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We report here the results of a preliminary study of coherent microwave radiation emitted by a small array of interacting Josephson tunnel junctions, including the first direct observation of the superradiant state in pairs of junctions.

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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.

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