National Physical Laboratory

Josephson Voltage Research

Binary Josephson Junction Array Chip
Chip containing an array of 8192 Josephson
junctions that are divided into binary segments.
The individual wiring tracks for biasing each
segment individually can be seen.
This array was fabricated at PTB in Germany
DC voltage measurements at NPL are traceable to the SI volt via the Josephson effect, which establishes an accurate potential difference based on the fundamental constants, h and e, and frequency. Such a standard is referred to as a ‘representation’ of the SI volt. It is a standard that is based on a quantum effect and, having been measured against the SI system of units, has an agreed value, with an uncertainty (see Quantum Electrical Standards and the Metrological Triangle). This quantum standard is an intrinsic standard; such a standard does not need repeated measurement against the realisation of the SI unit (in this case the volt) in order to validate its accuracy. Intrinsic standards are important tools in disseminating accurate measurement in a way that is efficient and economic.

AC voltages are currently compared to DC voltages, at the most accurate level, in a measurement of the equality of the heating effect, and hence the root mean square (rms) value is found. Thus the AC rms voltage is derived from the DC standard using a transfer standard, in this case a thermal converter. Thermal converters require calibration, and measurements using thermal converters can take several days. In order to eliminate the AC-DC transfer, in collaboration with other leading NMIs, NPL is researching the generation of accurate voltage-waveforms from an intrinsic standard, using the Josephson effect. NPL has developed world-leading control instrumentation for Josephson-derived waveform-synthesis, and has supplied Josephson voltage systems to many other NMIs.

NPL Binary Bias Source
15-channel binary bias source used
at NPL to bias binary Josephson Junction
arrays in order to simulate voltage
waveforms with quantum accuracy

Josephson-derived waveform synthesis follows the principle of the binary-weighted digital-to-analogue converter. An array of Josephson junctions is divided into sections, connected in series such that the  potential diffrerence across each junction is summed. The number of junctions in each section increases from section to section in binary series: 1, 2, 4, 8, 16… . thus the required voltage can be produced by converting to a binary number and switching on the required sections, which can be done individually, independent of the other sections.
It is possible to generate a time-varying  voltage-waveform by biasing appropriate combinations of junctions as a function of time. The waveform is is a stepwise-approximation to the desired waveform but, crucially, all of the steps are Josephson-derived voltages. The value and uncertainty of  the rms and peak-to-peak voltage of the waveform is calculable.

A limitation of this technique has its origin in the time taken by the voltage to switch between two quantised levels. During this time, believed to be from 10 ns to 50 ns in duration, the potential difference across the array is unknown and this contributes to the uncertainty in the  rms voltage. Factors contributing to the duration of the transient are the RC time constants of the array sections, the response of the control instrumentation, and the connecting cables.

AC Waveform Measurement – Quantum Traceability

As well as research into producing a standard source of voltage-waveforms, NPL is carrying out research into schemes for measuring voltage-waveforms, up to 10 V amplitude and 100 Hz bandwidth, traceable to quantum standards. Our approach uses a Delta-Sigma (DS) converter incorporating a pulsed reference voltage provided by a Josephson array. The challenges here include those encountered in the generation of waveforms, i.e. transient contributions when the reference voltage is not quantised, as well as understanding the uncertainties introduced by the differencing part of the DS modulator.

Last Updated: 6 Aug 2015
Created: 8 Jun 2007


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