What makes quantum computing so hard to explain?

Quantum computers, you You may have heard, they are magical Uber machines that will soon cure cancer and global warming by trying all possible answers in different parallel universes. for 15 years my blog And in other places, I have contested this cartoonish vision, trying to explain what I see as the most accurate, but ironically the most wonderful truth. I treat this as a public service and it is almost my moral duty as a quantum computing researcher. Alas, the work seems absurd: the noteworthy hype around quantum computers has only grown over the years, as companies and governments have invested billions, and as technology advances to the 50 programmable qubits that (on certain contrived standards) could truly give rise to the largest supercomputers. In the world Running for their money. And just as in cryptocurrency, machine learning, and other trendy fields, the money has come to shoppers.

Despite that, I understood it in the reflective moments. The truth is that even if you remove all bad urges and greed, it will still be difficult to explain quantum computing succinctly and honestly without mathematics. As quantum computing pioneer Richard Feynman once said of his Nobel Prize-winning work in quantum electrodynamics, if it could be described in a few sentences, it wouldn’t deserve a Nobel Prize.

Not that this stopped people from trying. Ever since Peter Shore discovered in 1994 that a quantum computer can crack most of the encryption that protects transactions on the Internet, excitement about the technology has been driven by more than just intellectual curiosity. In fact, developments in this field are usually covered as business or technology stories rather than as science stories.

That would be fine if a business or technology 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 that quantum computers won’t revolutionize everything.

Yes, they might one day solve some specific problems in minutes that (we think) would take longer than the lifetime of the universe on classic computers. But there are so many other important problems that most experts believe quantum computers will only help modestly, if any. Also, while Google and others have recently made credible claims that they have achieved artificial quantum acceleration, this was only for specific esoteric criteria (the ones I require) Help evolve). A quantum computer would likely be large and reliable enough to outperform classical computers in practical applications such as cracking ciphers and simulating the elusive chemistry.

But how can a programmable computer be faster for only some problems? Do we know any of them? And what does a “large and reliable” quantum computer mean in this context? To answer these questions, we have to delve deeper.

Let’s start with quantum mechanics. (What could be deeper?) It is very difficult to introduce the concept of superposition in everyday words. So, it’s not surprising that many writers choose an easy way out: they say superposition means “both at once,” so a quantum bit, or qubit, can be “both 0 and 1 at the same time,” whereas The classic bit can only be one or the other. They add that a quantum computer will achieve its speed by using qubits to try all possible solutions in superposition — that is, simultaneously or in parallel.

This is what I consider the fundamental error of popularizing quantum computing, which leads to all the rest. From here, it’s just a short jump to quantum computers to solve something like Travel salesperson problem By trying all possible answers at once – something almost all experts think they won’t be able to do.

The thing is, for the computer to be useful, at some point you need to look at it and read the output. But if you look at an equal superposition of all possible answers, the rules of quantum mechanics say that you’ll see and read a random answer. And if that’s all you want, you can pick one yourself.