Coherent Control of Strontium in an Optical Lattice: Spectroscopy of a M2 Transition and Demonstration of a Fine-Structure Qubit

The coherent excitation of ultra-narrow optical transitions between long-lived atomic states is fundamental for optical atomic clocks, quantum information processing, and quantum simulation. The exceptional frequency resolution offered by such transitions provides the basis for remarkable stability and accuracy. Neutral strontium in particular has a rich level structure offering an opportunity to construct outstanding optical atomic clocks and to encode high-quality qubits.



Here, we present our results on the coherent excitation of the ultranarrow ¹S₀–³P₂ magnetic quadrupole (M2) transition in ⁸⁸Sr. 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 perform a precision measurement of the transition matrix element and find a linewidth of the M2 transition of 24(7) μHz, confirming longstanding theoretical predictions and establishing an additional clock transition in strontium.



Building on these results, we demonstrate coherent control of the strontium fine-structure qubit, which promises fast single- and two-qubit gates. This THz qubit is encoded in the metastable ³P₂ and ³P₀ states, which are coupled by a Raman transition. We use the ¹S₀–³P₂ M2 transition for coherent state-initialization and read-out. We demonstrate Rabi oscillations with more than 60 coherent cycles and single-qubit rotations on the μs scale. Our results pave the way for fast quantum information processors and highly tunable quantum simulators with two-electron atoms.