Qubit
Units of information |
In quantum computing, a qubit (/ˈkjuːbɪt/) or quantum bit is a 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
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.
Bit versus qubit
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} complex numbers (or a single point in a 2^{n}-dimensional vector space).^{[3]}
Standard representation
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 [math]\displaystyle{ | 0 \rangle = \bigl[\begin{smallmatrix} 1\\ 0 \end{smallmatrix}\bigr] }[/math] and [math]\displaystyle{ | 1 \rangle = \bigl[\begin{smallmatrix} 0\\ 1 \end{smallmatrix}\bigr] }[/math]. They are written in the conventional Dirac—or "bra–ket"—notation; the [math]\displaystyle{ | 0 \rangle }[/math] and [math]\displaystyle{ | 1 \rangle }[/math] are pronounced "ket 0" and "ket 1", respectively. These two orthonormal basis states, [math]\displaystyle{ \{|0\rangle,|1\rangle\} }[/math], 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. A set of qubits taken together is called a quantum register. For example, two qubits could be represented in a four-dimensional linear vector space spanned by the following product basis states: [math]\displaystyle{ | 00 \rangle = \biggl[\begin{smallmatrix} 1\\ 0\\ 0\\ 0 \end{smallmatrix}\biggr] }[/math], [math]\displaystyle{ | 01 \rangle = \biggl[\begin{smallmatrix} 0\\ 1\\ 0\\ 0 \end{smallmatrix}\biggr] }[/math], [math]\displaystyle{ | 10 \rangle = \biggl[\begin{smallmatrix} 0\\ 0\\ 1\\ 0 \end{smallmatrix}\biggr] }[/math], and [math]\displaystyle{ | 11 \rangle = \biggl[\begin{smallmatrix} 0\\ 0\\ 0\\ 1 \end{smallmatrix}\biggr] }[/math].
In general, n qubits are represented by a superposition state vector in 2^{n} dimensional Hilbert space.
Qubit states
A pure qubit state is a coherent superposition of the basis states. This means that a single qubit ([math]\displaystyle{ \psi }[/math]) can be described by a linear combination of [math]\displaystyle{ |0 \rangle }[/math] and [math]\displaystyle{ |1 \rangle }[/math]:
- [math]\displaystyle{ | \psi \rangle = \alpha |0 \rangle + \beta |1 \rangle }[/math]
where α and β are the probability amplitudes, and are both complex numbers. When we measure this qubit in the standard basis, according to the Born rule, the probability of outcome [math]\displaystyle{ |0 \rangle }[/math] with value "0" is [math]\displaystyle{ | \alpha |^2 }[/math] and the probability of outcome [math]\displaystyle{ |1 \rangle }[/math] with value "1" is [math]\displaystyle{ | \beta |^2 }[/math]. Because the absolute squares of the amplitudes equate to probabilities, it follows that [math]\displaystyle{ \alpha }[/math] and [math]\displaystyle{ \beta }[/math] must be constrained according to the second axiom of probability theory by the equation^{[4]}
- [math]\displaystyle{ | \alpha |^2 + | \beta |^2 = 1. }[/math]
The probability amplitudes, [math]\displaystyle{ \alpha }[/math] and [math]\displaystyle{ \beta }[/math], encode more than just the probabilities of the outcomes of a measurement; the relative phase between [math]\displaystyle{ \alpha }[/math] and [math]\displaystyle{ \beta }[/math] is for example responsible for quantum interference, as seen in the two-slit experiment.
Bloch sphere representation
It might, at first sight, seem that there should be four degrees of freedom in [math]\displaystyle{ | \psi \rangle = \alpha |0 \rangle + \beta |1 \rangle\, }[/math], as [math]\displaystyle{ \alpha }[/math] and [math]\displaystyle{ \beta }[/math] 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:
- [math]\displaystyle{ \begin{align} \alpha &= e^{i \delta} \cos\frac{\theta}{2}, \\ \beta &= e^{i (\delta + \varphi)} \sin\frac{\theta}{2}. \end{align} }[/math]
Additionally, for a single qubit the global phase of the state [math]\displaystyle{ e^{i\delta} }[/math] has no physically observable consequences,^{[lower-alpha 1]} so we can arbitrarily choose α to be real (or β in the case that α is zero), leaving just two degrees of freedom:
- [math]\displaystyle{ \begin{align} \alpha &= \cos\frac{\theta}{2}, \\ \beta &= e^{i \varphi} \sin\frac{\theta}{2}, \end{align} }[/math]
where [math]\displaystyle{ e^{i \varphi} }[/math] is the physically significant relative phase.^{[5]}^{[lower-alpha 2]}
The possible quantum states for a single qubit can be visualised using a Bloch sphere (see picture). Represented on such a 2-sphere, a classical bit could only be at the "North Pole" or the "South Pole", in the locations where [math]\displaystyle{ |0 \rangle }[/math] and [math]\displaystyle{ |1 \rangle }[/math] 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 [math]\displaystyle{ (|0 \rangle + |1 \rangle)/{\sqrt{2}} }[/math] would lie on the equator of the sphere at the positive X-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 observable state space of the pure qubit states. This state space has two local degrees of freedom, which can be represented by the two angles [math]\displaystyle{ \varphi }[/math] and [math]\displaystyle{ \theta }[/math].
Mixed state
- [math]\displaystyle{ |\psi\rangle = \alpha |0\rangle + \beta |1\rangle,\, }[/math] a coherent superposition, represented by a point on the surface of the Bloch sphere as described above. Coherence is essential for a qubit to be in a superposition state. With interactions, quantum noise 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 [math]\displaystyle{ \varphi }[/math] and [math]\displaystyle{ \theta }[/math], as well as the length [math]\displaystyle{ r }[/math] of the vector that represents the mixed state. A pure state is fully specified by a single ket,
Quantum error correction can be used to maintain the purity of qubits.
Operations on qubits
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 multiplying the quantum gates unitary matrix with the quantum state vector. The result from this multiplication is a new quantum state.
- 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 [math]\displaystyle{ |\psi\rangle = \alpha |0\rangle + \beta |1\rangle }[/math] will be either [math]\displaystyle{ |0\rangle }[/math] with probability [math]\displaystyle{ |\alpha|^2 }[/math] or [math]\displaystyle{ |1\rangle }[/math] with probability [math]\displaystyle{ |\beta|^2 }[/math]. Measurement of the state of the qubit alters the magnitudes of α and β. For instance, if the result of the measurement is [math]\displaystyle{ |1\rangle }[/math], α is changed to 0 and β is changed to the phase factor [math]\displaystyle{ e^{i \phi} }[/math] no longer experimentally accessible. If measurement is performed on a qubit that is entangled, the measurement may collapse the state of the other entangled qubits.
- Initialization or re-initialization to a known value, often [math]\displaystyle{ |0\rangle }[/math]. This operation collapses the quantum state (exactly like with measurement). Initialization to [math]\displaystyle{ |0\rangle }[/math] 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 [math]\displaystyle{ |1\rangle }[/math]. Physically, for example if it is a superconducting phase qubit, by lowering the energy of the quantum system to its ground state.
- Sending the qubit through a quantum channel to a remote system or machine (an I/O operation), potentially as part of a quantum network.
Quantum entanglement
- 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. An important distinguishing feature between qubits and classical bits is that multiple qubits can exhibit
The simplest system to display quantum entanglement is the system of two qubits. Consider, for example, two entangled qubits in the [math]\displaystyle{ |\Phi^+\rangle }[/math] Bell state:
- [math]\displaystyle{ \frac{1}{\sqrt{2}} (|00\rangle + |11\rangle). }[/math]
In this state, called an equal superposition, there are equal probabilities of measuring either product state [math]\displaystyle{ |00\rangle }[/math] or [math]\displaystyle{ |11\rangle }[/math], as [math]\displaystyle{ |1/\sqrt{2}|^2 = 1/2 }[/math]. 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 [math]\displaystyle{ |0\rangle }[/math] or [math]\displaystyle{ |1\rangle }[/math], 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 [math]\displaystyle{ |0\rangle }[/math], Bob must measure the same, as [math]\displaystyle{ |00\rangle }[/math] is the only state where Alice's qubit is a [math]\displaystyle{ |0\rangle }[/math]. 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 gate to construct the Bell state
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 [math]\displaystyle{ |1\rangle }[/math], and otherwise leaves it unchanged. With respect to the unentangled product basis [math]\displaystyle{ \{|00\rangle }[/math], [math]\displaystyle{ |01\rangle }[/math], [math]\displaystyle{ |10\rangle }[/math], [math]\displaystyle{ |11\rangle\} }[/math], it maps the basis states as follows:
- [math]\displaystyle{ | 0 0 \rangle \mapsto | 0 0 \rangle }[/math]
- [math]\displaystyle{ | 0 1 \rangle \mapsto | 0 1 \rangle }[/math]
- [math]\displaystyle{ | 1 0 \rangle \mapsto | 1 1 \rangle }[/math]
- [math]\displaystyle{ | 1 1 \rangle \mapsto | 1 0 \rangle }[/math].
A common application of the CNOT gate is to maximally entangle two qubits into the [math]\displaystyle{ |\Phi^+\rangle }[/math] Bell state. To construct [math]\displaystyle{ |\Phi^+\rangle }[/math], the inputs A (control) and B (target) to the CNOT gate are:
[math]\displaystyle{ \frac{1}{\sqrt{2}}(|0\rangle + |1\rangle)_A }[/math] and [math]\displaystyle{ |0\rangle_B }[/math]
After applying CNOT, the output is the [math]\displaystyle{ |\Phi^+\rangle }[/math] Bell State: [math]\displaystyle{ \frac{1}{\sqrt{2}}(|00\rangle + |11\rangle) }[/math].
Applications
The [math]\displaystyle{ |\Phi^+\rangle }[/math] 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.^{[6]} 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.^{[7]}
Quantum register
A number of qubits taken together is a qubit register. Quantum computers perform calculations by manipulating qubits within a register.
Qudits and qutrits
The term qudit denotes the unit of quantum information that can be realized in suitable d-level quantum systems.^{[8]} A qubit register that can be measured to N states is identical^{[lower-alpha 3]} to an N-level qudit. A rarely used^{[9]} synonym for qudit is quNit,^{[10]} since both d and N are frequently used to denote the dimension of a quantum system.
Qudits are similar to the integer types in classical computing, and may be mapped to (or realized by) arrays of qubits. Qudits where the d-level system is not an exponent of 2 can not be mapped to arrays of qubits. It is for example possible to have 5-level qudits.
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.^{[11]}
In 2022, researchers at the University of Innsbruck succeeded in developing a universal qudit quantum processor with trapped ions.^{[12]} In the same year, researchers at Tsinghua University's Center for Quantum Information implemented the dual-type qubit scheme in trapped ion quantum computers using the same ion species.^{[13]} 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.^{[14]}
Physical implementations
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 | [math]\displaystyle{ |0 \rangle }[/math] | [math]\displaystyle{ |1 \rangle }[/math] |
---|---|---|---|---|
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 |
Vibrational qubit^{[15]} | Vibrational states | Phonon/vibron | [math]\displaystyle{ |01 \rangle }[/math] superposition | [math]\displaystyle{ |10 \rangle }[/math] superposition |
van der Waals heterostructure^{[16]} | Electron localization | Charge | Electron on bottom sheet | Electron on top sheet |
Qubit storage
In 2008 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.^{[17]} This event can be considered the first relatively consistent quantum data storage, a vital step towards the development of quantum computing. In 2013, 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.^{[18]} 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.^{[19]} An increased coherence of qubits is being explored by researchers who are testing the limitations of a Ge hole spin-orbit qubit structure.^{[20]}
See also
- Ancilla bit
- Bell state, W state and GHZ state
- Bloch sphere
- Physical and logical qubits
- Quantum register
- Two-state quantum system
- The elements of the group U(2) are all possible single-qubit quantum gates^{[21]}
- The circle group U(1) define the phase about the qubits basis states
Notes
- ↑ This is because of the Born rule. The probability to observe an outcome upon measurement is the modulus squared of the probability amplitude for that outcome (or basis state, eigenstate). The global phase factor [math]\displaystyle{ e^{i\delta} }[/math] is not measurable, because it applies to both basis states, and is on the complex unit circle so [math]\displaystyle{ |e^{i\delta}|^2 = 1. }[/math]
Note that by removing [math]\displaystyle{ e^{i\delta} }[/math] it means that quantum states with global phase can not be represented as points on the surface of the Bloch sphere. - ↑ The Pauli Z basis is usually called the computational basis, where the relative phase have no effect on measurement. Measuring instead in the X or Y Pauli basis depends on the relative phase. For example, [math]\displaystyle{ (|0\rangle + e^{i\pi/2}|1\rangle)/{\sqrt{2}} }[/math] will (because this state lies on the positive pole of the Y-axis) in the Y-basis always measure to the same value, while in the Z-basis results in equal probability of being measured to [math]\displaystyle{ |0\rangle }[/math] or [math]\displaystyle{ |1\rangle }[/math].
Because measurement collapses the quantum state, measuring the state in one basis hides some of the values that would have been measurable the other basis; See the uncertainty principle. - ↑ Actually isomorphic: For a register with [math]\displaystyle{ n }[/math] qubits [math]\displaystyle{ N=2^n }[/math] and [math]\displaystyle{ (\mathbb{C}^2)^{\otimes n} \cong \mathbb{C}^N }[/math]
References
- ↑ "Quantum coding". Physical Review A 51 (4): 2738–2747. 1995. doi:10.1103/PhysRevA.51.2738. PMID 9911903. Bibcode: 1995PhRvA..51.2738S.
- ↑ Nielsen, Michael A.; Chuang, Isaac L. (2010) (in en-US). 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. doi:10.1137/S0097539795293172. Bibcode: 1995quant.ph..8027S.
- ↑ Colin P. Williams (2011). Explorations in Quantum Computing. Springer. pp. 9–13. ISBN 978-1-84628-887-6.
- ↑ Nielsen, Michael A.; Chuang, Isaac (2010). Quantum Computation and Quantum Information. Cambridge: Cambridge University Press. pp. 13–16. ISBN 978-1-10700-217-3. OCLC 43641333. https://www.cambridge.org/9781107002173.
- ↑ Horodecki, Ryszard (2009). "Quantum entanglement". Reviews of Modern Physics 81 (2): 865–942. doi:10.1103/RevModPhys.81.865. Bibcode: 2009RvMP...81..865H.
- ↑ Preskill, John (2018). "Quantum Computing in the NISQ era and beyond". Quantum 2: 79. doi:10.22331/q-2018-08-06-79.
- ↑ 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" (in en). New Journal of Physics 15 (5): 053007. doi:10.1088/1367-2630/15/5/053007. ISSN 1367-2630. Bibcode: 2013NJPh...15e3007N. http://stacks.iop.org/1367-2630/15/i=5/a=053007.
- ↑ As of June 2022 1150 uses versus 31 uses on in the quant-ph category of arxiv.org.
- ↑ Kaszlikowski, Dagomir; Gnaciński, Piotr; Żukowski, Marek; Miklaszewski, Wieslaw; Zeilinger, Anton (2000). "Violations of Local Realism by Two Entangled N-Dimensional Systems Are Stronger than for Two Qubits". Phys. Rev. Lett. 85 (21): 4418–4421. doi:10.1103/PhysRevLett.85.4418. PMID 11082560.
- ↑ Choi, Charles Q. (2017-06-28). "Qudits: The Real Future of Quantum Computing?" (in en-US). https://spectrum.ieee.org/tech-talk/computing/hardware/qudits-the-real-future-of-quantum-computing.
- ↑ Ringbauer, Martin; Meth, Michael; Postler, Lukas; Stricker, Roman; Blatt, Rainer; Schindler, Philipp; Monz, Thomas (21 July 2022). "A universal qudit quantum processor with trapped ions" (in en). Nature Physics 18 (9): 1053–1057. doi:10.1038/s41567-022-01658-0. ISSN 1745-2481. https://www.nature.com/articles/s41567-022-01658-0. Retrieved 21 July 2022.
- ↑ Fardelli, Ingrid (August 18, 2022). "Researchers realize two coherently convertible qubit types using a single ion species". Phys.org. https://phys.org/news/2022-08-coherently-qubit-ion-species.amp.
- ↑ Irving, Michael (2022-10-14). ""64-dimensional quantum space" drastically boosts quantum computing" (in en-US). https://newatlas.com/telecommunications/qudits-64-dimensional-quantum-space/.
- ↑ Eduardo Berrios; Martin Gruebele; Dmytro Shyshlov; Lei Wang; Dmitri Babikov (2012). "High fidelity quantum gates with vibrational qubits". Journal of Chemical Physics 116 (46): 11347–11354. doi:10.1021/jp3055729. PMID 22803619. Bibcode: 2012JPCA..11611347B.
- ↑ B. Lucatto (2019). "Charge qubit in van der Waals heterostructures". Physical Review B 100 (12): 121406. doi:10.1103/PhysRevB.100.121406. Bibcode: 2019PhRvB.100l1406L.
- ↑ J. J. L. Morton (2008). "Solid-state quantum memory using the ^{31}P nuclear spin". Nature 455 (7216): 1085–1088. doi:10.1038/nature07295. Bibcode: 2008Natur.455.1085M.
- ↑ Kamyar Saeedi (2013). "Room-Temperature Quantum Bit Storage Exceeding 39 Minutes Using Ionized Donors in Silicon-28". Science 342 (6160): 830–833. doi:10.1126/science.1239584. PMID 24233718. Bibcode: 2013Sci...342..830S.
- ↑ 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. doi:10.1038/ncomms12232. PMID 27426851. Bibcode: 2016NatCo...712232N.
- ↑ Wang, Zhanning; Marcellina, Elizabeth; Hamilton, A. R.; Cullen, James H.; Rogge, Sven; Salfi, Joe; Culcer, Dimitrie (April 1, 2021). "Qubits composed of holes could be the trick to build faster, larger quantum computers". npj Quantum Information 7 (1). doi:10.1038/s41534-021-00386-2. https://phys.org/news/2021-04-qubits-holes-faster-larger-quantum.html.
- ↑ Barenco, Adriano; Bennett, Charles H.; Cleve, Richard; DiVincenzo, David P.; Margolus, Norman; Shor, Peter; Sleator, Tycho; Smolin, John A. et al. (1995-11-01). "Elementary gates for quantum computation". Physical Review A (American Physical Society (APS)) 52 (5): 3457–3467. doi:10.1103/physreva.52.3457. ISSN 1050-2947. PMID 9912645. Bibcode: 1995PhRvA..52.3457B.
Further reading
- Nielsen, Michael A.; Chuang, Isaac (2000). Quantum Computation and Quantum Information. Cambridge: Cambridge University Press. ISBN 0521632358. OCLC 43641333.
- Colin P. Williams (2011). Explorations in Quantum Computing. Springer. ISBN 978-1-84628-887-6.
- Yanofsky, Noson S.; Mannucci, Mirco (2013). Quantum computing for computer scientists. Cambridge University Press. ISBN 978-0-521-87996-5.
- A 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).
- An 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, showcasing a Bell state and the measurement of it, is found in Quantum entanglement for babies, by Chris Ferrie (2017). ISBN:9781492670261.
Original source: https://en.wikipedia.org/wiki/Qubit.
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