Atomic and molecular systems for radiation thermometry

Atoms and simple molecules are excellent candidates for new standards and sensors because they are both all 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 $^{85}$Rb Rydberg atoms to probe the BBR spectrum near 130GHz. This primary, {\it 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 blackbody radiation atomic sensor (CoBRAS) uses a vapour of $^{85}$Rb 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 $u(T)\approx 0.13$~K, with a clear path toward implementing a primary

💡 Research Summary

This paper presents a comprehensive exploration of using atoms and simple molecules as primary standards and sensors for radiative temperature measurement, a field known as radiation thermometry. The core premise leverages the fundamental identity of all atoms of a given species and isotope, whose properties are dictated by immutable quantum physics laws. The proposed method is based on precisely measuring the rate at which blackbody radiation (BBR) drives transitions between specific quantum states within an atom or molecule, either through absorption or stimulated emission.

The theoretical foundation is established through Einstein’s relations, showing that the BBR-induced transition rate (Ω_ij) between two states is proportional to the product of the spectral energy density of the radiation (governed by Planck’s law) and the square of the dipole matrix element for the transition (|⟨i|d|j⟩|^2). Since this rate depends on fundamental constants and well-defined atomic parameters, measuring it allows for determining temperature without requiring calibration against another thermometer, thus enabling a primary temperature standard.

A significant portion of the paper is dedicated to developing the necessary framework for interpreting real-world experiments. Real atomic systems are multi-level, and the population dynamics are modeled using a set of coupled rate equations (Eq. 3). This model incorporates spontaneous emission rates (Γ_ij) and BBR-induced rates (Ω_ij) between all relevant levels. The paper demonstrates how careful experimental design can simplify this complex model, for instance, by isolating an effective two-level system. It also highlights the versatility of atoms like Rubidium, which possess a dense spectrum of transitions across microwave to infrared frequencies, allowing researchers to “choose” the BBR frequency band of interest by selecting an appropriate atomic transition.

The paper then details two recent experimental realizations that probe vastly different regions of the blackbody spectrum, both using ⁸⁵Rb atoms:

1. The Cold Atom Thermometer (CAT): This experiment uses a laser-cooled cloud of ⁸⁵Rb atoms excited to a Rydberg state (32S₁/₂). The BBR around 130 GHz drives transitions to the nearby 32P state. The populations in these states are monitored over time using state-selective field ionization. By analyzing the time evolution of the population ratio between the initial (32S+31P) and the target (32P) states, the BBR-induced transition rate and thus the environmental temperature are extracted. CAT currently achieves a total uncertainty of approximately 1% and operates as a primary measurement.

2. The Compact Blackbody Radiation Atomic Sensor (CoBRAS): This sensor uses a room-temperature vapor cell of ⁸⁵Rb. It monitors the BBR-induced population of the 7S state via a transition at 24.5 THz. The 7S state subsequently decays via spontaneous emission to 6P and 5D states, emitting fluorescence at 420 nm and 743 nm, respectively. The ratio of these fluorescence signals is sensitive to the initial 7S population, which in turn depends on the BBR intensity (temperature). CoBRAS demonstrates excellent relative precision of u(T) ≈ 0.13 K and has a clear pathway to becoming a primary standard.

The paper concludes by discussing the advantages and current limitations of this approach. Key advantages include the potential for primary standardization, inherent spectral filtering via the atomic transition’s natural linewidth, and good theoretical sensitivity. The main limitations are the accuracy of theoretical dipole matrix elements (currently around 1% for best-known transitions) and the complexity of modeling multi-level dynamics. Future directions include using laser-cooled molecules to create clean two-level systems and improving atomic theory calculations. Overall, the work establishes atom- and molecule-based systems as a promising and fundamentally grounded route for advancing radiation thermometry.