Josephson frequency-voltage measurement

Parker and Langenberg [6]. An important feature of the Josephson frequency-voltage measurements is their complete independence from geometry. It is precisely this fact which has permitted refinement to reach its present level near 0.01 ppm and there appears no obvious obstacle to further improvement. 

The RF SQUID is based on the AC Josephson effect, uses only one Josephson junction, and is less sensitive than the DC SQUID, but is cheaper and easier to manufacture its SQUID is inductively coupled to a resonant tank circuit. Depending on the external magnetic field, as the SQUID operates in the resistive mode, the effective inductance of the tank circuit changes, thus changing the resonant frequency of the tank circuit. These frequency measurements can be easily done, and thus the losses that appear as the voltage across the load resistor in the circuit are a periodic function of the applied magnetic flux with a period of 0. 

The first of the macroscopic quantum effects to be demonstrated was the ac Josephson effect, whereby 2e/h is established via the invariant ratio nV/V for irradiation at frequency V and noting the n th step voltage, V. This has been a powerful tool in the service of electrical measurements. The history, which has been well reviewed by Petley, shows a rapid evolution from laboratory curiosity through a significant determination of a to the role of maintenance of the operational unit of voltage [16]. Incidentally, the early history of this area of work is an instance in which electrical metrology made a distinct impact on atomic physics. It will be recalled that a key issue of the early 1960 s was the discrepancy between values of a obtained from fine-structure measurement and those deduced from hyperfine structure. This story is well told in the widely known summary, due to Taylor,... 

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