What makes quantum computing so hard to explain?

Quantum computers, you could hear, are magical super machines that will soon cure cancer and global warming by trying all possible answers in different parallel universes. For 15 years, the my blog and elsewhere, I’ve railed against this cartoonish vision, trying to explain what I consider to be the more subtle but ironically even more fascinating truth. I approach this as a public service and almost my moral duty as a researcher in quantum computing. Alas, the work seems sisyphus: The hype about quantum computers has only increased over the years, as companies and governments have invested billions, and technology has progressed to programmable devices of 50. qubits which (on some artificial benchmarks) can really give the biggest supercomputers in the world a run for their money. And just like in cryptocurrency, machine learning, and other hot fields, along with the money came the hucksters.

In moments of reflection, however, I understand. The reality is that even if you removed all the bad incentives and greed, quantum computing would still be difficult to explain briefly and honestly without math. As quantum computing pioneer Richard Feynman once said of the work in quantum electrodynamics that won him the Nobel Prize, if it had been possible to describe it in a few sentences, it wouldn’t have won a Nobel Prize.

It doesn’t stop people from trying. Ever since Peter Shor discovered in 1994 that a quantum computer could crack most of the encryptions that protect Internet transactions, enthusiasm for the technology has been driven by more than just intellectual curiosity. Indeed, developments in the field are generally treated as business or technological stories rather than scientific stories.

It would be nice if a business or tech reporter could honestly tell readers, “Look, there’s all this deep quantum stuff under the hood, but all you need to understand is the bottom line: Physicists are about to build faster computers that will revolutionize everything.

The problem is, quantum computers aren’t going to revolutionize everything.

Yes, they might one day fix a few specific problems in a matter of minutes that (we think) would take longer than the Age of the Universe on conventional computers. But there are many other important issues that most experts believe quantum computers will only help modestly, if at all. Moreover, while Google and others have recently credibly claimed that they have achieved artificial quantum accelerations, it was only for specific esoteric benchmarks (the ones I helped develop). A quantum computer big enough and reliable enough to outperform conventional computers in practical applications such as breaking cryptographic codes and simulating chemistry is probably still a long way off.

But how could a programmable computer be faster for just a few problems? Do we know which ones? And what does a “big and reliable” quantum computer mean in this context? To answer these questions, we need to get to the heart of the matter.

Let’s start with quantum mechanics. (What could be deeper?) The concept of layering is notoriously difficult to convey in everyday words. So, unsurprisingly, many authors take the easy way out: they say that the superposition means “both at the same time”, so that a quantum bit, or qubit, is just a bit that can be “at the same time”. times 0 and 1 at the same time. ”, Whereas a classic piece can only be one or the other. They go on to say that a quantum computer would reach its speed by using qubits to try all possible solutions in superposition, that is, at the same time or in parallel.

This is what I have come to see as the fundamental faux pas in popularizing quantum computing, the one that leads to everything else. From there, it’s only a short jump to quantum computers that quickly solve something like the roaming seller problem trying all the possible answers at once, which almost every expert thinks they won’t be able to do.

The point is that for a computer to be useful, at some point you have to look at it and read some output. But if you look at an equal overlay of all possible answers, the rules of quantum mechanics say you will just see and read a random answer. And if that was all you wanted, you could have picked one yourself.


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