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Quantum-based impedance bridges

Realizing electrical units in the AC voltage regime with quantum precision

PTBnews 3.2020
Especially interesting for

metrology institutes

AC current and voltage measuring technologies

calibration laboratories

manufacturers of high-precision electronics

Within the scope of a European metrology research project, PTB is investigating the utilizability of novel quantum Hall resistance standards based on graphene for impedance metrology. The activities aim to develop quantum-based impedance bridges to simplify the calibration of electrical AC voltage quantities for practice-oriented, flexible and efficient use, e.g. in calibration laboratories or in industry. The first measurements performed have already shown very good reproducibility and demonstrated the new method's potential.

Schematic diagram (strongly simplified) of a Josephson impedance bridge combined with a quantum Hall resistor (QHR). The current for the measurement is supplied by two current sources (left and right); voltage is measured in a quantum-based way via two pulse-operated Josephson voltage standards (U1 and U2). After aligning the bridge by adjusting the voltage ratios and phase angles, the voltage measured by the detector is zero; thus, the ratio of the impedances of the device under test (ZDUT) to those of the quantum Hall impedance standard (ZQHR) is equal to the ratio between the two bridge voltages U1 and U2 which is very precisely known.

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.



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)