Units of information |

Information-theoretic |
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Data storage |

Quantum information |

This article may be too technical for most readers to understand.(October 2019) |

In quantum computing, a **qubit** ( /ˈkjuːbɪt/ ) or **quantum bit** (sometimes **qbit**^{[ citation needed ]}) is the basic unit of quantum information—the quantum version of the classic binary bit physically realized with a two-state device. A qubit is a two-state (or two-level) quantum-mechanical system, one of the simplest quantum systems displaying the peculiarity of quantum mechanics. Examples include the spin of the electron in which the two levels can be taken as spin up and spin down; or the polarization of a single photon in which the two states can be taken to be the vertical polarization and the horizontal polarization. In a classical system, a bit would have to be in one state or the other. However, quantum mechanics allows the qubit to be in a coherent superposition of both states simultaneously, a property that is fundamental to quantum mechanics and quantum computing.

- Etymology
- Bit versus qubit
- Standard representation
- Qubit states
- Bloch sphere representation
- Mixed state
- Operations on qubits
- Quantum entanglement
- Controlled gate to construct the Bell state
- Applications
- Quantum register
- Qudits and qutrits
- Physical implementations
- Qubit storage
- See also
- Further reading
- References

The coining of the term *qubit* is attributed to Benjamin Schumacher.^{ [1] } In the acknowledgments of his 1995 paper, Schumacher states that the term *qubit* was created in jest 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.

A binary digit, characterized as 0 or 1, is used to represent information in classical computers. When averaged over both of its states (0,1), a binary digit can represent up to one bit of Shannon information, where a bit is the basic unit of information. However, in this article, the word bit is synonymous with a binary digit.

In classical computer technologies, a *processed* bit is implemented by one of two levels of low DC voltage, and whilst switching from one of these two levels to the other, a so-called "forbidden zone" between two logic levels must be passed as fast as possible, as electrical voltage cannot change from one level to another *instantaneously*.

There are two possible outcomes for the measurement of a qubit—usually taken to have the value "0" and "1", like a bit or binary digit. However, whereas the state of a bit can only be either 0 or 1, the general state of a qubit according to quantum mechanics can be a coherent superposition of both.^{ [2] } Moreover, whereas a measurement of a classical bit would not disturb its state, a measurement of a qubit would destroy its coherence and irrevocably disturb the superposition state. It is possible to fully encode one bit in one qubit. However, a qubit can hold more information, e.g., up to two bits using superdense coding.

For a system of *n* components, a complete description of its state in classical physics requires only *n* bits, whereas in quantum physics it requires (2^{n} - 1) complex numbers (or a single point in a 2^{n}-dimensional vector space).^{ [3] }

In quantum mechanics, the general quantum state of a qubit can be represented by a linear superposition of its two orthonormal basis states (or basis vectors). These vectors are usually denoted as and . They are written in the conventional Dirac—or "bra–ket"—notation; the and are pronounced "ket 0" and "ket 1", respectively. These two orthonormal basis states, , together called the computational basis, are said to span the two-dimensional linear vector (Hilbert) space of the qubit.

Qubit basis states can also be combined to form product basis states. For example, two qubits could be represented in a four-dimensional linear vector space spanned by the following product basis states: , , , and .

In general, *n* qubits are represented by a superposition state vector in 2^{n} dimensional Hilbert space.

A pure qubit state is a coherent superposition of the basis states. This means that a single qubit can be described by a linear combination of and :

where `α` and `β` are probability amplitudes and can in general both be complex numbers. When we measure this qubit in the standard basis, according to the Born rule, the probability of outcome with value "0" is and the probability of outcome with value "1" is . Because the absolute squares of the amplitudes equate to probabilities, it follows that and must be constrained by the equation

Note that a qubit in this superposition state does not have a value in between "0" and "1"; rather, when measured, the qubit has a probability of the value “0” and a probability of the value "1". In other words, superposition means that there is no way, even in principle, to tell which of the two possible states forming the superposition state actually pertains. Furthermore, the probability amplitudes, and , encode more than just the probabilities of the outcomes of a measurement; the *relative phase* of and is responsible for quantum interference, *e.g.*, as seen in the two-slit experiment.

It might, at first sight, seem that there should be four degrees of freedom in , as and are complex numbers with two degrees of freedom each. However, one degree of freedom is removed by the normalization constraint |*α*|^{2} + |*β*|^{2} = 1. This means, with a suitable change of coordinates, one can eliminate one of the degrees of freedom. One possible choice is that of Hopf coordinates:

Additionally, for a single qubit the overall phase of the state *e ^{i ψ}* has no physically observable consequences, so we can arbitrarily choose

where is the physically significant *relative phase*.

The possible quantum states for a single qubit can be visualised using a Bloch sphere (see diagram). Represented on such a 2-sphere, a classical bit could only be at the "North Pole" or the "South Pole", in the locations where and are respectively. This particular choice of the polar axis is arbitrary, however. The rest of the surface of the Bloch sphere is inaccessible to a classical bit, but a pure qubit state can be represented by any point on the surface. For example, the pure qubit state would lie on the equator of the sphere at the positive y-axis. In the classical limit, a qubit, which can have quantum states anywhere on the Bloch sphere, reduces to the classical bit, which can be found only at either poles.

The surface of the Bloch sphere is a two-dimensional space, which represents the state space of the pure qubit states. This state space has two local degrees of freedom, which can be represented by the two angles and .

A pure state is one fully specified by a single ket, a coherent superposition as described above. Coherence is essential for a qubit to be in a superposition state. With interactions and decoherence, it is possible to put the qubit in a mixed state, a statistical combination or incoherent mixture of different pure states. Mixed states can be represented by points *inside* the Bloch sphere (or in the Bloch ball). A mixed qubit state has three degrees of freedom: the angles and , as well as the length of the vector that represents the mixed state.

There are various kinds of physical operations that can be performed on qubits.

- Quantum logic gates, building blocks for a quantum circuit in a quantum computer, operate on a set of qubits (a register); mathematically, the qubits undergo a (reversible) unitary transformation described by the quantum gates' unitary matrix.
- Quantum measurement is an irreversible operation in which information is gained about the state of a single qubit (and coherence is lost). The result of the measurement of a single qubit with the state will be either (with probability ) or (with probability ). Measurement of the state of the qubit alters the magnitudes of
`α`and`β`. For instance, if the result of the measurement is ,`α`is changed to 0 and`β`is changed to the phase factor no longer experimentally accessible. When a qubit is measured, the superposition state collapses to a basis state (up to a phase) and the relative phase is rendered inaccessible (i.e., coherence is lost). Note that a measurement of a qubit state that is entangled with another quantum system transforms the qubit state, a pure state, into a mixed state (an incoherent mixture of pure states) as the relative phase of the qubit state is rendered inaccessible. - Initialization or re-initialization to a known value, often . This operation collapses the quantum state (exactly like with measurement), which may in turn if the qubit is entangled, collapse the state of other qubits. Initialization to may be implemented logically or physically: Logically as a measurement, followed by the application of the Pauli-X gate if the result from the measurement was . Physically, for example if it is a superconducting phase qubit, by lowering the energy of the quantum system to its ground state.

An important distinguishing feature between qubits and classical bits is that multiple qubits can exhibit quantum entanglement. Quantum entanglement is a nonlocal property of two or more qubits that allows a set of qubits to express higher correlation than is possible in classical systems.

The simplest system to display quantum entanglement is the system of two qubits. Consider, for example, two entangled qubits in the Bell state:

In this state, called an *equal superposition*, there are equal probabilities of measuring either product state or , as . In other words, there is no way to tell if the first qubit has value “0” or “1” and likewise for the second qubit.

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 or , i.e., she can now tell if her qubit has value “0” or “1”. Because of the qubits' entanglement, Bob must now get exactly the same measurement as Alice. For example, if she measures a , Bob must measure the same, as is the only state where Alice's qubit is a . In short, for these two entangled qubits, whatever Alice measures, so would Bob, with *perfect* correlation, in any basis, however far apart they may be and even though both can not tell if their qubit has value “0” or “1” — a most surprising circumstance that can *not* be explained by classical physics.

Controlled gates act on 2 or more qubits, where one or more qubits act as a control for some specified operation. In particular, the controlled NOT gate (or CNOT or cX) acts on 2 qubits, and performs the NOT operation on the second qubit only when the first qubit is , and otherwise leaves it unchanged. With respect to the unentangled product basis , , , , it maps the basis states as follows:

- .

A common application of the C_{NOT} gate is to maximally entangle two qubits into the Bell state. To construct , the inputs A (control) and B (target) to the C_{NOT} gate are:

and

After applying C_{NOT}, the output is the Bell State: .

The Bell state forms part of the setup of the superdense coding, quantum teleportation, and entangled quantum cryptography algorithms.

Quantum 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.^{ [4] } A major hurdle facing quantum computing, as of 2018, in its quest to surpass classical digital computing, is noise in quantum gates that limits the size of quantum circuits that can be executed reliably.^{ [5] }

A number of qubits taken together is a qubit register. Quantum computers perform calculations by manipulating qubits within a register.

The term "**qu- d-it**" (

In 2017, scientists at the National Institute of Scientific Research constructed a pair of qudits with 10 different states each, giving more computational power than 6 qubits.^{ [7] }

Similar to the qubit, the qutrit is the unit of quantum information that can be realized in suitable 3-level quantum systems. This is analogous to the unit of classical information trit of ternary computers.

Any two-level quantum-mechanical 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 that approximate two-level 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 | ||
---|---|---|---|---|

Photon | Polarization encoding | Polarization of light | Horizontal | Vertical |

Number of photons | Fock state | Vacuum | Single photon state | |

Time-bin encoding | Time of arrival | Early | Late | |

Coherent state of light | Squeezed light | Quadrature | Amplitude-squeezed state | Phase-squeezed 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 |

Gapped topological system | Non-abelian anyons | Braiding of Excitations | Depends on specific topological system | Depends on specific topological system |

van der Waals heterostructure ^{ [8] } | Electron localization | Charge | Electron on bottom sheet | Electron on top sheet |

In a paper entitled "Solid-state quantum memory using the ^{31}P nuclear spin", published in the October 23, 2008, issue of the journal * Nature *,^{ [9] } a team of scientists from the U.K. and U.S. reported the first relatively long (1.75 seconds) 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. Recently, a modification of similar systems (using charged rather than neutral donors) has dramatically extended this time, to 3 hours at very low temperatures and 39 minutes at room temperature.^{ [10] } Room temperature preparation of a qubit based on electron spins instead of nuclear spin was also demonstrated by a team of scientists from Switzerland and Australia.^{ [11] } An increased coherence of qubits is being explored by researchers who are testing the limitations of a Ge hole spin-orbit qubit structure.^{ [12] }

- A good introduction to the topic is
*Quantum Computation and Quantum Information*by Nielsen and Chuang.^{ [2] } - An excellent treatment of two-level quantum systems, decades before the term “qubit” was coined, is found in the third volume of The Feynman Lectures on Physics (2013 ebook edition), in chapters 9-11.
- A non-traditional motivation of the qubit aimed at non-physicists is found in Quantum Computing Since Democritus, by Scott Aaronson, Cambridge University Press (2013).
- A good introduction to qubits for non-specialists, by the person who coined the word, is found in Lecture 21 of ‘‘The science of information: from language to black holes’’, by Professor Benjamin Schumacher, The Great Courses, The Teaching Company (4DVDs, 2015).
- A picture-book introduction to entanglement, contrasting classical systems and a Bell state, is found in “Quantum entanglement for babies“, by Chris Ferrie (2017).

In physics, the **no-cloning theorem** states that it is impossible to create an independent and identical copy of an arbitrary unknown quantum state, a statement which has profound implications in the field of quantum computing among others. The theorem is an evolution of the 1970 no-go theorem authored by James Park, in which he demonstrates that a non-disturbing measurement scheme which is both simple and perfect cannot exist. The aforementioned theorems do not preclude the state of one system becoming entangled with the state of another as cloning specifically refers to the creation of a separable state with identical factors. For example, one might use the controlled NOT gate and the Walsh–Hadamard gate to entangle two qubits without violating the no-cloning theorem as no well-defined state may be defined in terms of a subsystem of an entangled state. The no-cloning theorem concerns only pure states whereas the generalized statement regarding mixed states is known as the no-broadcast theorem.

**Quantum teleportation** is a technique for transferring quantum information from a sender at one location to a receiver some distance away. While teleportation is commonly portrayed in science fiction as a means to transfer physical objects from one location to the next, quantum teleportation only transfers quantum information. Moreover, the sender may not know the location of the recipient, and does not know which particular quantum state will be transferred.

**Quantum decoherence** is the loss of quantum coherence. In quantum mechanics, particles such as electrons are described by a wave function, a mathematical representation of the quantum state of a system; a probabilistic interpretation of the wave function is used to explain various quantum effects. As long as there exists a definite phase relation between different states, the system is said to be coherent. A definite phase relationship is necessary to perform quantum computing on quantum information encoded in quantum states. Coherence is preserved under the laws of quantum physics.

In quantum mechanics, a **probability amplitude** is a complex number used in describing the behaviour of systems. The modulus squared of this quantity represents a probability density.

In quantum computing and specifically the quantum circuit model of computation, a **quantum logic gate** is a basic quantum circuit operating on a small number of qubits. They are the building blocks of quantum circuits, like classical logic gates are for conventional digital circuits.

The **Bell states** or **EPR pairs** are specific quantum states of two qubits that represent the simplest examples of quantum entanglement; conceptually, they fall under the study of quantum information science. The Bell states are a form of entangled and normalized basis vectors. This normalization implies that the overall probability of the particle being in one of the mentioned states is 1: . Entanglement is a basis-independent result of superposition. Due to this superposition, measurement of the qubit will collapse it into one of its basis states with a given probability. Because of the entanglement, measurement of one qubit will assign one of two possible values to the other qubit instantly, where the value assigned depends on which Bell state the two qubits are in. Bell states can be generalized to represent specific quantum states of multi-qubit systems, such as the GHZ state for 3 or more subsystems.

A **qutrit** is a unit of quantum information that is realized by a quantum system described by a superposition of three mutually orthogonal quantum states.

In quantum information theory, **superdense coding** is a quantum communication protocol to communicate a number of classical bits of information by only transmitting a smaller number of qubits, under the assumption of sender and received pre-sharing an entangled resource. In its simplest form, the protocol involves two parties, often referred to as Alice and Bob in this context, which share a pair of maximally entangled qubits, and allows Alice to transmit two bits to Bob by sending only one qubit. This protocol was first proposed by Bennett and Wiesner in 1992 and experimentally actualized in 1996 by Mattle, Weinfurter, Kwiat and Zeilinger using entangled photon pairs. Superdense coding can be thought of as the opposite of quantum teleportation, in which one transfers one qubit from Alice to Bob by communicating two classical bits, as long as Alice and Bob have a pre-shared Bell pair.

In physics, in the area of quantum information theory, a **Greenberger–Horne–Zeilinger state** is a certain type of entangled quantum state that involves at least three subsystems. It was first studied by Daniel Greenberger, Michael Horne and Anton Zeilinger in 1989. Extremely non-classical properties of the state have been observed.

In computational complexity theory, **PostBQP** is a complexity class consisting of all of the computational problems solvable in polynomial time on a quantum Turing machine with postselection and bounded error.

The **time-evolving block decimation** (**TEBD**) algorithm is a numerical scheme used to simulate one-dimensional quantum many-body systems, characterized by at most nearest-neighbour interactions. It is dubbed Time-evolving Block Decimation because it dynamically identifies the relevant low-dimensional Hilbert subspaces of an exponentially larger original Hilbert space. The algorithm, based on the Matrix Product States formalism, is highly efficient when the amount of entanglement in the system is limited, a requirement fulfilled by a large class of quantum many-body systems in one dimension.

**Time-bin encoding** is a technique used in quantum information science to encode a qubit of information on a photon. Quantum information science makes use of qubits as a basic resource similar to bits in classical computing. Qubits are any two-level quantum mechanical system; there are many different physical implementations of qubits, one of which is time-bin encoding.

In physics, the **no-deleting theorem** of quantum information theory is a no-go theorem which states that, in general, given two copies of some arbitrary quantum state, it is impossible to delete one of the copies. It is a time-reversed dual to the no-cloning theorem, which states that arbitrary states cannot be copied. This theorem seems remarkable, because, in many senses, quantum states are fragile; the theorem asserts that, in a particular case, they are also robust. Physicist Arun K. Pati along with Samuel L. Braunstein proved this theorem.

**Entanglement distillation** is the transformation of *N* copies of an arbitrary entangled state into some number of approximately pure Bell pairs, using only *local operations and classical communication* (LOCC).

In quantum physics, a **quantum state** is a mathematical entity that provides a probability distribution for the outcomes of each possible measurement on a system. Knowledge of the quantum state together with the rules for the system's evolution in time exhausts all that can be predicted about the system's behavior. A mixture of quantum states is again a quantum state. Quantum states that cannot be written as a mixture of other states are called **pure quantum states**, while all other states are called **mixed quantum states**. A pure quantum state can be represented by a ray in a Hilbert space over the complex numbers, while mixed states are represented by density matrices, which are positive semidefinite operators that act on Hilbert spaces.

In the theory of quantum communication, an **amplitude damping channel** is a quantum channel that models physical processes such as spontaneous emission. A natural process by which this channel can occur is a spin chain through which a number of spin states, coupled by a time independent Hamiltonian, can be used to send a quantum state from one location to another. The resulting quantum channel ends up being identical to an amplitude damping channel, for which the quantum capacity, the classical capacity and the entanglement assisted classical capacity of the quantum channel can be evaluated.

**Linear Optical Quantum Computing** or **Linear Optics Quantum Computation** (**LOQC**) is a paradigm of quantum computation, allowing universal quantum computation. LOQC uses photons as information carriers, mainly uses linear optical elements, or optical instruments to process quantum information, and uses photon detectors and quantum memories to detect and store quantum information.

Being a component part of network science the study of quantum complex networks aims to explore the impact of complexity science and network architectures in quantum systems. According to quantum information theory it is possible to improve communication security and data transfer rates by taking advantage of quantum mechanics. In this context the study of quantum complex networks is motivated by the possibility of quantum communications being used on a massive scale in the future. In such case it is likely that quantum communication networks will acquire non trivial features as is common in existing communication networks today.

**Optical cluster states** are a proposed tool to achieve quantum computational universality in linear optical quantum computing (LOQC). As direct entangling operations with photons often require nonlinear effects, probabilistic generation of entangled resource states has been proposed as an alternative path to the direct approach.

In quantum information science, **monogamy** is a fundamental property of quantum entanglement that describes the fact that entanglement cannot be freely shared between arbitrarily many parties.

- ↑ B. Schumacher (1995). "Quantum coding".
*Physical Review A*.**51**(4): 2738–2747. Bibcode:1995PhRvA..51.2738S. doi:10.1103/PhysRevA.51.2738. PMID 9911903. - 1 2 Nielsen, Michael A.; Chuang, Isaac L. (2010).
*Quantum Computation and Quantum Information*. Cambridge University Press. p. 13. ISBN 978-1-107-00217-3. - ↑ Shor, Peter (1997). "Polynomial-Time Algorithms for Prime Factorization and Discrete Logarithms on a Quantum Computer∗".
*SIAM Journal on Computing*.**26**(5): 1484–1509. arXiv: quant-ph/9508027 . Bibcode:1995quant.ph..8027S. doi:10.1137/S0097539795293172. S2CID 2337707. - ↑ Horodecki, Ryszard; et al. (2009). "Quantum entanglement".
*Reviews of Modern Physics*.**81**(2): 865–942. arXiv: quant-ph/0702225 . Bibcode:2009RvMP...81..865H. doi:10.1103/RevModPhys.81.865. S2CID 59577352. - ↑ Preskill, John (2018). "Quantum Computing in the NISQ era and beyond".
*Quantum*.**2**: 79. arXiv: 1801.00862 . doi:10.22331/q-2018-08-06-79. S2CID 44098998. - ↑ Nisbet-Jones, Peter B. R.; Dilley, Jerome; Holleczek, Annemarie; Barter, Oliver; Kuhn, Axel (2013). "Photonic qubits, qutrits and ququads accurately prepared and delivered on demand".
*New Journal of Physics*.**15**(5): 053007. arXiv: 1203.5614 . Bibcode:2013NJPh...15e3007N. doi:10.1088/1367-2630/15/5/053007. ISSN 1367-2630. S2CID 110606655. - ↑ "Qudits: The Real Future of Quantum Computing?".
*IEEE Spectrum*. 2017-06-28. Retrieved 2017-06-29. - ↑ B. Lucatto; et al. (2019). "Charge qubit in van der Waals heterostructures".
*Physical Review B*.**100**(12): 121406. arXiv: 1904.10785 . Bibcode:2019PhRvB.100l1406L. doi:10.1103/PhysRevB.100.121406. S2CID 129945636. - ↑ J. J. L. Morton; et al. (2008). "Solid-state quantum memory using the
^{31}P nuclear spin".*Nature*.**455**(7216): 1085–1088. arXiv: 0803.2021 . Bibcode:2008Natur.455.1085M. doi:10.1038/nature07295. S2CID 4389416. - ↑ Kamyar Saeedi; et al. (2013). "Room-Temperature Quantum Bit Storage Exceeding 39 Minutes Using Ionized Donors in Silicon-28".
*Science*.**342**(6160): 830–833. Bibcode:2013Sci...342..830S. doi:10.1126/science.1239584. PMID 24233718. S2CID 42906250. - ↑ Náfrádi, Bálint; Choucair, Mohammad; Dinse, Klaus-Pete; Forró, László (July 18, 2016). "Room temperature manipulation of long lifetime spins in metallic-like carbon nanospheres".
*Nature Communications*.**7**: 12232. arXiv: 1611.07690 . Bibcode:2016NatCo...712232N. doi:10.1038/ncomms12232. PMC 4960311 . PMID 27426851. - ↑ Council, Australian Research (April 2, 2021). "Qubits composed of holes could be the trick to build faster, larger quantum computers".
*Phys.org*. Retrieved April 2, 2021.

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