Qubits and Other Concepts
To understand the modern classical computer, we must understand the concept that information is physical and is stored by a physical means, be it a light that is turned on or off, an electrical circuit being open or closed, or a wheel spinning one way or the other. These are different formats for representing information. In a classical computer, the elementary unit of information is the bit, which is measured as a 0 or a 1. A bit has only two possible states and can only represent one unit of information. Computers perform calculations and storage operations using arrays of bits. For example, if a computer has a 3-bit memory register, that register can store only one state out of eight possible combinations (011, 001, and so on). So if the object were to apply some operations to the range of possible numbers, the operations would be carried out in a linear fashion; the computer would perform eight different operations on eight different permutations.
Quantum bits, or "qubits" for short, behave in an entirely different manner. A qubit is the basic representation of data in a quantum system. Like the classical bit, it is a physical representation of physical information that can be measured and manipulated. Qubits can be combined into quantum registers, and operations can be carried out on them in essentially the same manner as on a classical computer. The differences between the qubit and the bit come from what the qubit can store when it is not being measured. Although a classical bit can exist in only one of its two possible states, a two-state qubit can be prepared in multiple possible states. In other words, a qubit can exist as a zero, a 1, or simultaneously as both 0 and 1, with a numerical coefficient representing the probability for each state. This means that the state of the particle can generally be any combination of the base states. In this manner, a qubit is totally unlike a bit because a bit can exist in only the 0 or 1 state, but the qubit can exist in any combination of its states. Therein lies the secret of quantum computing: incredible mind-blowing speed in processing information.