Primary thermometry below 25 K: a decade of low-uncertainty measurements linked to the new kelvin

In 2019, the basic unit of thermodynamic temperature, the kelvin, was redefined by fixing the value of the Boltzmann constant, opening new avenues for implementing and disseminating the kelvin with lower uncertainty, especially at temperatures below 25 K. In response, the Mise en Pratique for the definition of the kelvin (MeP-K) (2019) has recommended several primary thermometry methods, including acoustic gas thermometry (AGT), dielectric-constant gas thermometry (DCGT), refractive index gas thermometry (RIGT) and Johnson noise thermometry (JNT), as viable alternatives for realizing and disseminating the kelvin. Since the International System of Units (SI) revolution, significant progress on implementing the new kelvin has been made below 25 K. This progress indicates that primary thermometry, particularly its relative variants, can offer promising practical options for realizing and disseminating thermodynamic temperature directly linking to the new kelvin below 25 K with lower uncertainty. This is very important for metrological applications of science and industry, which require precise and accurate temperature calibrations. This article is part of the Theo Murphy meeting issue 'The redefined kelvin: progress and prospects'.