In a brightly lit subterranean lab at the National Institute of Standards and Technology (NIST) sits a room-sized electromechanical machine called the NIST-4 Kibble balance.

The instrument can already measure the mass of objects of roughly 1 kilogram, about as heavy as a quart of milk, as accurately as any device in the world. But now, NIST researchers have further improved their Kibble balance’s performance by adding a custom-built device that provides an exact definition of electrical resistance. The device is called the quantum Hall array resistance standard (QHARS); it consists of a set of several smaller devices that use a quirk of quantum physics to generate extremely precise amounts of electrical resistance. The researchers describe their work in a Nature Communications paper.

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