Smallest unit of information in a quantum computer. A qubit is named from the words "Quantum Binary Digit". It is actually a superposition of an infinite number of 0 and 1 states.

A physical (quantum-mechanical) system that can be configured in (at least) 2 different states (i.e. 0 and 1). It is used in quantum computers to store the information of 1 bit. Because it obeys the rules of quantum mechanics, it can be prepared in a superposition of the states 0 and 1 at the same time

Electrons serve well as qubits because they are spin 1/2 particles. Spin 1/2 particles have two (!) distinct spin eigenstates which can be used to encode binary information.

A quantum bit is a measure of data stored in a quantum information system. Simply put, 1 qubit can store an infinite amount of classical data (numbers, words, etc.) but only 1 bit can ever be extracted. This is because a qubit is a set of two numbers (a vector in 3d space,) which can go on forever. For example, the numbers could be (3.1415926535897..., 2.7182818...) The data is mostly useless, because when measured, a qubit gives a value: 1 or 0. There are algorithms, however, that allow quantum computing to use this "hidden" data more effectively than classical data, for instance by factoring large pseudoprimes.

Hardy, Yorick. Steeb, Willi Hans. "Introduction" Classical and Quantum Computing: With C++ and Java Simulations. Pg. ii-xxv.
Nielsen, Michael A. "Rules for a Complex Quantum World" Scientific American, November 2002.

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