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The GPS is a very handy example in explaining to a broad audience why it is useful for humanity to know the laws of general relativity. It nicely bridges the abstract theory with daily life technologies! I'd like to know an analogous example of a technology which could not have been developed by engineers who didn't understand the rules of quantum mechanics. (I guess that I should say quantum mechanics, because asking for a particle physics application could be too early.)

To bound the question:

  • No future applications (e.g. teleportation).
  • No uncommon ones (for, who has a quantum computer at home?).
  • A less frequently-cited example than the laser, please.
  • If possible, for sake of simplicity, we'll allow that the quantum theory appears in form of a small correction to the classical one (just like one doesn't need the full apparatus of general relativity to deduce the gravitational red-shift).
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    $\begingroup$ I guess you wouldn't be happy with "The Universe", - without fermions and their PEP, life would be a bit boring... :-) But on a more serious note, I think you might run into the "list questions are discouraged" problem $\endgroup$
    – twistor59
    Commented Jul 10, 2013 at 6:49
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    $\begingroup$ As a lynch pin , you would not be asking this question here. The web and all the internet technology and all the electronic technology from cathode ray tubes to transistors to ....you name it, depend on our exploration and utilization of quantum physics . $\endgroup$
    – anna v
    Commented Jul 10, 2013 at 7:11
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    $\begingroup$ I find it appropriate to ask the question because it might be that these days the forrest is so dense, we can't recognize the trees no more. One might think of a way to give a generic set of question that can be asked of a device and if all are answered positively, it's involving quantum mechanics. $\endgroup$
    – TMOTTM
    Commented Jul 10, 2013 at 8:11
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    $\begingroup$ Can I get some clarification here: Do you mean "If QM wasn't a feature of the world," or do you mean "If we did not understand QM" ? $\endgroup$
    – shieldfoss
    Commented Jul 10, 2013 at 9:45
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    $\begingroup$ @dmckee I think this question is completely different from the linked question. It speaks about technologies, but those questions speak about everyday-observed phenomena. $\endgroup$
    – Mostafa
    Commented Jul 10, 2013 at 14:57

6 Answers 6

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How about diagnostic methods in modern medicine?

Nuclear magnetic resonance (NMR) - it wouldn't even make sense to talk about it without quantum mechanics, because it depends on the quantum mechanical concept of spin

Positron emission tomography - hey, the name says it all, not only do you apply quantum mechanics, but you have a direct application of antimatter

X-ray scanning, scintigraphy and many, many more... nuclear medicine is full of direct applications of nuclear, particle and quantum physics... It's even common to find particle accelerators in oncology departments for cancer therapy! And what's a better application to mention to a common layman than "curing cancer"?

I'm sure you'll find lots of examples from medicine on the Internet :)

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    $\begingroup$ From the way the question is stated, I believe the OP rather askes for everyday boxes that we carry around (like GPS navigators) and which make use of quantum mechanics. $\endgroup$
    – TMOTTM
    Commented Jul 10, 2013 at 12:53
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    $\begingroup$ Application of general relativity in GPS technology is primarily in satellites and I'm not quite sure how handy it actually is to carry one around :) Regardless, I still think it's a pretty interesting example, for laymen, of quantum mechanics helping us stay alive, without having to explain too much why it's important, it's a no-brainer that medicine helps you stay alive ^^ $\endgroup$
    – user20250
    Commented Jul 10, 2013 at 13:04
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    $\begingroup$ I would not count X-ray because that was used way before QM but in NMR a classical approach fails relatively early and a lot of injuries require a MRI scan so +1. $\endgroup$
    – Alexander
    Commented Jul 10, 2013 at 14:29
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    $\begingroup$ you're right, I was thinking about how you need quantum mechanics to have a complete description of bremsstrahlung in X-ray tubes, but hey, silly me... you need quantum mechanics to have a complete description of pretty much everything xD $\endgroup$
    – user20250
    Commented Jul 10, 2013 at 15:41
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The first common application that comes to my mind would definitely be the LED. From there on, everything that has even remotely to do with a semi-conductor. Furthermore, these days all chemical reactivity is understood in terms of quantum mechanics.

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  • $\begingroup$ I think this applies to semiconductor devices in general (including integrated circuits such as the ones used to type this message) $\endgroup$
    – Andre Holzner
    Commented Jul 10, 2013 at 7:53
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    $\begingroup$ I hesitate to edit the nice answer by @TMOTTM so for the time beeing I add this reference as a comment : physikdidaktik.uni-karlsruhe.de/publication/ajp/diode_ajp.pdf which I find an excellent pedagogical explanation of the semiconductor diode as a light source or solar cell dealing with a simple chemical analogy: I would call "electrophotonic reactions"(?) (I ignore if the term is commonly used). $\endgroup$
    – laboussoleestmonpays
    Commented Jul 10, 2013 at 9:07
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    $\begingroup$ Semiconductors were used in actual, practical devices long before quantum mechanics was understood by engineers. In fact, I think that even the first 'LED' (light emission by current passing through a semiconductor) was observed in 190x. Granted, the progress of semiconductors would have slowed down (or come to a halt) if QM was not there to provide a good model of them, but I'm not sure whether "technology which couldn't exist if humankind didn't know QM" applies to LEDs/semiconductors. $\endgroup$
    – us2012
    Commented Jul 10, 2013 at 13:49
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    $\begingroup$ Semiconductors were used before most engineers knew quantum mechanics. But the guys who invented the LED, laser diode, and so on were as much physicists and chemists as engineers. The guy who invented the first "practically useful visible" (says Wiki) LED had studied under Bardeen, for example. $\endgroup$
    – The Photon
    Commented Jul 11, 2013 at 4:36
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Quantum theory is so integrated into every day life that I think most people would find it very difficult to imagine a world in which we'd never developed quantum mechanics.

First, without quantum physics, we would probably not understand the behavior of materials well enough to have invented modern semi-conductors. No modern electronics. No computers. No internet. No video games. No dot-com boom. No Facebook. No mass video communication. (We'd still have transistors, so we'd still have radio and 1960's quality TVs.)

We also wouldn't have developed anything like modern chemistry without quantum mechanics. We wouldn't understand why elements behave the way they do. We'd be lacking the basis for biochemistry. I think it's quite plausible that we wouldn't know about DNA or have a clue how proteins work. No hope of rational drug design. No clue how diseases work.

It's not just quantum mechanics that shapes our world either. Without quantum field theory, we wouldn't have any clue about nuclear physics. No nuclear power plants. (That's about 10% of the world's electricity budget. Doesn't sound like much, but I don't think we'd be happy if it abruptly disappeared.) Also, no nuclear weapons. Maybe this is a good thing, but I think even Harry Turtledove would have trouble imagining what the past 70 years of history would have been like without them.

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    $\begingroup$ I daresay nuclear power plant design is very much more concerned with engineering details, than with nuclear-physics ones. The principle idea predates even QM, not to speak of QFT. $\endgroup$
    – leftaroundabout
    Commented Jul 10, 2013 at 18:29
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    $\begingroup$ @leftaroundabout Could you elaborate on the history? I'd be interested in the details $\endgroup$
    – user1504
    Commented Jul 10, 2013 at 18:33
  • $\begingroup$ Actually this was a bit exaggerated. The first experiments with radioactive stuff may have been motivated partly by possible use as energy sources, but certainly no one knew how a chain reaction could be implemented. That, using the newly-discovered neutrons, was suggested by Leó Szilárd in 1933, so well after QM but still before the field theories had been settled. $\endgroup$
    – leftaroundabout
    Commented Jul 10, 2013 at 18:46
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To list some more applications:

  • UltraPrecise clocks. The most precise one, was built at NIST in 2010, based on a single aluminum atom (ion), in an ion trap. As is reported here The clock would neither gain nor lose one second in about 3.7 billion years. These clocks have many applications, from fundamental physics researches to GPS and navigation systems.

  • Quantum random number generators,(see here) which are available now, have many applications. In cryptography, you need a random source of numbers (for example for key generation). Quantum random number generators are used to provide these random numbers, using the random nature of quantum world. (much better than traditional pseudorandom numbers) Also, the quality of stochastic simulations depends on the quality (randomness) of used random numbers. ( BTW, there are online quantum random bit generators available, which generate random numbers in a lab by measuring some quantum quantity. For example see here and here. This (2nd) site has some other *fun stuff*s too!)

  • Quantum communications. Already has short-range applications.(for example, in Australia)

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  • $\begingroup$ Quantum Random number generator are not really useful for stochastic simulation. But they can be really useful for crypto. These, as well as quantum crypto would not qualify because the OP specifically excludes uncomon applications. However, your precise clock example is a good one, and is useful in GPS, which is the very example the OP likes. $\endgroup$
    – Frédéric Grosshans
    Commented Jul 10, 2013 at 12:06
  • $\begingroup$ @Mostafa: Can you explain what you mean with the last point? I only know the group of A. Zeilinger working on Quantum Cryptography and this is in Austria, not Australia..;-) $\endgroup$
    – Noldig
    Commented Jul 10, 2013 at 13:31
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    $\begingroup$ @Noldig See here. $\endgroup$
    – Zorich
    Commented Jul 10, 2013 at 13:38
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    $\begingroup$ Hardware RNGs are an example of where quantum effects may be used as entropy sources in random number generation. However, the ones that do use quantum effects (usually, having a high throughput is rather indicative of this) are unfortunately rather expensive. $\endgroup$
    – Reid
    Commented Jul 10, 2013 at 18:48
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    $\begingroup$ @Mostafa They are not useful for stochastic simulation, because pseudo-random number generator have a much higher rate, and are much cheaper, especially when we don't need cryptographic security. $\endgroup$
    – Frédéric Grosshans
    Commented Jul 11, 2013 at 15:11
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The quantum mechanical semiconductor-based transistor is the technological backbone behind all modern computers. So the internet runs off quantum mechanics.

It is a correction in the sense that you don't need quantum mechanical principals to build computers, or even transistors, but semiconductor technology makes the computers small enough to be as ubiquitous as they've become.

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    $\begingroup$ Technically, I think you do need QM to build semiconductor transistors. It's true that you don't need QM to build transistors generally, but non-semiconductor transistors have an economic value of maybe a few million dollars, whereas semiconductor transistors are probably 10s of trillions of dollars of impact, and we haven't seen the end yet. $\endgroup$
    – Alan Rominger
    Commented Jul 10, 2013 at 14:17
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    $\begingroup$ Transistors utilize QM. The things we had to do the same job before transistors were called valves. $\endgroup$
    – DJClayworth
    Commented Jul 10, 2013 at 16:09
  • $\begingroup$ @DJClayworth Or Vacuum tubes for us American's. en.wikipedia.org/wiki/Vacuum_tube $\endgroup$
    – Zoredache
    Commented Jul 10, 2013 at 16:44
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I think quantum engineering of magnetism could be an appropriate answer to the question.

Indeed the microscopic origin of magnetic field produced by iron for instance can be explained thanks to the old microscopic model by Ampere relying on a classical macroscopic electrodynamical analogy. But this model is not fully consistent with classical physics of course. It requires a quantum explanation. I use to say to my students at high school that natural magnetism is a nice exemple of quantordinary phenomenon (weak diamagnetism) while superconductivity for instance is quantastic (strong diamagnetism)!

To be more definite about quantum engineering I am basically thinking about high-tech making it possible to select, organize, design materials at the nanoscale thus implying quantum effects.

As a good example of quantum engineering of magnetism I would mention : giant magnetoresistance whose discovery was rewarded with a Noble prize to Albert Fert and Peter Grünberg in 2007!. This totally technology became standard in the read-out heads of hard disks and was of crucial importance to the accelerating trend of hard disk miniaturization.

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    $\begingroup$ You don't need to master QM to use magnets. Similarly you can work with stable matter regardless of how well you've mastered QM. $\endgroup$
    – Dan Piponi
    Commented Jul 11, 2013 at 0:39
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    $\begingroup$ Dear @DanPiponi, you are absolutely right, one doesn't need to master QM to use magnets but to shape the magnetic properties of compounds or nano or meta-materials making them dia, (super)para, (anti)ferro, hard, soft ... mastering QM definitely helps a lot ! $\endgroup$
    – laboussoleestmonpays
    Commented Jul 11, 2013 at 12:04
  • $\begingroup$ To help the reader understand the relevant point from Dan Piponi I add the following reference that was removed from my answer after some editing : "...quantum mechanics garantees the stability of matter from atoms to stars (projecteuclid.org/…) ..." $\endgroup$
    – laboussoleestmonpays
    Commented Jul 11, 2013 at 12:08

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