132nd  Top portmanteaus 
representation of a qubit]]
Fundamental units of information 

In quantum computing, a qubit (pronounced /ˈkjuːbɪt/) or quantum bit is a unit of quantum information —the quantum analogue of the classical bit —with additional dimensions associated to the quantum properties of a physical atom. The physical construction of a quantum computer is itself an arrangement of entangled^{[clarification needed]} atoms, and the qubit represents^{[clarification needed]} both the state memory and the state of entanglement in a system. A quantum computation is performed by initializing a system of qubits with a quantum algorithm —"initialization" here referring to some advanced physical process that puts the system into an entangled state.^{[citation needed]}
The qubit is described by a state vector in a twolevel quantummechanical system, which is formally equivalent to a twodimensional vector space over the complex numbers.
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A bit is the basic unit of computer information. Regardless of its physical realization, a bit is always understood to be either a 0 or a 1. An analogy to this is a light switch— with the off position representing 0 and the on position representing 1.
A qubit has some similarities to a classical bit, but is overall very different. Like a bit, a qubit can have two possible values—normally a 0 or a 1. The difference is that whereas a bit must be either 0 or 1, a qubit can be 0, 1, or a superposition of both.
The states in which a qubit may be measured are known as basis states (or vectors). As is the tradition with any sort of quantum states, Dirac, or braket notation is used to represent them. This means that the two computational basis states are conventionally written as $\; 0\; \backslash rangle$ and $\; 1\; \backslash rangle$ (pronounced "ket 0" and "ket 1").
A pure qubit state is a linear superposition of those two states. This means that the qubit can be represented as a linear combination of $0\; \backslash rangle$ and $1\; \backslash rangle$ :
where α and β are probability amplitudes and can in general both be complex numbers.
When we measure this qubit in the standard basis, the probability of outcome $0\; \backslash rangle$ is $\; \backslash alpha\; ^2$ and the probability of outcome $1\; \backslash rangle$ is $\; \backslash beta\; ^2$. Because the absolute squares of the amplitudes equate to probabilities, it follows that α and β must be constrained by the equation
simply because this ensures you must measure either one state or the other.
The state space of a single qubit register can be represented geometrically by the Bloch sphere. This is a twodimensional space which has an underlying geometry of the surface of a sphere. This essentially means that the single qubit register space has two local degrees of freedom. Represented on such a sphere, a classical bit could only be on the zaxis at the top or (a single point) on the equator of the sphere, in the locations where $1\; \backslash rangle$ and $0\; \backslash rangle$ are respectively. The rest of the surface of the sphere is inaccessible to a classical bit.
There are various kinds of physical operations that can be performed on pure qubit states.^{[citation needed]}
An important distinguishing feature between a qubit and a classical bit is that multiple qubits can exhibit quantum entanglement. Entanglement is a nonlocal property that allows a set of qubits to express higher correlation than is possible in classical systems. Take, for example, two entangled qubits in the Bell state
In this state, called an equal superposition, there are equal probabilities of measuring either $00\backslash rangle$ or $11\backslash rangle$, as $1/\backslash sqrt\{2\}^2\; =\; 1/2$.
Imagine that these two entangled qubits are separated, with one each given to Alice and Bob. Alice makes a measurement of her qubit, obtaining—with equal probabilities—either $0\backslash rangle$ or $1\backslash rangle$. Because of the qubits' entanglement, Bob must now get the exact same measurement as Alice; i.e., if she measures a $0\backslash rangle$, Bob must measure the same, as $00\backslash rangle$ is the only state where Alice's qubit is a $0\backslash rangle$.
Entanglement also allows multiple states (such as the Bell state mentioned above) to be acted on simultaneously, unlike classical bits that can only have one value at a time. Entanglement is a necessary ingredient of any quantum computation that cannot be done efficiently on a classical computer.
Many of the successes of quantum computation and communication, such as quantum teleportation and superdense coding, make use of entanglement, suggesting that entanglement is a resource that is unique to quantum computation.
A number of entangled qubits taken together is a qubit register. Quantum computers perform calculations by manipulating qubits within a register. A qubyte is a collection of eight entangled qubits. It was first demonstrated by a team at the Institute of Quantum Optics and Quantum Information at the University of Innsbruck in Austria in December 2005.^{[1]}
Similar to the qubit, a qutrit is a unit of quantum information in a 3level quantum system. This is analogous to the unit of classical information trit. The term "qudit" is used to denote a unit of quantum information in a dlevel quantum system. A quiet qubit refers to a qubit that can be efficiently decoupled from the environment.^{[2]}
Any twolevel system can be used as a qubit. Multilevel systems can be used as well, if they possess two states that can be effectively decoupled from the rest (e.g., ground state and first excited state of a nonlinear oscillator). There are various proposals. Several physical implementations which approximate twolevel systems to various degrees were successfully realized. Similarly to a classical bit where the state of a transistor in a processor, the magnetization of a surface in a hard disk and the presence of current in a cable can all be used to represent bits in the same computer, an eventual quantum computer is likely to use various combinations of qubits in its design.
The following is an incomplete list of physical implementations of qubits, and the choices of basis are by convention only.
Physical support  Name  Information support  $0\; \backslash rangle$  $1\; \backslash rangle$ 

Single photon (Fock states)  Polarization encoding  Polarization of light  Horizontal  Vertical 
Photon number  Photon number  Vacuum  Single photon state  
Timebin encoding  Time of arrival  Early  Late  
Coherent state of light  Squeezed light  Quadrature  Amplitudesqueezed state  Phasesqueezed state 
Electrons  Electronic spin  Spin  Up  Down 
Electron number  Charge  No electron  One electron  
Nucleus  Nuclear spin addressed through NMR  Spin  Up  Down 
Optical lattices  Atomic spin  Spin  Up  Down 
Josephson junction  Superconducting charge qubit  Charge  Uncharged superconducting island (Q=0)  Charged superconducting island (Q=2e, one extra Cooper pair) 
Superconducting flux qubit  Current  Clockwise current  Counterclockwise current  
Superconducting phase qubit  Energy  Ground state  First excited state  
Singly charged quantum dot pair  Electron localization  Charge  Electron on left dot  Electron on right dot 
Quantum dot  Dot spin  Spin  Down  Up 
In a paper entitled: “Solidstate quantum memory using the ^{31}P nuclear spin,” published in the October 23, 2008 issue of the journal Nature^{[3]}, an international team of scientists that included researchers with the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) reported the first relatively long (1.75 segs.) and coherent transfer of a superposition state in an electron spin 'processing' qubit to a nuclear spin 'memory' qubit. This event can be considered the first relatively consistent quantum Data storage, a vital step towards the development of quantum computing.
The origin of the term qubit is attributed to a paper by Benjamin Schumacher.^{[4]} In the acknowledgments of his paper, Schumacher states that the term qubit was invented in jest (due to its phonological resemblance with an ancient unit of length called cubit), during a conversation with William Wootters. The paper describes a way of compressing states emitted by a quantum source of information so that they require fewer physical resources to store. This procedure is now known as Schumacher compression.

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Blend of quantum and bit, influenced by cubit
Singular 
Plural 
qubit (plural qubits)
