Inner-shell magnetic dipole transition in Tm atom as a candidate for optical lattice clocks

Abstract

We consider a narrow magneto-dipole transition in the ${}^{169}$Tm atom at the wavelength of $1.14\mu$m as a candidate for a 2D optical lattice clock. Calculating dynamic polarizabilities of the two clock levels $[\text{Xe}]4f^{13}6s^2(J=7/2)$ and $[\text{Xe}]4f^{13}6s^2(J=5/2)$ in the spectral range from $250$nm to $1200$nm, we suggest the "magic" wavelength for the optical lattice at $807$nm. Frequency shifts due to black-body radiation (BBR), the van der Waals interaction, the magnetic dipole-dipole interaction and other effects which can perturb the transition frequency are calculated. The transition at $1.14\mu$m demonstrates low sensitivity to the BBR shift corresponding to $8\times 10^{-17}$ in fractional units at room temperature which makes it an interesting candidate for high-performance optical clocks. The total estimated frequency uncertainty is less than $5\times 10^{-18}$ in fractional units. By direct excitation of the $1.14\mu$m transition in Tm atoms loaded into an optical dipole trap, we set the lower limit for the lifetime of the upper clock level $[\text{Xe}]4f^{13}6s^2(J=5/2)$ of $112$ms which corresponds to a natural spectral linewidth narrower than $1.4$Hz. The polarizability of the Tm ground state was measured by the excitation of parametric resonances in the optical dipole trap at $532$nm.