Josephson Effects and Josephson Voltage Standards

Division 2

In 1962, the 22-year-old British student Brian D. Josephson predicted two effects which could experimentally verified shortly afterwards. The modern Josephson voltage standard is based on one of these effects.

The Josephson effects occur if two superconductors are weakly coupled, e.g. by separating them by an insulating layer of a few nanometer in thickness. Irradiation of this Josephson junction with microwaves (being an electromagnetic wave) creates between the superconductors discrete voltage levels depending only on the ratio of two fundamental constants and the frequency f of the microwaves. These steps of constant voltage are generated at voltages Vn :

Vn = n x h/2e x f

n = 1, 2, 3, ... denotes the number of the steps, h the Planck constant and e the elementary charge. The term Josephson constant KJ is nowadays used for the quotient 2e/h instead of its reciprocal value for historical reasons.

The distance between neighbouring steps amounts to about 150 µV using a typical microwave frequency around 70 GHz.

Left picture: Current-voltage characteristics of a conventional superconductor-insulator superconductor (SIS) Josephson contact (X: 100 μA / div. Y: 1 mV / div.)
Right picture: Current-voltage characteristics of the same contact under microwave irradiation. The stages of constant voltage (Shapiro steps) are clearly visible.

By means of Josephson junctions, voltages can be reproduced with relative uncertainties of less than one part in ten billion (1 : 1010, i.e. 1 nV at 10 V). The Josephson effect therefore since 1990 was is world-wide used as a basis for constant reference voltages in metrological national institutes and in calibration laboratories of industry. As the Josephson constant is was not known with the needed accuracy by that time, the same agreed value is was used everywhere for these purposes from 1. January 1990 until 19. May 2019: KJ-90 = 483 597.9 GHz/V

Since May 20, 2019, the day the revision of the International System of Units (SI) came into force, a new reference value is valid. It is determined by the defined values for the Planck constant h and for the elementary charge e:

KJ = 2e/h = 483 597.848 416 984 … GHz/V

In the Clean Room Centre of PTB, large series arrays are fabricated consisting of several ten thousand Josephson junctions which are connected so that voltages up to 10 V can be generated. These series arrays are the main components of modern Josephson voltage standards.

In 1973, the Nobel prize for physics was awarded B. D. Josephson for his discovery.

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Original paper of B.D. Josephson

• B.D. Josephson, "Possible new effects in superconductive tunnelling", Phys. Lett. 1 (1962) 251-253.

Further papers dealing with Josephson voltage standards

• J. Kohlmann, "Quantum Standards for Voltage", in "Applied Superconductivity: Handbook on Devices and Applications", P. Seidel (Ed.), Wiley-VCH, (2015) 807-827.
• J. Kohlmann, "Programmable Josephson voltage standards: from DC to AC”, in “100 Years of Superconductivity", H. Rogalla und P. Kes (Eds.), Taylor & Francis, (2011) 540-545.
• J. Kohlmann, R. Behr und T. Funck, "Josephson voltage standards", Meas. Sci. Techn. 14 (2003) 1216-1228.
• R. Behr, F. Müller und J. Kohlmann, "Josephson junction arrays for voltage standards", in "Studies of Josephson junction arrays", Vol. 2, (Studies of high temperature superconductors: 40), A.V. Narlikar (Ed.), Nova Science Publ.: Huntington, NY (2002) 155-184.
• C.A. Hamilton, "Josephson voltage standards", Rev. Sci. Instrum., 71 (2000) 3611-3623.
• J. Niemeyer, "Das Josephsonspannungsnormal - Entwicklung zum Quantenvoltmeter", PTB-Mitteilungen 110 (2000) 169-177.
• J. Niemeyer, "Josephson voltage standards", in: Handbook of Applied Superconductivity, B. Seeber (Ed.), Institute of Physics Publishing, Bristol (1998) 1813-1834.

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