Coherent excitation of a $\mu$Hz scale optical magnetic quadrupole transition

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We report on the coherent excitation of the ultranarrow $^{1}\mathrm{S}_0$-$^{3}\mathrm{P}_2$ magnetic quadrupole transition in $^{88}\mathrm{Sr}$.

• 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 linewidth of the M2 transition to 24(7) $\mu$Hz, confirming longstanding theoretical predictions.