A practical

quantum computer A quantum computer is a computer that exploits quantum mechanical phenomena. On small scales, physical matter exhibits properties of both particles and waves, and quantum computing takes advantage of this behavior using specialized hardware. ...

must use a physical system as a programmable

quantum register In quantum computing, a quantum register is a system comprising multiple qubits. It is the quantum analogue of the classical processor register. Quantum computers perform calculations by manipulating qubits within a quantum register. Definitio ...

. Researchers are exploring several technologies as candidates for reliable

qubit In quantum computing, a qubit () 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 syste ...

implementations. *

Superconducting quantum computing Superconducting quantum computing is a branch of Solid-state physics, solid state physics and quantum computing that implements superconductivity, superconducting electronic circuits using superconducting qubits as artificial atoms, or quantum dot ...

(qubit implemented by the state of nonlinear resonant superconducting circuits containing Josephson junctions) *

Trapped ion quantum computer A trapped-ion quantum computer is one proposed approach to a large-scale quantum computer. Ions, or charged atomic particles, can be confined and suspended in free space using electromagnetic fields. Qubits are stored in stable electronic state ...

(qubit implemented by the internal state of trapped ions) *

Neutral atom quantum computer A neutral atom quantum computer is a List of proposed quantum registers, modality of quantum computers built out of Rydberg atoms; this modality has many commonalities with trapped-ion quantum computers. As of December 2023, the concept has been u ...

(qubit implemented by internal states of neutral atoms trapped in an optical lattice or an array of dipole traps, i.e. optical tweezers) *

Quantum dot Quantum dots (QDs) or semiconductor nanocrystals are semiconductor particles a few nanometres in size with optical and electronic properties that differ from those of larger particles via quantum mechanical effects. They are a central topic i ...

computer,

spin Spin or spinning most often refers to: * Spin (physics) or particle spin, a fundamental property of elementary particles * Spin quantum number, a number which defines the value of a particle's spin * Spinning (textiles), the creation of yarn or thr ...

-based (e.g. the Loss-DiVincenzo quantum computer) (qubit given by the spin states of trapped electrons) * Quantum dot computer, spatial-based (qubit given by electron position in double quantum dot) * Quantum computing using engineered

quantum well A quantum well is a potential well with only discrete energy values. The classic model used to demonstrate a quantum well is to confine particles, which were initially free to move in three dimensions, to two dimensions, by forcing them to occup ...

s, which could in principle enable the construction of a quantum computer that operates at room temperature * Coupled

quantum wire In mesoscopic physics, a quantum wire is an electrically conducting wire in which quantum effects influence the transport properties. Usually such effects appear in the dimension of nanometers, so they are also referred to as nanowires. Quantum ...

(qubit implemented by a pair of quantum wires coupled by a

quantum point contact A quantum point contact (QPC) is a narrow constriction between two wide electrically conducting regions, of a width comparable to the electronic wavelength (nano- to micrometer). The importance of QPC lies in the fact that they prove quantisation o ...

) *

Nuclear magnetic resonance quantum computer Nuclear magnetic resonance quantum computing (NMRQC) is one of the several proposed approaches for constructing a quantum computer, that uses the spin states of nuclei within molecules as qubits. The quantum states are probed through the nuclear m ...

(NMRQC) implemented with the

nuclear magnetic resonance Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are disturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with a ...

of molecules in solution, where qubits are provided by

nuclear spin Nuclear may refer to: Physics Relating to the nucleus of the atom: * Nuclear engineering * Nuclear physics * Nuclear power * Nuclear reactor * Nuclear weapon * Nuclear medicine *Radiation therapy *Nuclear warfare Mathematics * Nuclear space * ...

s within the dissolved molecule and probed with radio waves * Solid-state NMR Kane quantum computer (qubit realized by the nuclear spin state of

phosphorus Phosphorus is a chemical element; it has Chemical symbol, symbol P and atomic number 15. All elemental forms of phosphorus are highly Reactivity (chemistry), reactive and are therefore never found in nature. They can nevertheless be prepared ar ...

donors in

silicon Silicon is a chemical element; it has symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a tetravalent metalloid (sometimes considered a non-metal) and semiconductor. It is a membe ...

) * Vibrational quantum computer (qubits realized by vibrational superpositions in cold

molecule A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...

s) * Electrons-on-helium quantum computer (qubit is the electron spin) *

Cavity quantum electrodynamics Cavity Quantum Electrodynamics (cavity QED) is the study of the interaction between light confined in a reflective cavity and atoms or other particles, under conditions where the quantum nature of photons is significant. It could in principle be ...

(CQED) (qubit provided by the internal state of trapped atoms coupled to high-finesse cavities) * Molecular magnet (qubit given by spin states) *

Fullerene A fullerene is an allotropes of carbon, allotrope of carbon whose molecules consist of carbon atoms connected by single and double bonds so as to form a closed or partially closed mesh, with fused rings of five to six atoms. The molecules may ...

-based ESR quantum computer (qubit based on the electronic spin of atoms or molecules encased in fullerenes) * Nonlinear optical quantum computer (qubits realized by processing states of different

modes Mode ( meaning "manner, tune, measure, due measure, rhythm, melody") may refer to: Arts and entertainment * MO''D''E (magazine), a defunct U.S. women's fashion magazine * ''Mode'' magazine, a fictional fashion magazine which is the setting fo ...

of light through both linear and

nonlinear In mathematics and science, a nonlinear system (or a non-linear system) is a system in which the change of the output is not proportional to the change of the input. Nonlinear problems are of interest to engineers, biologists, physicists, mathe ...

elements) * Linear optical quantum computer (LOQC) (qubits realized by processing states of different modes of light through linear elements e.g. mirrors,

beam splitter A beam splitter or beamsplitter is an optical instrument, optical device that splits a beam of light into a transmitted and a reflected beam. It is a crucial part of many optical experimental and measurement systems, such as Interferometry, int ...

s and phase shifters). Quantum microprocessor based on laser photonics at room temperature made possible. *

Diamond-based quantum computer The nitrogen-vacancy center (N-V center or NV center) is one of numerous Photoluminescence, photoluminescent Crystallographic defects in diamond, point defects in diamond. Its most explored and useful properties include its spin-dependent photolumi ...

(qubit realized by the electronic or nuclear spin of

nitrogen-vacancy center The nitrogen-vacancy center (N-V center or NV center) is one of numerous Photoluminescence, photoluminescent Crystallographic defects in diamond, point defects in diamond. Its most explored and useful properties include its spin-dependent photolumi ...

s in diamond) *

Bose–Einstein condensate In condensed matter physics, a Bose–Einstein condensate (BEC) is a state of matter that is typically formed when a gas of bosons at very low Density, densities is cooled to temperatures very close to absolute zero#Relation with Bose–Einste ...

-based quantum computer * Transistor-based quantum computer (string quantum computers with entrainment of positive holes using an electrostatic trap) * Rare-earth-metal-ion-doped inorganic crystal based quantum computer (qubit realized by the internal electronic state of

dopant A dopant (also called a doping agent) is a small amount of a substance added to a material to alter its physical properties, such as electrical or optics, optical properties. The amount of dopant is typically very low compared to the material b ...

s in

optical fiber An optical fiber, or optical fibre, is a flexible glass or plastic fiber that can transmit light from one end to the other. Such fibers find wide usage in fiber-optic communications, where they permit transmission over longer distances and at ...

s) * Metallic-like carbon nanospheres-based quantum computer

References

{{Reflist, 30em Quantum computing