PRO VOLT: Programmable voltage standard for AC and DC metrology

• Forschungsstelle: METAS
• Projektnummer: 211.98.03
• Projekttitel Englisch: PRO VOLT: Programmable voltage standard for AC and DC metrology

Schlüsselwörter (Englisch)

Josephson effect, Josephson array voltage standard, representation of the volt.

Kurzbeschreibung (Englisch)

Nowadays, superconductor-normal metal-superconductor (SIS) Josephson junction arrays are routinely used to represent the SI unit of voltage, the volt. The intrinsic physical properties of this type of junctions make them sensitive to external noise and prone to chaotic behavior. With the advancement of the technology, several new types of Josephson junction arrays have recently been developed. Theses arrays are based on superconductor-normal metal-superconductor (SNS) and SINIS junctions. The new types of junctions are intrinsically more stable and allow a faster voltage selection. Therefore, this technological progress will broaden the application of the Josephson effect in metrology. In particular, programmable voltage standards and D/A converters of fundamental accuracy can be envisioned. Nine European national laboratories and an electronic company are co-operating in the ProVolt project. The research objective for the first year was to manufacture integrated superconducting circuits for low output voltage and to develop the instrumentation needed to test, characterize and make precision measurements with these new Josephson arrays.
The design and the fabrication of a new cryoprobe for the measurements of SINIS arrays and the characterization of SINIS arrays fabricated at the Physikalisch-Technische Bundesanstalt, Germany, are the major tasks OFMET has to accomplish within the project.

Projektziele (Englisch)

Development of an intrinsically stable Josephson array voltage standard.

Abstract (Englisch)

At OFMET, the first task was to design and fabricate a new cryoprobe for the measurements of SINIS arrays. This cryoprobe allows to measure binary arrays at a frequency of 75 GHz. An oversized circular wave guide brings the microwave to the array. The wiring allows to feed up to 13 segments on the array. In addition, two precision wires used for voltage measurement are provided. The whole probe is inserted in a transportable helium Dewar at a temperature of 4 K.
In 2000, the measurements with a SINIS array provided by the Physikalisch-Technische Bundesanstalt (PTB, DE) were started. A thorough characterization of the physical properties of the array was performed. The array was found to perform very well. Clear voltage steps were observed. It was found that the voltage steps of the various segments of the binary array had different widths depending on their location in the array. This effect was clearly not caused by a spread in the junction's parameters during the fabrication process. Instead, it was due to an inhomogeneous distribution of the power to the array. Due to the large number of junctions, this is a difficult and complex problem that might be solved in the next generation of arrays.
In addition, a precise test of the plateau flatness was performed by making a comparison between the SINIS array and our SIS voltage standard. It was found that the SINIS voltage step was not perfectly flat when observed at the nanovolt level. That is not what was observed earlier when the array was measured at PTB. Aging of the array or its high sensitivity to external noise might be the cause of that slight deviation. This point will be further investigated in a close future.
In conclusion, the ProVolt project was very successful for OFMET. A new measurement system was developed that allows to test SINIS Josephson junction arrays. Precision measurements were carried out to assess the accuracy of the new SINIS arrays. A collaboration has been initiated between OFMET and PTB. This collaboration will certainly continue because more measurements are needed before the new arrays can reliably be used as primary voltage standards.

Umsetzung und Anwendungen (Englisch)

A new primary voltage standard based on SINIS Josephson junction arrays was developed. Precision measurements were carried out to assess the accuracy of the new SINIS arrays. The new system is fully programmable and allows a fast and very accurate linearity calibration of high-end digital voltmeters.