Editing Qubit (section)

==Physical implementations==
Any [[two-state quantum system|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., the 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 have been 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.

All physical implementations are affected by noise. The so-called ''T''<sub>1</sub> lifetime and ''T''<sub>2</sub> dephasing time are a time to characterize the physical implementation and represent their sensitivity to noise. A higher time does not necessarily mean that one or the other qubit is better suited for [[quantum computing]] because gate times and fidelities need to be considered, too.

Different applications like [[quantum sensing]], [[quantum computing]] and [[quantum communication]] use different implementations of qubits to suit their application.

The following is an incomplete list of physical implementations of qubits, and the choices of basis are by convention only.

{| class="wikitable" align="center"
|-
! scope="col" | Physical support
! scope="col" | Name
! scope="col" | Information support
! scope="col" style="background: white;" | <math>|0 \rangle</math>
! scope="col" style="background: white;" | <math>|1 \rangle</math>
|-
| rowspan=3 |[[photon]]
| [[Polarization (waves)|polarization]] [[Encoding (memory)|encoding]]
| [[polarization of light]]
| horizontal
| vertical
|-
| [[Amount of substance|number of photons]]
| [[Fock state]]
| [[vacuum]]
| single-photon state
|-
| [[time-bin encoding]]
| [[time of arrival]]
| early
| late
|-
| [[coherent state]] of [[light]]
| [[Squeezed coherent state|squeezed light]]
| [[Optical phase space|quadrature]]
| [[amplitude]]-[[Squeezed coherent state|squeezed]] [[Quantum state|state]]
| phase-squeezed state
|-
| rowspan=2|[[electron]]s
| [[Spin quantum number|electronic spin]]
| [[Spin (physics)|spin]]
| up
| down
|-
| [[electron]] [[Amount of substance|number]]
| [[Charge (physics)|charge]]
| no electron
| two electron
|-
| [[Nuclear magnetic resonance quantum computer|nucleus]]
| [[nuclear spin]] [[Address|addressed]] [[Preposition and postposition|through]] [[Nuclear magnetic resonance|NMR]]
| [[Spin (physics)|spin]]
| up
| down
|-
| [[Neutral atom quantum computer|neutral atom]]
| atomic [[energy level]]
| [[Spin (physics)|spin]]
| up
| down
|-
| trapped [[ion]]
| atomic [[energy level]]
| [[Spin (physics)|spin]]
| up
| down
|-
| rowspan=3| [[Josephson junction]]
| [[Superconductivity|superconducting]] [[charge qubit]]
| [[Charge (physics)|charge]]
| uncharged [[Superconductivity|superconducting]] island ({{nowrap|1=''Q'' = 0}})
| charged superconducting island ({{nowrap|1=''Q'' = 2''e''}}, one extra [[Cooper pair]])
|-
| [[Superconductivity|superconducting]] [[flux qubit]]
| [[Current source|current]]
| [[clockwise]] [[Current source|current]]
| counterclockwise current
|-
| [[Superconductivity|superconducting]] [[phase qubit]]
| [[energy]]
| [[ground state]]
| first excited state
|-
| singly charged [[quantum dot]] pair
| [[Electron localization function|electron localization]]
| [[Charge (physics)|charge]]
| electron on left dot
| electron on right dot
|-
| [[quantum dot]]
| [[Dot product|dot]] [[Spin (physics)|spin]]
| [[Spin (physics)|spin]]
| down
| up
|-
| [[Topological order|gapped topological system]]
| [[Non-abelian group|non-abelian]] [[anyon]]s
| [[Braid group|braiding of excitations]]
| depends on specific [[Topology|topological]] [[system]]
| depends on specific topological system
|-
| vibrational qubit<ref>{{cite journal |last1=Berrios |first1=Eduardo |last2=Gruebele |first2=Martin |last3=Shyshlov |first3=Dmytro |last4=Wang |first4=Lei |last5=Babikov |first5=Dmitri |year=2012 |title=High fidelity quantum gates with vibrational qubits |journal=Journal of Chemical Physics |volume=116 |issue=46 |pages=11347–11354 |bibcode=2012JPCA..11611347B |doi=10.1021/jp3055729 |pmid=22803619}}</ref>
| [[Vibrational bond|vibrational]] [[Quantum state|states]]
| [[phonon]]/[[Vibronic spectroscopy|vibron]]
| <math>|01 \rangle</math> [[Superposition principle|superposition]]
| <math>|10 \rangle</math> superposition
|-
| [[van der Waals heterostructure]]<ref>
{{cite journal |author=Lucatto |first=B. |display-authors=etal |year=2019 |title=Charge qubit in van der Waals heterostructures |journal=Physical Review B |volume=100 |issue=12 |pages=121406 |arxiv=1904.10785 |bibcode=2019PhRvB.100l1406L |doi=10.1103/PhysRevB.100.121406 |s2cid=129945636}}</ref>
| [[Electron localization function|electron localization]]
| [[Charge (physics)|charge]]
| [[electron]] on bottom sheet
| electron on top sheet
|}