From 11 to 22 of april another research visit occured. Vitor Cabral from IPQ (Instituto Português da Qualidade) visited NPL (National Physical Laboratory, United Kingdom) and studied Josephson Voltage Systems. The system setup used for the experimental work was a “quantum-referenced waveform synthesizer” as a standard to calibrate directly a Voltmeter. The reference system was formed by a Josephson array of 1 V biased by a current source developed by NPL. As transfers standards was used a commercial DAC that was compared to the Josephson voltage by a sampling process of the differential voltage measured by commercial ADC. The signal of the referenced DAC was used as standard signal to be measured by a DATRON 4920 Voltmeter. The measurements were conducted in the way to exploring different configurations of the setup, equipment and/or measurements conditions in order to observe the repeatability and/or reproducibility of the results. These experimental work allowed the acquisition of practical knowledge in several operating features of the system as the: operation of the Josephson array by the control of the bias source associated; the way to check and recognize the happening of flux trapped and the process to deal with it, by the heating of the array; the major role of the control of the synchronism between the different signals to assure the observation of valid data; the influence in the results of the sampling time instant related to its position in the step of the voltage array; the dependence of the system to the interconnections used and to the consequent current flow.
Javier Díaz de Aguilar Rois from CEM (Centro Español de Metrología) visited NPL. The visit was mainly related on the feasibility of an ac source based on Digital-to-Analog-Converter (DAC) directly corrected by a Programable Josephson Voltage Standard (PJVS). The initial characterization of the system shows the improvement of the incorporation an ultrastable DAC on the system. More work is necessary but preliminary results are very promising to use this configuration as a new standard. This new standard can be also used for the intercomparison of AC quantum standards.
ACJVS in NPL laboratory is shown on follwing picture.