Volt Maintenance at NBS via 2e/h: A New Definition of the NBS Volt

Abstract

This paper describes in detail the procedures, methods and measurements used to establish a new definition of the U.S. legal volt via the ac Josephson effect. This new definition has been made possible by the use of thin film tunnel junctions (capable of producing 10 mV outputs) and high accuracy voltage comparators. The Josephson junction is used as a precise frequency-to-voltage converter with a conversion factor equal to 2e/h. A series of measurements of 2e/h has been carried out at NBS referenced to the as-maintained unit of emf based on a large group of standard cells. Measurements made at regular intervals over a one year period (1971 to 1972) indicate that the mean emf of this group of standard cells has decreased about 4 parts in 107. Primarily to remove the effects of this drift, on July 1, 1972 a new as-maintained unit was defined by choosing a value of 2e/h consistent with the existing unit of emf. The adopted value of 2e/h is 483593.420 GHz/VNBS. The precision (one standard deviation) with which the new unit of emf can be maintained with the present techniques and apparatus is about 2 parts in 108. The accuracy of the present system is estimated to be 4 parts in 108. Comparisons of 2e/h systems at different national laboratories have been limited by uncertainties associated with the physical transfer of standard cells. In order to determine the relative agreement of the various 2e/h systems with precision better than 1 or 2 parts in 107, it appears desirable to compare 2e/h systems directly by transporting one of them.

The ampere and the electrical units in the quantum era

“…Towards the Josephson voltage standards. Since a Josephson junction acts as a perfect frequency-to-voltage converter based on fundamental constants, it was soon proposed to use these steps to improve the voltage standards [46,47,48] taking advantage of the high accuracy of time references. Today, microwave sources can be referenced and locked to atomic clocks to a level of a few parts in 10 11 .…”

Quantum Hall Effect: Discovery and Application

“…Brian Josephson's groundbreaking publication in 1962 entitled "Possible New Effects in Superconductive Tunneling" (27) pointed out the connection between fundamental constants and voltage V. Experiments confirmed that the predicted relation V = h/2e·f, where f = microwave frequency, is universal (i.e., independent of the superconducting material) and reproducible with high precision (28). The US National Bureau of Standards (NBS) demonstrated that a voltage generated by the Josephson effect is more stable and more reproducible than the international voltage standard realized by the Weston cadmium cell, an electrochemical cell with a voltage of V = 1.01864 V. In 1972, the NBS decided to define the legal volt in the US via the Josephson effect by using a fixed value for the Josephson constant K J = 2e/h = 483593.420 GHz/V NBS (29). However, scientists in France and Russia had measured slightly different Josephson constants because the Weston cells at the various metrology institutes were not identical at the level of 3 ppm.…”

A Practical Josephson Voltage Standard at One Volt

“…The use of superconducting-quantuminterference device (SQUID) null detectors in the early 1970s allowed this to be tested to a few parts in 10 9 , and thus the Josephson effect had obvious potential for use as a voltage standard [14]. By the early 1970s, NIST staff had set up a potentiometric measurement system in Gaithersburg that compared 2 mV to 10 mV dc Josephson junction voltages with 1.018 V standard cells to a few parts in 10 8 [15,16]. International comparisons in 1971-72 among national metrology institutes (NMIs), including NBS, the National Physical Laboratory (NPL) in the U.K., the National Research Council (NRC) in Canada, the National Standards Laboratory (NSL) in Australia, and the PhysikalischTechnische Bundesanstalt (PTB) in Germany, as well as the International Bureau of Weights and Measures (BIPM), found that the measured values of 2e/h agreed with each other to within 2ϫ10 -7 [17].…”