Spectrometers: Breakthrough spectrometer defines Kelvin temperature

Researchers have developed an ultrasensitive absorption spectrometer that can measure Boltzmann's constant.

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Researchers from the University of Adelaide, the University of Western Australia (Perth), and the University of Queensland (Brisbane) have developed an ultrasensitive absorption spectrometer that can measure Boltzmann's constant—the parameter that relates energy to temperature.1 Because the technique can be duplicated in laboratories worldwide, it will allow scientists to create a universally agreed-upon temperature scale and define a standard for Kelvin temperature based on laser measurements of the speed of atoms in a gas rather than the freezing point of water (which varies because the chemical composition of water varies worldwide).

Quantum-limited spectroscopy

Accurate measurement of the spectral linewidths and transition frequencies of atoms can be used not only for temperature determination, but also in gas sensing, planetary atmosphere studies, thermometry in tokamak fusion reactors, and countless other atomic physics applications.

The spectrometer devised by the team measures the characteristics of a spectral line in cesium (Cs) with 2 ppm precision, which correlates to a measure of Boltzmann's constant (kB) with 6 ppm precision and 71 ppm uncertainty—a factor of 16 better than alternative measurements.

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A highly accurate linear absorption spectrometer for probing the D1 transition in atomic cesium (Cs) vapor uses a near-ideal light source. A probe laser (blue box) is frequency-tuned with respect to a highly stable master laser (brown box) that is itself locked to a highly stable optical frequency comb (rainbow box) to give frequency accuracy. The output signal of the probe laser is frequency-purified by passing through an optical cavity before passing through an acousto-optic modulator (gold box) to control its power. A Glan-Taylor prism (green box) then purifies the polarization and the beam is split into two by a Wollaston beamsplitter (purple box). One output of the beamsplitter is measured using a photodetector to estimate the incident power on the Cs cell while a second photodetector measures the transmitted light. The Cs cell is suspended inside a thermal and magnetic shield that is independently monitored with high-accuracy conventional thermometers (standard platinum resistance thermometers). (Image credit: University of Adelaide)

To measure the D1 Cs transition, a frequency-tunable probe laser is locked at a user-adjusted frequency difference away from a highly stable master laser. The master laser is locked to an atomic transition in Cs and its frequency is continuously monitored by an optical frequency comb (see figure). An optical cavity and a length of optical fiber are used to spectrally and spatially filter the probe laser to reduce the spontaneous emission of the probe beam from 1.6% down to 0.01% and an acousto-optic modulator further stabilizes the optical power. The probe light is split by a beamsplitter and one arm enters the Cs-filled vacuum cell while the other is used to estimate the incident light on the cell. The ratio of the photodiode measurements from these two arms yields the transmission ratio.

With this configuration, the transmission spectrum of Cs can be measured at 895 nm with shot-noise-limited precision, enabling highly precise determination of Boltzmann's constant and a highly accurate determination of absolute Kelvin.

The extreme precision of the measurement technique allows direct determination of subtle perturbations in the line shape of the atomic spectra based on a theoretical model that can discriminate between internal atomic state dynamics and external motions of the atoms. This model looks at the Lorentzian or Gaussian line shape of the transition, its convoluted forms, and also takes into account etalon oscillations in the optical cavity, Doppler broadening, and pumping perturbations to perform a lineshape-fitting routine that better predicts the transmission parameters of the atomic transitions.

"Traditionally, scientists kept a set of special clocks, rules, and standard masses to define units such as the second, meter, and kilogram," says professor Andre Luiten, director of the Institute of Photonics and Advanced Sensing, University of Adelaide. "Over the last 50 years, we have been getting rid of these standards and replacing them with universal quantities such as the speed of light or the frequency at which certain atoms vibrate. This program is completed for time, electrical quantities, and length, but mass and temperature still make use of special objects."

Luiten continues, "Our work will bring a universally agreed temperature scale to the globe. As with any upgrade, this one will be deemed successful if people hardly notice the transition on a day-to-day basis. But for those at the cutting edge—whether developing new metal alloys at very high temperatures, or measuring the temperatures of the coldest substances, the need for absolute temperature is critical."


1. G.-W. Truong et al., NatureCommun., 9345, 1–6 (Oct. 14, 2015).

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