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Over the past decade, quantum computing<\/a> has grown into a billion-dollar industry. Everyone seems to be investing in it, from tech giants, such as IBM and Google, to the US military.<\/p>\n But Ignacio Cirac<\/a> at the Max Planck Institute of Quantum Optics in Germany, a pioneer of the technology, has a more sober assessment. \u201cA quantum computer is something that at the moment does not exist,\u201d he says. That is because building one that actually works \u2013 and is practical to use \u2013 is incredibly difficult.<\/p>\n This story is part of our Concepts Special, in which we reveal how experts think about some of the most mind-blowing ideas in science. Read more here<\/a><\/p>\n Rather than the \u201cbits\u201d of conventional machines, these computers use quantum bits, or qubits<\/a>, to encode information. These can be made in several ways, from tiny superconducting circuits to extremely cold atoms, but all of them are complex to build.<\/p>\n The upside is that their quantum properties can be used to do certain kinds of computation more quickly than standard computers.<\/p>\n Such speed-ups are attractive for a range of problems that normal computers struggle with, from simulating exotic physics systems to efficiently scheduling passenger flights or grocery deliveries to supermarkets. Five years ago, it seemed quantum computers would ameliorate these and many other computational challenges.<\/p>\n Today, the situation is a lot more nuanced. Progress in building ever bigger quantum computers has, admittedly, been stunning, with several companies developing machines with more than 1000 qubits. But this has also revealed impossible-to-ignore difficulties.<\/p>\n One major problem is that, as these computers get larger, they tend to make more errors, and finding ways to prevent or fix these has proven to be harder than expected. Last year, Google\u2019s researchers made the most notable dent in this problem so far<\/a>, but even so, fully fledged, useful quantum computers aren\u2019t here yet \u2013 as Cirac points out.<\/p>\n Because of this, the list of realistic applications for these machines may be shorter than we once hoped. Weigh the cost of building one against the smaller-than-imagined savings it could deliver, and, for many use cases, it may not make economic sense. \u201cThe biggest misconception is that a quantum computer can accelerate any problem,\u201d says Cirac.<\/p>\n So, which problems might still benefit from quantum computation? Quantum computers could break the cryptography systems<\/a> we currently use for secure communication, and this makes the technology interesting to governments and other institutions whose security could be imperiled by it, says Scott Aaronson<\/a> at the University of Texas at Austin.<\/p>\n Another place where quantum computers should still be useful is in modelling materials and chemical reactions. This is because quantum computers, themselves a system of quantum objects, are perfectly suited to simulate other quantum systems, such as electrons, atoms and molecules.<\/p>\n \u201cThese will be simplified models; they won\u2019t represent real materials. But if you design the system appropriately, they\u2019ll have enough properties of the real materials that you can learn something about their physics,\u201d says Daniel Gottesman<\/a> at the University of Maryland.<\/p>\n Quantum chemistry simulations may sound more niche than scheduling flights, but some of the possible outcomes \u2013 finding a room-temperature superconductor<\/a>, say \u2013 would be transformative.<\/p>\n