Quantum Computing in the NISQ era and beyond

I am a theoretical physicist with a background in particle physics and cosmology, but for more than 20 years much of my research effort has been directed toward quantum information science. I’m drawn to this field because I feel we are now in the early stages

1 of exploring a new frontier of the physical sciences, what we might call the complexity frontier or the entanglement frontier. This new frontier, while different than the frontier of particle physics or cosmology, is very fundamental and exciting. Now, for the first time in human history, we are acquiring and perfecting the tools to build and precisely control very complex, highly entangled quantum states of many particles, states so complex that we can’t simulate them with our best digital computers or characterize them well using existing theoretical tools. This emerging capability will open the door to new discoveries.

For a physicist like me, what is really exciting about quantum computing is that we have good reason to believe that a quantum computer would be able to efficiently simulate any process that occurs in Nature. We don’t think that’s not true for classical (i.e., non-quantum) digital computers, which (as far as we know) can’t simulate highly entangled quantum systems. With quantum computers we should be able to probe more deeply into the properties of complex molecules and exotic materials, and also to explore fundamental physics in new ways, for example by simulating the properties of elementary particles, or the quantum behavior of a black hole, or the evolution of the universe right after the big bang.

Our confidence that exploration of the entanglement frontier will be rewarding rests largely on two principles: (1) quantum complexity (our basis for thinking that quantum computing is powerful), and (2) quantum error correction (our basis for thinking that quantum computers are scalable to large devices solving hard problems). Underlying both of these principles is the idea of quantum entanglement. Entanglement is the word we use for the characteristic correlations among the parts of a quantum system, which are quite different from the correlations we encounter in everyday life. To understand the concept of entanglement, imagine a system with many parts, for example a book which is 100 pages long. For an ordinary classical 100-page book, every time you read another page you learn another 1% of the content of the book, and after you have read all of the pages one by one you know everything that’s in the book. But now suppose instead that it’s a quantum book, where the pages are very highly entangled with one another. Then when you look at the pages one at a time you see only random gibberish, and after you have read all the pages one by one you know very little about the content of the book. That’s because the information in the quantum book is not imprinted on the individual pages; it is encoded almost entirely in how the pages are correlated with one another. If you want to read the book you have to make a collective observation on many pages at once. That’s quantum entanglement, the essential feature making information carried by quantum systems very different from information processed by ordinary digital computers.