Long-Lived Coherence on a μHz Scale Optical Magnetic Quadrupole Transition

We report on the coherent excitation of the ultranarrow S01–P23 magnetic quadrupole transition in Sr88. By confining atoms in a state insensitive optical lattice, we achieve excitation fractions of 97(1)% and observe linewidths as narrow as 58(1) Hz. With Ramsey spectroscopy, we find coherence times of 14(1) ms, which can be extended to 266(36) ms using a spin-echo sequence. We determine the lifetime of the P23 level for spontaneous emission of magnetic quadrupole radiation to be 110(31) min, confirming long-standing theoretical predictions. These results establish an additional clock transition in strontium and pave the way for applications of the metastable P23 state in quantum computing and quantum simulations. Published by the American Physical Society 2024