> Comparing to classic computers, their practical usability has been clear since Babbage time
Is this true? I thought that in the 40s-50s there was some argument over whether practical computers could really be built given that vacuum tubes were so unreliable. Von Neumann wrote gave a series of lectures in 1952 (eventually transcribed into an article) showing how it could be done: http://arep.med.harvard.edu/gmc/Von_Neumann_1956ro.pdf , and the argument became more or less moot once transistors arrived).
There's perhaps an analogy here to be made with quantum error correction, but that seems to be a lot harder...
The first transistors are 1947. We don't use that sort of transistor today, but unlike the vacuum tube it would have been obvious that a computer built from these transistors was viable. You can't get to the iPhone from there, but room size machines that do arbitrary Turing computation are only expensive. So that means it's in the realm of supersonic flight. If you must do it you can, but maybe you'll decide it's too expensive.
By 1950 then, the only question is whether we should, not whether we can.
Check out this one: https://en.wikipedia.org/wiki/Tabulating_machine
Yes, not exactly a computer, definitely not a von Neumann one, but close enough to see how such thing would be useful. Also according to Wikipedia, this is the first thing to ever be called "super computing" device. That's what I mean - there's a long history of continuosly improving practical devices that led to the modern computer. For QC, we don't have that history.
Depends on what you mean by "computer", but being able to reliably calculate even just simple arithmetic has clear benefits. In the 1800s there would be countless clerks doing some sort of bookkeeping who'd benefit from it.
I suppose double entry would be your error correction. That also benefits from having a calculating machine.
Is this true? I thought that in the 40s-50s there was some argument over whether practical computers could really be built given that vacuum tubes were so unreliable. Von Neumann wrote gave a series of lectures in 1952 (eventually transcribed into an article) showing how it could be done: http://arep.med.harvard.edu/gmc/Von_Neumann_1956ro.pdf , and the argument became more or less moot once transistors arrived).
There's perhaps an analogy here to be made with quantum error correction, but that seems to be a lot harder...