The Josephson Effect and e/k

A Josephson junction, consisting of two superconductors weakly coupled together, is an example of a very simple macroscopic quantum system. When there is a voltage V across the junction, the supercurrents flowing through it oscillate at a frequency given very precisely by the relation v=2eVlh. The general validity of this formula has been used recently by Parker, Langenberg, Denenstein, and Taylor as the basis for a new determination of the fundamental constant ratio elh. The new value is 38±10 ppm smaller than the previously accepted value. This result has far-reaching implications concerning the values of the fundamental constants and quantum electrodynamics. This article begins with a simple derivation of the Josephson voltage-frequency relation. Theory suggests that there should be no significant corrections to this equation, a result which is well confirmed by experiment. Various types of junction are then described, together with their applications to the measurement of elh. The impact of this measurement upon the fundamental constants and quantum electrodynamics is briefly discussed. It seems likely that Josephson junctions will have an important application in comparing and maintaining standards of electromotive force. [This is another in the series of review-tutorial papers that are written at the invitation of the Editor and that will be published from time to time in the American Journal of Physics. Articles in this series will be of an expository, critical nature and are intended to be readable by students and professionals alike as a first introduction to a specialized topic. ]