Editing Volt (section)

=== Josephson junction definition ===
{{Main|Josephson voltage standard}}

Historically the "[[Conventional electrical unit|conventional]]" volt, ''V''<sub>90</sub>, defined in 1987 by the 18th [[General Conference on Weights and Measures]]<ref name="cgpm-18">{{cite web|url=https://www.bipm.org/documents/20126/33145736/CGPM18.pdf/f461df63-75c1-c14d-e6b7-69867b79382f|title=Resolutions of the CGPM: 18th meeting (12–15 October 1987)|access-date=27 February 2022|archive-date=27 February 2022|archive-url=https://web.archive.org/web/20220227150143/https://www.bipm.org/documents/20126/33145736/CGPM18.pdf/f461df63-75c1-c14d-e6b7-69867b79382f|url-status=live}}</ref> and in use from 1990 to 2019, was implemented using the [[Josephson effect]] for exact frequency-to-voltage conversion, combined with the [[Caesium standard|caesium frequency standard]]. Though the Josephson effect is still used to realize a volt, the constant used has changed slightly.

For the [[Magnetic flux quantum|Josephson constant]], ''K''<sub>J</sub> = 2''e''/''h'' (where ''e'' is the [[elementary charge]] and ''h'' is the [[Planck constant]]), a "conventional" value ''K''<sub>J-90</sub> = {{val|0.4835979|u=GHz/μV}} was used for the purpose of defining the volt. As a consequence of the [[2019 revision of the SI]], as of 2019 the Josephson constant has an exact value of {{math|''K''<sub>J</sub>}} = {{val|483597.84841698|end=...|u=GHz/V}}, which replaced the conventional value ''K''<sub>J-90</sub>.

This standard is typically realized using a series-connected array of several thousand or tens of thousands of [[Electrical junction|junctions]], excited by microwave signals between 10 and 80&nbsp;GHz (depending on the array design).<ref name=ieee-josephson>{{Citation |title=1 Volt DC Programmable Josephson Voltage Standard |first1=Charles J. |last1=Burroughs |first2=Samuel P. |last2=Bent |first3=Todd E. |last3=Harvey |first4=Clark A. |last4=Hamilton |journal=IEEE Transactions on Applied Superconductivity |date=1 June 1999 |volume=9 |number=3 |pages=4145–4149 |issn=1051-8223 |publisher=[[Institute of Electrical and Electronics Engineers]] (IEEE) |doi=10.1109/77.783938 |bibcode=1999ITAS....9.4145B |s2cid=12970127 |url=https://zenodo.org/record/1232191}}</ref> Empirically, several experiments have shown that the method is independent of device design, material, measurement setup, etc., and no correction terms are required in a practical implementation.<ref>{{Citation |title=Current status of the quantum metrology triangle |first=Mark W. |last=Keller |url=http://qdev.boulder.nist.gov/817.03/pubs/downloads/set/Metrologia%2045,%20102.pdf |journal=Metrologia |volume=45 |number=1 |pages=102–109 |date=18 January 2008 |issn=0026-1394 |doi=10.1088/0026-1394/45/1/014 |quote=Theoretically, there are no current predictions for any correction terms. Empirically, several experiments have shown that ''K''<sub>J</sub> and ''R''<sub>K</sub> are independent of device design, material, measurement setup, etc. This demonstration of universality is consistent with the exactness of the relations, but does not prove it outright. |bibcode=2008Metro..45..102K |s2cid=122008182 |access-date=11 April 2010 |archive-url=https://web.archive.org/web/20100527094953/http://qdev.boulder.nist.gov/817.03/pubs/downloads/set/Metrologia%2045,%20102.pdf |archive-date=27 May 2010 |url-status=dead}}</ref>