Bits and Qubits

The most fundamental building block of the classical computer is the bit. A bit is a variable with only two possible values: 0 or 1. The smallest conceivable storage for a bit involves a single elementary particle of some sort. Consider a particle with a spin-1/2 characteristic that when measured is either $+1/2$ or $-1/2$. We could encode 1 to be $+1/2$ and 0 to be $-1/2$, and if we could measure and manipulate the spin of such a particle, then we could theoretically use this particle to store one bit of information. The spin-1/2 particle, or any two state system that behaves in a quantum manner, could instead be the fundamental building block of a quantum computer. We will call a two state quantum system a qubit, to denote that it is analogous to a classical bit. When a classical bit is measured, the value observed will always be the value stored. Quantum physics states that when we measure a qubit we will find it in one of two states, which we can label 0 and 1. The differences between the qubit and the bit arise from the possible state of a qubit between measurements.