Blackbody Radiation Shift in Optical Lattice Clocks

Short answer: In optical lattice clocks, thermal radiation from the apparatus shifts the clock transition through the blackbody Stark shift, and knowing that shift is a key limit on accuracy. Papers from 2012, 2014 and 2021 describe ways to measure and model it in strontium and ytterbium clocks. The 2014 and 2021 papers report uncertainty contributions in the 10^-18 range.

What is the blackbody radiation shift?

The 2014 Physical Review Letters abstract describes the Stark shift due to blackbody radiation as the key factor limiting the performance of many atomic frequency standards. The difficulty is that the thermal radiation environment inside a clock apparatus is hard to characterize at a high level of precision. The 2021 paper in Metrologia calls it a key obstacle limiting the frequency uncertainty of optical lattice clocks, and says a well-characterized environment is needed to know exactly the temperature felt by the cold atoms.

What did the 2012 strontium work do?

A 2012 Physical Review Letters paper determined the frequency shift that blackbody radiation induces on the clock transition in strontium. The abstract says that its uncertainty previously limited the uncertainty of strontium lattice clocks, and that the new evaluation reduces the relative frequency uncertainty at room temperature. The numerical values did not render in the page text we could read, so this post does not quote them. The authors based their evaluation on a measurement of the differential dc polarizability of the two clock states and on modelling of the dynamic contribution using that value and experimental data for other atomic properties.

How did the 2014 ytterbium experiment handle it?

The authors demonstrate an in-vacuum radiation shield that furnishes a uniform, well-characterized blackbody environment for the atoms in an ytterbium optical lattice clock. Operated at room temperature, the shield enables specification of the environment to a fractional clock uncertainty contribution of 5.5 x 10^-19. Combined with uncertainty in the atomic response, they report a total blackbody Stark shift uncertainty of 1 x 10^-18. Operating the shield at elevated temperatures also let them measure the temperature dependence of the shift directly and showed consistency between their evaluated environment and the expected atomic response.

What did the 2021 study add?

A different ytterbium clock group monitors vacuum chamber temperature in real time with calibrated platinum resistance temperature detectors, and uses finite element analysis of the thermal radiation of the chamber to obtain the effective temperature at the atoms' position. They report that temperature inhomogeneity in the chamber and limited knowledge of air convection contribute the largest part of the uncertainty. For a typical room temperature environment, the effective temperature can be determined to 160 mK, corresponding to a fractional frequency uncertainty of 5.3 x 10^-18 for the shift. They also give a simple formula relating the effective temperature to the monitored points, which allows real-time correction without finite element analysis.

What to keep in mind

These are results from specific laboratory clocks, each with its own apparatus and method, so the 10^-19 and 10^-18 figures are not directly comparable. They are the authors' own uncertainty evaluations. This post is based on abstracts and visible summary text and the full papers were not checked. It makes no claim about products or deployed timekeeping systems.

FAQ

Why does thermal radiation matter for an atomic clock?

It causes a Stark shift of the clock transition, and the 2014 abstract calls it the key factor limiting many atomic frequency standards.

What is a radiation shield doing here?

In the 2014 experiment it gives the atoms a uniform, well-characterized thermal environment, so the shift can be specified more precisely.

Can the shift be corrected in real time?

The 2021 authors say a simple formula relating effective temperature to monitored temperatures enables real-time correction.

What was the 2012 strontium result?

The authors report a reduced uncertainty for the strontium shift. The numbers did not render, so none are given here.