Quantum physics gets a bad rap. The behaviour of the atoms and particles it describes is often said to be weird, and that weirdness has given rise to all manner of esoteric notions – that we live in a multiverse, say, or that the reality we see isn’t real at all. As a result, we often overlook the fact that quantum physics has had a real effect on our lives: every time you glance at your smartphone, for instance, you are benefiting from quantum phenomena. But the story of what quantum theory is good for doesn’t end there. As our mastery of quantum phenomena advances, a new of crop of technologies designed to harness them more directly promises to have a huge impact on science and society. While quantum teleportation and quantum sensing sound exotic and intriguing, the technology that holds the most transformative potential is the one you have probably already heard of: quantum computing. This article is part of a special series celebrating the 100th anniversary of the birth of quantum theory. Read more here. If you believe the hype, quantum computers could accelerate drug development, discover revolutionary new materials and even help mitigate climate change. But while the field has come a long way, its future isn’t entirely clear. Engineering hurdles abound, for starters. And what often gets lost in the race to overcome these challenges is that the very nature of quantum computing makes it difficult to know exactly what the machines will be useful for. For all the bombast, researchers are quietly confronting the same existential question: if we could build the quantum…