Some Other Directions for Quantum Computing

This book is about gate-based quantum computing. In each example, we prepare qubits and apply operations with gates. Most of the gates perform reversible quantum operations. The main exceptions to the reversibility rule are the measurement gates, which destroy superposition and yield solutions to our intricate, mathematical problems.

Gate-based quantum computing is great for certain problems such as cracking RSA encryption, but, as far as we know, many other problems are more easily solved using classical computing. To search for a value in an unsorted list, you can run Grover’s algorithm on a quantum computer, but to find out how much tax you owe the government this year, you should run a Java program on a classical computer. (In fact, doing all your tax calculations by hand would probably be more efficient than relying on any kind of quantum computer for help.)

Nevertheless, gate-based quantum computing is an example of general...