more complex undertaking, but it's much more efficient. Namely, you need much less fissile material, and more of that fissile material is able to participate in the chain reaction. This design is what allows for nuclear tipped missiles. The same principles can be applied to a U-235 based weapon as well.

The implosion based design is super interesting to read about. One memorable aspect is that the designers realized that applying a tamper of uranium (U-238) around the fissile material allows for significant improvement in yield. The chain reaction is exponential, so the few extra nanoseconds that the uranium keeps the fissile material together leads to significant increase in yield.

https://en.wikipedia.org/wiki/Little_Boy

https://en.wikipedia.org/wiki/Fat_Man

The Manhattan project employed some significant % of all of America. A project of that scale will likely never happen again.

It was also about far more than the science. It was about industrializing the entire production process and creating industrial capability that simply did not exist before.