As I write this book, quantum computers are in their infancy. There are no more than 20 quantum computers in the entire world, none of which contain more than about 50 qubits, or quantum bits. I write this chapter knowing that much or all of it may be outdated, or proven wrong, even before the book gets released. Much of the mathematics used in quantum mechanics and quantum computing is well beyond the scope of this book, so parts of this chapter will simply mention quantum methods and algorithms without any explanation of how they work.
The basis for quantum computing is the quantum bit, or qubit. A qubit has two basis states that are denoted |0〉 and |1〉, corresponding to the 0 and 1 states of an ordinary bit in a conventional computer. The notation |1〉 is called bra-ket notation. When the angled brace is on the left, like 〈0| it is called bra, so 〈0| is read “bra-0.” When the angled brace is on the right it is called ket, so |1〉 is read “ket-1.” The notation was invented by Nobel Prize winner English physicist Paul Adrien Maurice Dirac.