Editing Magnetic flux quantum (section)

== Superconducting magnetic flux quantum ==
{| class="wikitable" style="float: right;"
! colspan=2 | CODATA values
! Units
|-
| {{math|Φ}}<sub>0</sub> || {{physconst|Phi0|unit=no}} || [[Weber (unit)|Wb]] 
|-
| {{math|''K''}}<sub>J</sub> || {{physconst|KJ|unit=no}} || [[Hertz|Hz]]/[[volt|V]]
|-
|}
If one deals with a superconducting ring<ref>{{cite journal | url=https://www.nature.com/articles/nphys813 | doi=10.1038/nphys813 | title=Magnetic flux periodicity of h/E in superconducting loops | date=2008 | last1=Loder | first1=F. | last2=Kampf | first2=A. P. | last3=Kopp | first3=T. | last4=Mannhart | first4=J. | last5=Schneider | first5=C. W. | last6=Barash | first6=Y. S. | journal=Nature Physics | volume=4 | issue=2 | pages=112–115 | arxiv=0709.4111 | bibcode=2008NatPh...4..112L }}</ref> (i.e. a closed loop path in a [[superconductor]]) or a hole in a bulk [[superconductor]], the magnetic flux threading such a hole/loop is quantized. 

The (superconducting) '''magnetic flux quantum''' {{nowrap|{{math|1=Φ<sub>0</sub> = ''h''/(2''e'')}} ≈ {{physconst|Phi0}}}} is a combination of fundamental physical constants: the [[Planck constant]] {{math|''h''}} and the [[electron charge]] {{math|''e''}}. 
Its value is, therefore, the same for any superconductor.

To understand this definition in the context of the Dirac flux quantum one shall consider that the effective quasiparticles active in a superconductors are [[Cooper pairs]] with an effective charge of 2 electrons {{math|1=''q'' = 2''e''}}.

The phenomenon of flux quantization was first discovered in superconductors experimentally by B. S. Deaver and W. M. Fairbank<ref name=Deaver:1961:FluxQuantum /> and, independently, by R. Doll and M. Näbauer,<ref name=Doll:1961:FluxQuantum /> in 1961. The quantization of magnetic flux is closely related to the [[Little–Parks effect]],<ref>{{Cite journal|last=Parks|first=R. D.|date=1964-12-11|title=Quantized Magnetic Flux in Superconductors: Experiments confirm Fritz London's early concept that superconductivity is a macroscopic quantum phenomenon|url=https://www.science.org/doi/10.1126/science.146.3650.1429|journal=Science|language=en|volume=146|issue=3650|pages=1429–1435|doi=10.1126/science.146.3650.1429|issn=0036-8075|pmid=17753357|s2cid=30913579|url-access=subscription}}</ref> but was predicted earlier by [[Fritz London]] in 1948 using a [[Phenomenology (particle physics)|phenomenological model]].<ref>{{Cite book|url=https://books.google.com/books?id=VNxEAAAAIAAJ|title=Superfluids: Macroscopic theory of superconductivity|last=London|first=Fritz|date=1950|publisher=John Wiley & Sons|pages=152 (footnote)|language=en}}</ref><ref name=":0" />

The inverse of the flux quantum, {{math|1/Φ<sub>0</sub>}}, is called the '''Josephson constant''', and is denoted {{math|''K''}}<sub>J</sub>. It is the constant of proportionality of the [[Josephson effect]], relating the [[potential difference]] across a Josephson junction to the [[frequency]] of the irradiation. {{anchor|KJ-1990}}The Josephson effect is very widely used to provide a standard for high-precision measurements of potential difference, which (from 1990 to 2019) were related to a fixed, [[conventional electrical unit|conventional value]] of the Josephson constant, denoted {{math|''K''}}<sub>J-90</sub>.  With the [[2019 revision of the SI]], the Josephson constant has an exact value of {{math|''K''}}<sub>J</sub> = {{val|483597.84841698|end=...|u=GHz⋅V{{sup|−1}}}}.<ref>{{cite web|url=https://www.bipm.org/utils/en/pdf/si-mep/MeP-a-2018.pdf|title=''Mise en pratique'' for the definition of the ampere and other electric units in the SI|publisher=[[BIPM]] |archive-url=https://web.archive.org/web/20210308034514/https://www.bipm.org/utils/en/pdf/si-mep/MeP-a-2018.pdf |archive-date=2021-03-08 |url-status=dead}}</ref>