The redefined kelvin: progress and prospects

The kelvin redefinition has stimulated new and disruptive approaches to deliver temperature traceability, namely practical primary thermometry at the point of measurement. The most innovative ways to provide such traceability could be the small-scale optical-based approaches. Whilst in their infancy, these photonic/optomechanical thermometry approaches are rapidly gaining ground and have the potential to become practical solution to primary thermometry for real in-situ applications in particular thanks to its high chipset integration capacity. The photonic/optomechanical temperature sensors can offer micrometer spatial resolution, large temperature range (from few K up to room temperature) and additionally can be self-calibrated.

In this presentation, after a brief introduction on photonic-based approaches in thermometry, a special emphasis will be put on different thermometry techniques with optomechanical resonator. An overview of the following techniques will be given: photonic thermometry (which explores the frequency shift of optical resonance due to the thermo-optic effect); optomechanical noise thermometry (where the Brownian motion of a mechanical oscillator is probed by optical phase measurement); quantum correlation thermometry (which uses quantum radiation pressure noise to calibrate the thermal noise of the resonator) and sideband asymmetry technique (where the ratio between the amplitudes of Stokes and anti-Stokes scattering peaks is linked to the temperature of the resonator). The advantages and limitations of each technique, as well as the possibility of combination of these different relative and absolute techniques in a practical self-calibrated temperature sensor operating in the large temperature range will be discussed. 

Dr Olga Kozlova, Laboratoire National de Métrologie et d'essais, France

Dr Olga Kozlova, Laboratoire National de Métrologie et d'essais, France

Dr Olga Kozlova received her PhD degree from Université Pierre et Marie Curie - UPMC (Paris 6) in 2012. Her PhD work at SYRTE - Observatoire de Paris was focused on the improvement of mid- and long-term stability of Cs atomic clocks with all optical interrogation. 

She joined LNE (Laboratoire national de métrologie et d'essais) in 2012 as a research engineer in the Temperature Department, where she has worked on the development of tuneable wavelength spectroradiometers in visible and infrared ranges, on Doppler broadening thermometry, and on thermodynamic temperature measurements with radiometric technique. 

Since 2018, she has been involved in the development of optical-based primary thermometry using optomechanical resonators.

Radiation (non-contact) thermometry relies on classical radiation detectors, which must be calibrated. Maintaining an accurate calibration can be difficult, as they rely upon long traceability chains, or even impossible, in applications where the detector cannot be retrieved for recalibration. Radiation thermometers that rely instead on immutable constants of physics, or even better, realise the kelvin directly, can improve the overall quality of radiation detectors. Here, we describe efforts to realise several such detectors using atoms and molecules. Blackbody radiation (BBR) induces two effects on the internal states of these quantum systems: it shifts the energies of the internal states and drives transitions them. In this talk, we will discuss two experiments attempting to use the latter effect to infer temperature: the compact blackbody radiation atom sensor (CoBRAS), which is sensitive to BBR in the infrared regime ( roughly 10 µm); and a related experiment using Rydberg atoms, which is sensitive to BBR in the microwave regime (roughly 300 GHz). Both experiments use Rb atoms, demonstrating the versatility of even a single atomic system for radiation thermometry. Finally, we will debate the advantages and disadvantages of this method to radiation thermometry, compare to other atomic-based thermometry techniques, and conclude by mentioning other, new experiments that are on the horizon.

Dr Stephen Eckel, National Institute of Standards and Technology, USA

Dr Stephen Eckel, National Institute of Standards and Technology, USA

An expert in cold atom sensing and precision measurement with over 50 published papers and six patents, Stephen Eckel’s current research focuses on using the immutable properties of atoms and molecules to make calibration-free sensors for both temperature and pressure. In 2016, he started as a permanent research physicist at the National Institute of Standards and Technology (NIST) in the Fundamental Thermodynamics Group developing the cold atom vacuum standard, the only primary standard of vacuum pressure in the ultra-high and extreme-high vacuum regimes. Prior to 2016, he was National Research Council Postdoctoral Fellow at the Joint Quantum Institute, a collaborative institute between NIST and the University of Maryland, where he was a working on inertial sensing using ring-shaped Bose-Einstein condensates. He graduated from Yale University with a PhD in Physics in 2012 where his research focused on two different searches for the electron’s electric dipole moment.

Semiconductors are useful materials for temperature measurements due to their sensitivity to thermal changes in their basic properties. Silicon, the most used semiconductor, is essential in modern electronics and is of growing importance in integrated photonics, providing a cost-effective platform for optical sensors. Silicon-based ring resonator temperature sensors utilise the dependence of the optical modes of oscillation propagating in the ring to the change in refractive index (n) of the silicon with temperature (dn/dT). However, silicon has two main drawbacks: it is fundamentally a poor light emitter due to its indirect bandgap, requiring external light sources, and as an elemental semiconductor, its thermal properties are largely fixed.

In contrast, compound semiconductors such as InP, GaAs, GaN, and InAs can efficiently emit light due to their direct bandgap and are commonly used in semiconductor-based light emitters, such as LEDs and lasers. They also have controllable thermal properties which can be manipulated through alloying. These characteristics make them well-suited for "active resonator" temperature sensors with embedded light sources, which can be optimised for different temperature ranges. 

This discussion will focus on InP-based alloys, demonstrating their basic properties, how they can be designed into active quantum well-based semiconductor heterostructures and fabricated into micro-ring and other resonators. The ability to integrate light sources within the sensor itself simplifies the design and enhances the overall functionality of the temperature sensing system. This approach has the potential for the development of versatile temperature sensors that can be used in a multitude of applications.

Professor Stephen Sweeney, University of Glasgow, UK

Professor Stephen Sweeney, University of Glasgow, UK

Stephen Sweeney is a semiconductor physicist specialising in photonic devices for applications including communications, space, energy, metrology and sensing with over 25 years’ experience in academia and industry. He is Professor of Photonics and Nanotechnology at the University of Glasgow and holds visiting professorships at the University of Surrey, University of Wollongong, Australia, and the Ferdinand Braun Institute, Germany. He is CTO for ZiNIR Ltd. (UK) and is Editor of the Journal of Materials Science: Materials in Electronics. He was previously Head of the Physics Department at the University of Surrey, Lead Scientist for Marconi Optical Components, President of the British Science Association (Physics & Astronomy division), visiting scientist at Arizona State University, USA, Philipps-Universität, Marburg, Germany, and JSPS International Invitational Fellow with the Kyoto Institute of Technology, Japan. He is a Chartered Physicist, a Fellow of The Institute of Physics and a Fellow of SPIE.

The redefinition of the kelvin has ushered in a new era for thermometry, emphasising the need for robust global interoperability and traceability. This presentation will explore the multifaceted challenges associated with maintaining these standards in a rapidly evolving scientific landscape. Central to this discussion is the role of the Consultative Committee for Thermometry (CCT), which oversees the establishment and realisation of the international temperature scales and the thermodynamic temperature. The CCT's efforts in harmonising international standards and ensuring the accuracy of temperature measurements are crucial for global consistency.

Furthermore, the evolving role of National Metrology Institutes (NMIs) in the International System of Units post-redefinition world highlight the increasing importance of decentralised and quantum-based measurement techniques. NMIs are now tasked with developing and disseminating advanced, intrinsically accurate sensors that could operate independently of traditional standards, thus enhancing the traceability and reliability of temperature measurements across diverse applications.

This presentation will also delve into these critical topics, examining the collaborative efforts and the long-term CCT strategy required to sustain global interoperability in thermometry.

Dr Dolores del Campo, Centro Español de Metrología, Spain

Dr Dolores del Campo, Centro Español de Metrología, Spain

Dr Dolores del Campo studied Physics in the Universidad Complutense de Madrid and holds a PhD from the Universidad Politécnica de Madrid. She has been dedicated to thermal metrology for more than 30 years and she is currently Deputy Director of the Centro Español de Metrología. Their research activity has been focused in the field of thermometry, having published numerous articles in prestigious metrology journals and coordinating international and national research projects. She has been chair of the EURAMET Technical Committee for Thermometry from 2018 to 2022. In 2018 she was elected as member of the International Committee of Weights and Measures (CIPM); currently she is coordinating the CIPM Sectorial Task Group on Climate Change and Environment and she is the president of the CIPM Consultative Committee for Thermometry. She is the EURAMET Chairperson for the period June 2024 to June 2027 and chairperson of the European Quality Infrastructure Network. 

The 2019 redefinition of the kelvin and its accompanying Mise en Pratique (MeP-K) herald a bold new future for the thermometry community, in which users will reap the benefits of direct dissemination of thermodynamic temperature T, practical primary thermometry, and zero-chain measurement traceability at the point of use. The MeP-K offers a blueprint for putting the kelvin into practice, yet the proliferation of new realisation options it enables will lead to practical inconsistencies for thermometer users. For nearly 100 years, the thermometry community has been accustomed to a straightforward International Temperature Scale measurement chain with traceability to the kelvin and trust assured by National Metrology Institutes (NMIs), culminating in the present T90 International Temperature Scale of 1990. In contrast, the emerging world of coexisting T and T90 thermometers, with some offering traceability outside of the traditional NMI chain, will challenge this stability. Users will need to reconcile devices with varied traceability paths, that return temperatures disagreeing by much more than their measurement uncertainties. In this talk, practical solutions to these practical inconsistencies will be discussed, focusing on harmony between T, T90, and three pillars of metrology—traceability, equivalence and competence—in order to allow thermometry end users to broadly benefit from new technologies while maintaining trust in measurements and minimising net disruption.

Dr Patrick Rourke, National Research Council Canada, Canada

Dr Patrick Rourke, National Research Council Canada, Canada

Dr Patrick Rourke joined the National Research Council Canada in 2012, following a postdoctoral fellowship at the University of Bristol and doctoral studies at the University of Toronto.

His research focuses are primary thermometry, defined temperature scales, and the interplay between them. This has included work on refractive-index gas thermometry, acoustic gas thermometry, the present and future of the International Temperature Scale of 1990 (ITS-90), alternatives to mercury fixed points, and implications of coexisting thermodynamic temperature T and ITS-90 temperature T90 thermometers.

Dr Rourke is currently the Canadian delegate to the CIPM Consultative Committee for Thermometry (CCT), member of the CCT Working Group for Contact Thermometry, chair of the CCT Working Group on Digitalisation, and CCT Digitalisation Representative to the CIPM Forum on Metrology and Digitalisation.

The revised definition of the kelvin has created new interest in alternative paths of traceability to the SI. In particular, development of methods for the direct realisation of the kelvin are ongoing, establishing traceability without the need to go through the international-consensus temperature scale, the ITS-90. These direct realisation methods are in contrast to the ITS-90 and the measurement infrastructure of reference materials and artefact thermometer calibration chains. This infrastructure has been built up over a century or more of industrialisation and scientific progress, and while very robust, is not well suited to certain categories of application environments. We examine how direct traceability to the kelvin can be applied to address those application requirements. We assess the relative practical merits of various direct realisation approaches from the standpoint of achievable uncertainties, practical realisations and selected measurement performance metrics over the range 4 K to 1235 K. This assessment points to a narrow application space of long-term missions, or those occurring in remote or hazardous environments where humans are largely absent, as the most compelling cases for the direct realisation approach. We examine two examples of special application environments where those performance advantages could be the best suited for point-of-use measurement.

Dr Weston Tew, National Institute of Standards and Technology, USA

Dr Weston Tew, National Institute of Standards and Technology, USA

Dr Weston Tew has held the position of staff Physicist in the NIST Sensor Science Division since 2012. His work there includes special applications in temperature measurement; new fixed points; high-temperature thermocouples, industrial standards and conformity assessment. From 1993-2010 he served as a staff physicist in the NIST Process Measurements Division working on temperature sensors, cryogenic thermometry, interpolation, gas-based fixed points, noise thermometry, thermodynamic temperature and isotopic effects in phase equilibria. Dr Tew worked as a guest researcher at the BIPM in Sevres France in 1992 and was a National Research Council postdoctoral fellow at NIST from 1989 to 1991. Dr Tew is a Fellow of the ASTM International; a member the IEC SC65B Working Group 5 on Temperature Sensors; a member of The IEEE; and a member of the American Physical Society.