Atomic and molecular systems for radiation thermometry

Atoms and simple molecules are excellent candidates for new standards and sensors because they are both identical and their properties are determined by the immutable laws of quantum physics. Here, we introduce the concept of building a standard and sensor of radiative temperature using atoms and molecules. Such standards are based on precise measurement of the rate at which blackbody radiation (BBR) either excites or stimulates emission for a given atomic transition. We summarize the recent results of two experiments while detailing the rate equation models required for their interpretation. The cold atom thermometer (CAT) uses a gas of laser-cooled 85Rb Rydberg atoms to probe the BBR spectrum near 130 GHz. This primary, i.e. not traceable to a measurement of like kind, temperature measurement currently has a total uncertainty of approximately 1%, with clear paths toward improvement. The compact BBR atomic sensor (CoBRAS) uses a vapour of 85Rb and monitors fluorescence from states that are either populated by BBR or populated by spontaneous emission to measure the blackbody spectrum near 24.5 THz. The CoBRAS has an excellent relative precision of u(T) ≈ 0.13 K, with a clear path toward implementing a primary measurement. This article is part of the Theo Murphy meeting issue 'The redefined kelvin: progress and prospects'.