Inner-shell clock transition in atomic thulium with small BBR shift

TL;DR

This study demonstrates that the inner-shell clock transition in thulium atoms has an exceptionally low sensitivity to black-body radiation and external electric fields, making it highly promising for stable atomic clock applications.

Contribution

The paper provides the first direct polarizability measurements of Tm's clock transition, revealing an ultra-low BBR shift and identifying the magic wavelength for optimal clock operation.

Findings

Measured differential polarizabilities at multiple wavelengths.

Inferred a static scalar differential polarizability of -0.047(18) a.u.

Determined the magic wavelength at 813.320(6) nm.

Abstract

With direct polarizability measurements we demonstrated extremely low sensitivity of the inner-shell clock transition at $1.14\,\mu$m in Tm atoms to external dc electric fields and black-body radiation (BBR). We measured differential polarizabilities of clock levels in Tm at wavelengths of 810--860\,nm and at 1064\,nm and inferred the static scalar differential polarizability of the inner-shell clock transition of $-0.047(18)$ atomic units corresponding to only $2\times10^{-18}$ fractional frequency shift from BBR at the room temperature. This is a few orders of magnitude smaller compared to the BBR shift of the clock transitions in the neutral atoms (Sr, Yb, Hg) and competes with the least sensitive ion species (e.g. Al$^+$ or Lu$^+$). For the $1.14\,\mu$m clock transition, we experimentally determined the "magic" wavelength of $813.320(6)$\,nm, recorded the transition spectral linewidth of $10$\,Hz, and measured its absolute frequency of $262\,954\,938\,269\,213(30)$\,Hz.