Quantum-based impedance bridges
Realizing electrical units in the AC voltage regime with quantum precision
Today, realizing electrical units – or rather calibrating electrical quantities – in the AC voltage regime (i.e. impedances) is mostly done by means of calculable, conventional artefacts or via DC-AC voltage transfer based on quantum resistance standards exploiting the quantum Hall effect. The bridges used for this purpose are based on inductive voltage dividers and achieve excellent measurement uncertainties in the range of only a few parts in a billion. The use of such bridges is, however, limited with regard to the realizable voltage ratios, phase angles and frequencies; moreover, they are complex and require manual operation, thus making them accessible to experts only.
To overcome these limitations, PTB, together with 10 other partner institutes, is developing flexible and more easily automatable impedance bridges within the scope of a European metrology research project titled “Graphene impedance quantum standards (GIQS)”. Hereby, modern quantum voltage sources based on pulse-driven Josephson voltage standards are used instead of inductive voltage dividers to generate the bridge voltages. These so-called Josephson impedance bridges allow the flexible and accurate realization of AC voltages at different frequencies, with any phase angles, and with high stability over time. Combined with a quantum Hall resistor, these bridges allow nearly any random impedance to be calibrated accurately.
The aim pursued by PTB, which consists in developing a quantum Hall resistor made of graphene, offers even more advantages for the practice-oriented use of Josephson impedance bridges. Conventional quantum Hall resistors made of semiconducting heterogeneous structures must typically be operated at high magnetic fields and low temperatures, which makes expensive and complex cryogenic magnetic systems and the supply with liquid helium indispensable. In contrast to this, the particular properties of graphene enable use in less cost-intensive and more easily operable magnetic systems with small cooling units.
However, the graphene circuits used for this purpose still have to be further optimized. The high accuracy of the Josephson impedance bridge provides ideal preconditions for these investigations. The first measurements performed at PTB on a 10 nF capacitance standard have already shown very good reproducibility of the results in the range of a few parts in 108 and have thus confirmed the great potential of this new measurement method. Further optimization of the bridge measuring technology and of the graphene standard resistors is currently being worked on.
Contact
Stephan BauerDepartment 2.6 Electrical Quantum MetrologyPhone: +49 531 592-2633stephan.bauer@ptb.de
Scientific publication
S. Bauer, Y. Pimsut, R. Behr, O. Kieler, M. Kruskopf, L. Palafox, J. Lee, J. Schurr: AC quantum Hall resistance combined with a four-terminal pair pulse-driven Josephson impedance bridge. Accepted for publication in IEEE Trans. Instrum. Meas. (2020)