Physics

Shell (structure)

Laser

Optics

Radiation

Atomic physics

Black-body radiation

Composite material

Atomic clock

Materials science

Thulium

Nature Communications

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.

Inner-shell clock transition in atomic thulium with a small blackbody radiation shift