With the benefit of hindsight, it is easy to agree with what you say. But from the point of view of the scientists creating the first classical computers, classical fault-tolerance seemed just as difficult to them as quantum fault-tolerant computation seems to us. See von Neumann's "Probabilistic Logics and the Synthesis of Reliable Organisms from Unreliable Components" from 1952 https://static.ias.edu/pitp/archive/2012files/Probabilistic_...

Not that it is impossible for OP to be right, but the argument he is using used to be applied to classical computation and ultimately turned out to be wrong in that context.

This idea that multiple redundant voting computing units would be magic to Johnny VN seems unlikely. This is because that exact scheme was used with manual "computers" (people with desk calculators) for example in code breaking and nuclear weapon simulation. You'll find descriptions of these techniques for example in Feynman talks.

> Team member Raphael Some of JPL explains: "One way to use faster, consumer CPUs in space is simply to have three times as many CPUs as you need: The three CPUs perform the same calculation and vote on the result. If one of the CPUs makes a radiation-induced error, the other two will still agree, thus winning the vote and giving the correct result."

https://science.nasa.gov/science-news/science-at-nasa/2005/1...

Yes. In practice it can be worthwhile doing this in distributed compute. These methods (and those from sibling comments) do rely on the error rate being low, and the data compared being much smaller than the amount of compute (eg bytes in final answer << number of CPU instructions used). AFAICT such faults can be treated much like network and storage errors.

What we can’t do is have a set of CPUs where roughly every 1000th instruction fails and hope that plugging together a bunch of computers together to check each other’s progress will work. The specific issue is that the checking is wrong so frequently that you can’t “win” to solve a problem involving trillions of instructions by adding more computers. The overhead of “checking the checkers” just blows up.

What’s interesting about quantum computation is this is exactly what is proposed - have qubit error rates of (I think) around 0.1% and lots of measurement, classical compute, and feedback control to keep the computation “on track”. The whole scheme relies on the error rate for all of that classical compute step to be negligible.

There have actually been a surprising number of computing platforms that implement fault-tolerant processing. It's often called "lock-step" execution.

https://www.vmware.com/content/dam/digitalmarketing/vmware/e... (throughout)

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

https://www.intel.sg/content/dam/doc/product-brief/high-perf... (page 5)

This isn't really accurate. Fault-tolerant classical computing absolutely does exist as a field, and plenty of work has been done there. Some of the early work was done by von Neumann [1] because early computing hardware was extremely error-prone. Over time it turned out that these techniques were not really needed due to the fact that modern solid-state hardware is extremely reliable. The field of quantum computing actually resurrected a handful of ideas that were originally developed for classical computers.

More generally, nobody needs "perfect" classical computing either to make quantum computing work. Given a (quantum or classical) processor with some degree of error, the idea behind these techniques is to "boost" that into a processor with arbitrarily small error. It just turns out that with modern classical processors the error is so small that we suffer it rather than pay the cost of using these techniques.

[1] https://www.cs.ucf.edu/~dcm/Teaching/COP5611-Spring2013/Pape...