[[Classical Information|Classical information]] is usually stored in terms of [[Bit|bits]], a system that can take two values. It does not really matter what these bits are made out of. They could be black and white playing cards, electric current flowing or not, or a magnetic domain on a hard drive. The important part is that all classical information can be encoded in bits and a bit is the smallest amount of classical information. The *qubit* (short for **quantum bit**) is the analog for [[Quantum Information|quantum information]]: the smallest unit of quantum information that all quantum information can be decomposed into. Just as a bit, a qubit has to special states: the two [[Quantum State| quantum mechanical states]] $\ket{0}$ and $\ket{1}$. In physics, such a system is called a two-level system since we are considering two energy levels. ![[two-level-system.excalidraw.light.svg]] The quantum state of a *qubit* can be conveniently visualized on a [[Bloch Sphere|Bloch sphere]]. In contrast to a classical [[bit]], a qubit can be in [[Superposition|superposition]]: the state is not only $0$ or $1$, but there is a probability to be in either state. The qubit only takes a distinct value (with a certain probability) once it is [[Measurement|measured]]. At this point the [[Wave Function|wave function]] of the qubit collapses and we see a classical outome. >[!read]- Further Reading >- [[Bit]] >- [[Quantum Mechanics]] >- [[Quantum State]] >- [[Measurement]] >- [[Superposition]] >[!ref]- References