Optical telecommunications-band inner-shell orbital clock transition in neutral erbium atoms

The pursuit of the most advanced optical atomic lattice clocks has greatly influenced fundamental physics. One of the biggest systematic effects currently limiting their performance is the blackbody radiation (BBR) shift. At the same time, its applications require the development of a long-distance time transfer and clock network. Here, we investigate an inner-shell orbital clock transition in neutral erbium atoms in the optical telecommunications band. The transition exhibits much smaller BBR than is typical for alkaline earth atoms and a small quadratic Zeeman shift. We evaluate the relative uncertainties of the room temperature blackbody radiation shift, quadratic Zeeman shift, dipole–dipole interaction shift, and ac Stark shift of the probe light as 5.6×10−19, 2.0×10−18, 1.5×10−18, and 1.0×10−19, respectively. We also precisely measure the differential polarizability of the clock transition and determine a magic wavelength νmagic=354,780,036.2 (7.9) MHz. Our work paves the way for realizing a higher precision optical atomic frequency standard with inner-shell transitions in the direct access telecommunications-band.