High-accuracy measurements of electrical quantities are essential to modern engineering, manufacturing, research, and quality assurance, among many other pursuits. Accordingly, one major goal of the NIST-on-a-Chip program is to place miniaturized, quantum-based electrical standards in the hands of users. NIST has decades of experience in inventing and testing quantum voltage standards, and has more recently made progress in a highly promising technology for a quantum current standard. The sensors in both kinds of device are inherently chip-scale.
Voltage. The universally accepted standard for voltage relies on large arrays of tiny devices called Josephson junctions (JJ), each comprising two superconducting elements separated by a thin barrier of non-superconducting material. Irradiating those junctions with microwave energy at various frequencies, or applying current pulses to the junctions, produces different outputs which serve as standards that are intrinsically accurate because they are based on quantum constants and frequencies that can be controlled to exquisite precision.
Applying microwave radiation to a JJ produces step-wise increments of dc current and voltage that take on exact, discrete (quantized) values that depend only on the applied frequency and fundamental invariants of nature. By applying a bias current across the junctions, arrays of JJs generate exact ac voltages as well as spectrally pure waveforms. (Click here for one technical description.)
NIST scientists developed and continuously improved the first programmable Josephson voltage standard, which is accurate to a few parts in 10 billion, and allows measurements of dc voltage in the commercially relevant range of ±10 V by precisely synchronizing inputs to about 300,000 JJs. Ongoing improvements to ac voltage standards and waveform generation will soon make the same kind of quantum-based accuracy available to industries such as wireless telecommunications that depend critically on high-accuracy transmission of microwave signals.
Current. Although the ampere is a SI base unit, it is difficult to realize directly. At present, the most accurate measurements are made using quantum-based determinations of the volt and the ohm, and then calculating the ampere from those values. The impending redefinition of the ampere in terms of the elementary electrical charge (e) will change that practice, and NIST researchers are at work on a chip-scale current-measurement device that is ideally suited to the new definition.
The goal is to develop a compact, robust, quantum-based standard for current that can be widely deployed on-site wherever it is needed to streamline the calibration of critical equipment. To that end, the NIST team is fabricating and testing devices based on both single-electron transport and superconducting Cooper pair transport. These can be configured so that they can count and collect individual electrons (or pairs of electrons) with extraordinary accuracy, operate at gigahertz frequencies, and can be combined into parallel arrays of hundreds of devices to boost the output.
One grand challenge is to understand how to operate these devices fast enough that they pass more than 1 nanoampere with at most 1 error for every hundred million electrons that pass. This level of performance demands an exquisite understanding of the role that each step in the fabrication process and each part of the device operation plays in determining the accuracy of the resulting current.
For a more detailed description of NoaC research on quantum electrical standards, visit these pages: