All In One: NIST Develops Single Device to Realize Electrical Standards

Researchers at the National Institute of Standards and Technology (NIST) have developed one of the first all-in-one instruments for realizing the most up-to-date standards for voltage, resistance, and current. The prototype instrument—a sort of Swiss Army knife for electrical standards—could pave the way for a compact tool that would save both time and money by enabling engineers in their own laboratories to directly calibrate electrical equipment to international standards.

Currently, engineers must periodically ship devices that act as calibration standards to NIST or other calibration labs to ensure the measuring instruments are directly tied to international standards. The new instrument would eliminate the need to ship such devices.

In accordance with the International System of Units (SI), the standards are based on the fundamental constants of nature and are critical for applications ranging from the proper operation of electrical power grids to advanced military equipment.

Since 1990, the ohm (the SI unit of resistance) has been based on the quantum Hall effect (QHE), in which the resistance of certain atomically-thin sheets of material takes on discrete (quantized) values, dictated by the fundamental constants of nature. To create the quantized values of resistance, the sheets must be cooled to 4 degrees above absolute zero (4 Kelvin or -269 Celsius) and subject to a high magnetic field oriented perpendicular to the flow of current in the material.

For years, NIST metrologists have sought to realize the quantum electrical standards for current, voltage, and resistance in a single instrument, an endeavor that would greatly simplify the dissemination of these standards to industry, government, and academia. That’s been a challenge because the large magnetic field required for the QHE resistance standard – six to nine times greater than the field used in a typical medical MRI machine – would cause superconducting devices employed for the voltage standard to fail. As a result, researchers have had to use separate cryostats – or even separate laboratories – to measure voltage, current, and resistance.

In 2013, however, scientists discovered a new type of quantized resistance, the quantum anomalous Hall effect (QAHE), which is also based on the fundamental constants. Like the QHE, the QAHE occurs in atom-thin sheets of material, but the sheets must be cooled to even lower temperatures, about one-hundredth of a degree above absolute zero (0.01 K). Significantly, the QAHE is induced with a magnetic field only one-fifth to one-fortieth of that required for the QHE. Moreover, the field is only required to magnetize the sample; it can then be switched off.

With a magnetic field no longer needed, the researchers were able to insert the quantum device that generates precision voltages into the same cryostat that houses the QAHE resistance standard.

The precision voltage standard, known as a programmable Josephson voltage standard, consists of an integrated circuit chip containing pairs of miniature superconductors separated by a thin metal barrier (known as a Josephson junction). Inside these junctions, electrons pair up and may tunnel back and forth across the barrier. When a microwave signal is applied, the tunneling produces a highly accurate voltage that depends only on the signal’s frequency and the fundamental constants.

To house these instruments, the researchers built a cryostat that occupies a floor space of about 4 square meters, comparable to those commonly used in quantum computing laboratories.

“This early prototype is proof that practical integration is feasible,” said NIST researcher Jason Underwood.

While the achievement is an important milestone in quantum standards development, widespread distribution of the prototype is still years away. That’s mainly because most materials now known to exhibit QAHE only do so at extremely low temperatures, requiring a massive refrigeration system. Scientists, including those at NIST, are searching for materials that reliably show the QAHE at higher temperatures. Such materials would allow the electrical standards to be realized in a more portable cryostat that would be easier for industrial laboratories to use.

“As the performance of QAHE materials advances, we can shrink the size of the cryostat substantially, especially if we can achieve a robust QAHE at temperatures above 0.1 Kelvin (a tenth of a degree above absolute zero),” added Underwood. “The cryogenic hardware at those higher temperatures is far more compact and transportable. Like NIST’s standard reference instruments, a future unified instrument could fit inside of a standard 19” equipment rack, calibrating multiple SI units in a space that currently offers just one.”

The relative accuracy of the new instrument for voltage, resistance, and current is several parts in a million, comparable to the best calibration and measurement capabilities of national metrology institutes across the globe.  But researchers say a less expensive version of the instrument with lower accuracy would also suffice for creating the necessary electrical standards.

“While we always seek to reduce our uncertainties, a deployable calibration instrument doesn’t necessarily have to achieve uncertainties as small as national metrology institute standards,” Underwood said.  “The uncertainties just have to be low enough to meet the customer’s calibration goals.”

NIST researchers developed the new instrument in collaboration with scientists from Stanford University; the University of Maryland, College Park; and the University of California, Los Angeles (UCLA). The scientists described their study online in Nature Electronics on Aug. 12.

Paper:

Rodenbach, L.K., Underwood, J.M., Tran, N.T. M., Panna, A. R., Andersen, M.P., Barcikowski, Z.S., Payagala, S.U., Zhang, P., Tai, L., Wang, K.L., Elmquist, R.E., Jarrett, D.G., Newell, D.B., Rigosi, A.F., and Goldhaber-Gordon, D. A unified realization of electrical quantities from the quantum International System of Units. Nature Electronics, 2025, posted online Aug. 12, 2025. DOI: https://dx.doi.org/10.1038/s41928-025-01421-2