Processors used to be mechanical, using macroscopic switches and gears. Then they began to use valve switches. Then they used microscopic circuitry that you could only see with a powerful magnifying glass or a microscope. Now, they're using nanoscale transistors and all manner of quantum tricks so the manufacturers can cram more power into a smaller area.

The same can be said of storage media: there was a time when modified televisions and sound waves travelling through mercury were used as volatile storage. Now we have tiny little SRAM chips that can only be viewed with an electron microscope.

Modern integrated circuits are designed taking into account quantum effects, down to the electron. There must have been a starting point for this, when an effect discovered, identified, isolated or explained using quantum mechanics was first used in hardware as a result of this discovery.

At what point were quantum effects first deliberately designed into classical computing hardware (as a result of their aforementioned discovery, identification, isolation or explanation using quantum mechanics) in order to accomplish a specific effect that could not have been predicted using scientific models not involving quantum mechanics, so that the quantum effect occurs in normal use of the hardware?

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    I'm not sure this is actually an answerable question - Even watching a dial on a mechanical computer and explaining what you (exactly) see somehow involves quantum mechanics, explaining the behaviour of a simple component like a diode or an electronic tube as well - quantum effects are everywhere. Building a computer, however, does not require understanding or even being able to explain what exactly is happening on atomic and sub-atomic level. It's enough that you know the effects you are observing to build a machine that uses them. – tofro Jun 16 '18 at 14:01
  • ...we don't exactly know what causes gravity, for example, even today. Still a mechanical device can use it. – tofro Jun 16 '18 at 14:04
  • @tofro Those effects can be predicted using classical mechanics. The question is badly worded at present; any suggestions for improvement? – wizzwizz4 Jun 16 '18 at 14:12
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    I think your question can only be fixed by asking for quantum effects that have to be understood before applied. But the answer would be simply none. Engineering is, unlike Physics, not trying to explain things - We're happy if something works even if we don't understand why ;) – tofro Jun 16 '18 at 14:24
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    I suspect that the OP has heard that modern CPU designs must take quantum effects into account because at such small scales, quantum effects become significant and cannot be ignored. But it's my understanding that designs take these effects into account by sizing and spacing features to avoid these effects, rather than to actually make use of them. – Ken Gober Jun 17 '18 at 15:19

The question makes it sound as if quantum effects were something that is a natural extension of miniaturization. It might seem like it since quantum effects are usually considered minor, but that's not related. Quantum effects - and their handling can well be observed in macroscopic systems and need to be accounted for. So throughout history, it was way more a fight to avoid quantúm effects in computing than to 'use' them. Given that, that fight got intensified a lot with shrinking dimensions.

Already tube computers had to work around quantum effect - or more correct benefited from work done before. In this case, it's about the field emission. In a vacuum tube, a sufficiently strong electric field can make electrons tunnel out of an unheated kathode. This effect was discovered (and documented) several times between the 1880s and 1930, but it wasn't until Fowler and Nordheim published their equations in 1928, explaining its quantum nature.

Basically, it's one of many quantum tunneling effects - and electron tunneling is, due to this, maybe the best researched of all.

As for practical tube design, this meant that any filament (not only kathodes) must avoid pointy areas, as this would lead to higher local voltage, resulting in increased tunneling. this has been discovered by trial and error. So next time you look at a tube you might see way more than just wires.

Oh, and it's not just a thing of the past. The constantly lowered voltage of modern CPUs is also due to this effect (*1) as otherwise too many electrons would just tunnel through the isolation - an effect called leaking, and one of the main features to be handled when designing a new process.

There are many more such effects in electronics that had to be taken into account during the development of today's computers. So pick your choice.

*1 - Among others effects, as lowering the voltage for example also lowers the current thus reducing the generated heat and stoping circuits from melting.

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This is extremely hard to answer. Very probably the first computer actually using quantum effects was the first one that worked electrically:

Everybody knows that in order to be able to use electricity, you need conductive and non-conductive materials (a conductor and an isolator), to guide the flow of electrons where you want them, and keep them away from places you don't want them.

Modern Physics is only able to explain why some materials are conductors and some aren't (and some are semiconductors) using quantum mechanics (see Fermi Energy, valence, electronic bands theory, band gaps). The why, however, was never really relevant to Engineering (and isn't even strictly necessary today). While Physicists want to know exactly why something behaves like it does, Engineering is happy when something can predictably work (like copper being conductive, rubber not). So, even if nobody could really explain exactly what happened and why, Volta and Galvani were able to successfully build isolators and conductors, building the foundations for modern electricity. Very often, in order to build something, you do not need to understand what exactly happens, it is enough to be able to reliably reproduce an effect.

The Zuse Machines and the Eniac which are considered the first "real" computers both used electricity, based on conductors and isolators - So it could be said they also were the first to actively use quantum effects.

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