An EMPIR project’s quantum standard allows SI traceability to AC voltage measurements for the first time

Since May 2019 all seven base units of the SI have been defined by unchanging, fundamental constants of the universe. However, whilst DC voltage measurements can be directly traced to the standard volt until recently alternating voltage measurements could not. This was due a lengthy traceability chain involving several steps leading to larger sources of uncertainty. This measurement problem was addressed by the EMPIR project Waveform metrology based on spectrally pure Josephson voltages’ (15SIB04, QuADC).

To achieve this a new quantum-based voltage standard was developed, which generates spectrally pure arbitrary waveforms with quantised output voltages.

The new standard not only demonstrates high transportation stability but unlike traditional thermal transfer standards it can make measurements in minutes rather than hours and with 100x less uncertainty.

The new quantum standard

The new quantum standard derives its accuracy and low uncertainty from the integration of ‘state of the art’ instrumentation developed in the project.

At its heart is a Josephson Arbitrary Waveform Synthesiser constructed from thousands of series-connected superconducting Josephson junctions held at extremely low temperatures.  When arbitrary current pulses are applied it generates quantized voltage-time pulses which are used to construct voltage waveforms directly traceable to the SI.

The production of the standard was made possible through the combined international expertise and collaboration of all the partners in the EMPIR project consortium:

• The initial Josephson Arbitrary Waveform Synthesiser system was developed at PTB (Germany) and verified through inter-comparison work performed at NPL (UK) with further validation work performed by PTB, NPL, VTT MIKES (Finland), INTI (Argentina), CMI (Czech Republic), APPLICOS (Netherlands) and Signal Conversion Ltd (UK).
• Two types of an optical pulse pattern generator were developed to drive the synthesiser. NPL developed a system based on FPGA (field programmable gate array) technology and VTT MIKES developed a system based on a pulsed laser.
• Two project partners, JV (Norway) and USN (Norway), cooperated to optimise the low temperature photodiode mounting procedure for the new system, making it reliable and robust to thermal cycling.
• CMI, with input from PTB and NPL, led the development of the user-friendly software for integrating automation techniques and algorithms in the new quantum AC voltage standard.
• METAS (Switzerland), with input from VSL and PTB, led the development of ‘impedance matching’ techniques. These techniques help cancel out the loading effect caused by cables and decreased the difference between the voltage applied to the device under test and the calculated voltage at the synthesiser.
• The architecture of the voltage dividers, which are used to increase the voltage range of the new quantum-based voltage standard, were based on work by RISE (Sweden)  and TUBITAK UME TUBITAK UME (Turkey) with input from CEM (Spain), CMI and VSL.

This new standard is now available from PTB and it is anticipated to be used in work with CERN to help validate their new HPM7177 digitizer, for the measurement of the current in the new magnets installed as part of the Large Hadron Collider’s (LHC) high luminosity upgrade - the HL-LHC.

The NPL optical pulse pattern generator, with its ability to synthesis voltage waveforms and make real time measurements due to the 20 MHz feedback circuit developed in QuADC, was presented at the 2020 CPEM conference (under the title: “Real-time quantum-accurate voltage waveform synthesis”). This capability is opening up the development of new measurement techniques such as the characterisation of the total harmonic distortion of high-performance analogue to digital converters (ADCs).

The prototype optical pulse pattern generator that VTT developed in QuADC is based on a custom-made mode-locked-laser and a fibre optic time-division multiplexer. As well as being successfully used to drive a Josephson junction array its development continues. VTT’s vision is that through the deeper understanding of the potential of the device it could be used for driving future quantum technological devices beyond its application to voltage metrology. This includes such things as projects to develop an optical ultrafast data bus for a quantum computer or data routing based on superconducting electronics.

RISE is continuing to develop their voltage divider in an internal project, combining the divider, phase adjustment and buffer amplifier into one unit. TUBITAK UME plans to use its new divider in the EMPIR Research potential project ‘A digital traceability chain for AC voltage and current’ (17RPT03, DIG-AC) which will advance the European capability for digital evaluation of dynamic AC voltage and current by utilising quantum standards and developing publicly available measurement systems.

These new measurement instrumentation and facilities will allow a range of industrial end-users to develop previously unachievable processes and products that rely on fast analogue-to-digital conversion, such as power quality monitors, which provide support to the smart grids of the future. 

Dr. Behr from PTB, who coordinated the project, said about the work

“This project has advanced AC quantum voltage standards in a variety of ways. Thus, it will soon lead to an improvement of modern, fast sampling methods, as they are used in digital sensors and instruments in almost all areas of our life.”

This EMPIR project is co-funded by the European Union's Horizon 2020 research and innovation programme and the EMPIR Participating States.

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