The most practical/precise volt standard.
It motivated the definition of the ampere in the 2019 redefinition of the SI base units
Quick NIST article about it: www.nist.gov/news-events/news/2013/04/primary-voltage-standard-whole-world (archive)
Can be used as a very precise magnetometer.
Two parallel Josephson junctions.
In Ciro's ASCII art circuit diagram notation:
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