Collinear Three-Photon Excitation of a Strongly Forbidden Optical Clock Transition

The ultranarrow ¹S₀ --> ³P₀ clock transition in alkaline-earth-like atoms serves as the foundation for the world’s best atomic clocks [1] and for long-baseline atom interferometer concepts for gravitational wave detection [2]. This transition is weakly allowed in fermions, but excitation in bosonic isotopes is strongly suppressed without a sizable magnetic bias field [3]. Here we demonstrate coherent excitation of the clock transition in bosonic ⁸⁸Sr using a novel collinear three-photon process in a weak magnetic field. We observe Rabi oscillations with frequencies up to 50 kHz using W/cm² laser intensities and Gauss-level magnetic fields. Additionally, we realize a Mach-Zehnder clock atom interferometer using three-photon transitions. The absence of nuclear spin in bosonic isotopes offers reduced sensitivity to magnetic fields and light polarization in optical lattices, which may reduce key systematics in optical atomic clocks. The copropagating geometry of the laser fields permits the interrogation of atomic ensembles separated by a baseline with identical laser pulses – a key requirement for dark matter searches and gravitational wave detection with clock atom interferometers [3].