A classical computer is a collection of information storage units called bits. These physical devices have two states each, denoted 0 and 1. Any computation that a computer performs is essentially the result of the manipulation of the states of bits.
Similarly, a qubit is a physical system with two quantum states, and it is the fundamental physical component of a quantum computer. A qubit can exist in one of the two states or – unlike classical computers – a superposed state with contributions from both states.
This superposition is a quantum feature that the bits in conventional computers don’t exhibit. Superposed states, also known as coherent superpositions, are important in quantum information-processing protocols. However, superpositions are fragile. The fragility arises out of the interaction between the qubit and other systems. The more the number of interaction channels, the faster the superposition “decoheres” and the qubit ends up in one of the two states.
