Quantum Computing

Abstract

Digital computers are based on devices that can take on only two states, one of which is denoted by 0 and the other by 1. By concatenating several 0s and 1s together, 0–1 combinations can be formed to represent as many different entities as desired. A combination containing a single 0 or 1 is called a bit. In general, n bits can be used to distinguish among 2ⁿ distinct entities and each addition of a bit doubles the number of possible combinations. Computers use strings of bits to represent numbers, letters, punctuation marks, and any other useful pieces of information. In a classical computer, the processing of information is done by logic gate. A logic gate maps the state of its input bits into another state according to a truth table. The simplest non-trivial classical gate is the NOT gate, a one-bit gate which negates the state of the input bit: 0 becomes 1 and vice versa. Thus digital computers contain switches called logical gates which open and give a high output voltage only when certian conditions are satisfied at their inputs (of which they have more than one). There are six types of gates AND, NAND, NOR, OR, NOT and XOR. The truth table for the first four given below for two inputs A and B, a high output or input (for example 5V) is shown by a 1 and a low output or input (for example 0V) by a 0.