Reading a Quantum Book

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.