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New Roles for National Metrology Institutes in the Post-Redefinition World

Ion Trap
Peek into a microscope to check out this ion trap, a means to confine atoms that have been ionized (electrons added or taken away). The trap itself is made from gold electrodes on a sapphire chip and was used to amplify and measure the tiny motions of a single magnesium ion using a "quantum squeezing" technique. The trap was also used to "entangle" multiple ions, a quantum feat that links their properties at a distance. NIST is at the forefront of research in the revolutionary field of quantum information, thanks to work like this. Thanks to work like this, NIST is at the forefront of research in the revolutionary field of quantum information, which includes efforts to build a practical quantum computer.
Credit: D. Slichter/NIST

Until the 2019 redefinition of the International System of Units (SI), accurate measurements of properties such as mass and temperature required the use of standards and instruments traceable – through a documented chain of calibrations – to a national metrology institute such as the U.S. National Institute of Standards and Technology (NIST).

For example, NIST maintained America’s copies of the metal international prototype kilogram from which all measures of mass were derived and the glass “triple-point of water” cells used to calibrate thermometers. Without traceability to those and other standards, measurements could not be authoritative.

But once all the SI base units were redefined in terms of fundamental constants of nature, that paradigm changed. It was suddenly possible in principle (and increasingly in practice, thanks to the advent of intrinsically accurate, portable quantum-based sensors) for workers in laboratories or even on a factory floor to create and use their own independent standards.

A group that can realize the Planck constant – which describes the properties of particles and waves on the atomic scale – can make mass measurements without comparison to an official metal standard. And so forth for other measurements. Indeed, NIST has a program, called NIST on a Chip, to encourage exactly that sort of independence by developing and distributing miniaturized, quantum-accurate devices.

So in those new circumstances, what is the function of NMIs? That is the question raised in an article written by NIST researchers Barbara Goldstein and Jay Hendricks with colleagues at nine different metrology organizations* around the world published in the journal Nature Physics on July 12, 2022.

The authors argue that “NMIs can continue to play an important role by providing a worldwide quality-ensuring metrology network.”

small atomic clock with coffee bean for scale
Next-generation technology, coffee bean for scale – the heart of this miniature atomic clock ticks at high optical frequencies based on the vibrations from rubidium atoms confined within. While standard atomic clocks use cesium atoms (which also currently provides the world’s definition of a second), optical clocks with rubidium atoms can be more precise (about 100x better) and tick for longer on their own, without outside help. This may make the optical clock a solid candidate for a future redefinition of the second. Until now, optical clocks have been bulky and complex, used only as experiments by metrological institutions. Chip-sized optical clocks like this could eventually replace traditional oscillators in navigation systems and telecommunications networks. Shoutout to our collaborators at Caltech, Stanford and Draper Lab!
Credit: M.T. Hummon/NIST

“There will be a growing need for NMIs to develop relevant, increasingly automated measurement procedures and to oversee their correct implementation to ensure accurate calibration with the quantum standards at the point of use,” they write. “Furthermore, designing, building and operating quantum standards remains a complex and demanding procedure. Things can go wrong leading potentially to incorrect operation, which — in the absence of a ‘classical’ traceability route — might not be obvious.”

The authors cite numerous examples in which NMI expertise will be critical, including quantum computing, the likely redefinition of the second, and the effort to achieve ultra-secure quantum communications. Those applications and more will continue to depend on the “immense expertise” of NMIs.

* The authors are members of a technical committee of IMEKO, an international measurement confederation. The committee subject area is quantum measurement and quantum information.

Released July 18, 2022