# 6

Qubits and Quantum Logic Gates

A computation can be broadly defined as a transformation of one memory state into another. Put slightly differently, a computation is a function that transforms information [281]. In the case of classical digital computing, the fundamental memory unit is a *binary digit *(bit) of information. Functions that operate on bits of information are called *logic gates*. Logic gates are Boolean functions that can be combined into *circuits *capable of performing addition and multiplication, as well as more complex operations. In logic gates, the number of output bits does not have to be the same as the number of input bits.

A computation may seem to be an abstract mathematical concept but it always requires some physical system in order to be executed. It does not matter what this physical system is: billiard balls, electric switches, transistors, or anything else – the computation is substrate independent. However, it is always some physical process...