Development of a Quantum Blackbody Thermometer toward Primary On-orbit Thermometry

TL;DR

The paper tackles the problem of drift-prone and biased on-orbit radiometric calibration by proposing a quantum blackbody thermometer (QBT) that uses rubidium fluorescence ratios to achieve a primary, intrinsically calibrated temperature readout in the $250\ \mathrm{K}$–$400\ \mathrm{K}$ range with target long-term accuracy of $30\ \mathrm{mK}$. The approach employs a dual-laser scheme and a comprehensive rate-equation model (across $59$ rubidium states) to self-calibrate the detection-efficiency and extract temperature from temperature-dependent fluorescence signals. A simplified analytic form and a prototype demonstrate the feasibility of self-calibration and rapid readout with fiber-coupled optics, suggesting achievable sensitivities on the order of $\delta T/T \sim 3\times10^{-4}/\sqrt{t}$ and potential improvements to inter-sensor radiometric biases. If space-qualified, the QBT could replace or augment existing thermometers in radiometric calibration sources, enhancing long-term data quality for climate and planetary science.

Abstract

We present a roadmap to a deployable, intrinsically calibrated thermometer with long-term accuracy of 30 mK, exceeding existing on-orbit resistance-based thermometers. Our quantum blackbody thermometer is based on measuring fluorescence ratios of optically excited rubidium atoms in microfabricated vapor cells. The key advantage of the quantum blackbody thermometer is that long-term stability of the fluorescence ratios is guaranteed by the immutable physical properties (transition strengths) of the rubidium atom. This should be compared against resistance-based thermometers, such as platinum resistance thermometers, which may be calibrated with exceptional accuracy but are susceptible to temporal drift and shifts due to improper handling.

Figures (3)

• Figure 1: Schematic of a quantum blackbody thermometer (QBT) measuring an on board radiometric calibration source blackbody. The QBT is based upon a microfabricated alkali vapor cell housed in a fiber-bundle-coupled sensor head. Fiber switches select between exciting the alkali atoms with the thermometer and calibration lasers, and fluorescence photons are detected by photomultiplier assemblies (PMAs). Earth and satellite symbols adapted from icons by Lusi Astianah and JK Lim via The Noun Project.
• Figure 2: Proposed self-calibrating QBT sensing scheme. a) Temperature sensing. Rb atoms are laser-excited to the 6$^2$P$_{3/2}$ state by the 421 nm thermometer laser. TR stimulates transitions to 7$^2$S$_{1/2}$ and 5$^2$D$_{3/2}$ at different temperature-dependent rates. Relative populations of 7$^2$S$_{1/2}$ and 5$^2$D$_{3/2}$ are determined from the 741 nm and 762 nm fluorescence, respectively. b) Line shows the calculated ratio of 741 nm to 762 nm fluorescence when laser-exciting Rb with the thermometer laser. Grey bands are 68% confidence intervals extracted from Monte Carlo simulations which include theoretical uncertainty in the Rb TDMEs UDportal. c) Detector calibration. Rb atoms are laser-excited to the 7$^2$P$_{3/2}$ state by the 359 nm calibration laser. In this case, populations of 7$^2$S$_{1/2}$ and 5$^2$D$_{3/2}$ are due to spontaneous decay from 7$^2$P$_{3/2}$. d) Thus, the calculated ratio of 741 nm to 762 nm fluorescence when exciting with the calibration laser is essentially temperature-independent, and provides sensitivity to the ratio of total detection efficiency at 741 nm to 762 nm. e) Example $1\sigma$ uncertainty plot constraining $T$ and $\eta_{741}/\eta_{762}$ (green) for the case of $r^{(6^2\rm{P_{3/2}})}_{741,762}$ and $r^{(7^2\rm{P_{3/2}})}_{741,762}$ each measured to a relative precision of $10^{-4}$ (shaded bands) at $T = 300$ K .
• Figure 3: Measured prototype QBT fluorescence (dots) as a function of laser frequency as the laser is scanned over the upper ground hyperfine manifold of the 5$^2$S$_{1/2} \rightarrow 7^2$P$_{3/2}$ transition for $^{87}$Rb and $^{85}$Rb. Fluorescence at 741 nm and 762 nm demonstrate a high signal-to-noise ratio for the self-calibration scheme in Fig. \ref{['fig:ratios']}. Strong fluorescence is also detected on the $5^2$P$_{3/2} \rightarrow 5^2$S$_{1/2}$ decay at 780 nm. The solid line is a fit of two Gaussian peaks (one for each isotope) plus constant offset to the 780 nm fluorescence, with excellent agreement to the data. The shaded dashed lines show the constituent single Gaussian peaks.