Two-Photon Cooling of Calcium Atoms

Alkaline-earth(-like) atoms trapped in optical tweezers and excited to Rydberg states have emerged as a promising platform for quantum simulation and computation, owing to the

high control and scalability of the system. In such systems, the long-lived metastable states can be used for motional ground state cooling, qubit readout and manipulation, as well as

providing access to single-photon Rydberg excitation and quantum erasure conversion. To trap individual atoms in optical tweezers, temperatures as low as tens of microkelvin are

desirable. The absence of hyperfine structure in alkaline-earth atoms precludes the use of standard sub-Doppler schemes developed for alkali atoms. In this work, we demonstrate

two-photon cooling of calcium atoms using a two-photon transition from the ¹S₀ ground state to the upper 4s5s ¹S₀ state via the ¹P₁ intermediate state. We achieve temperatures

as low as 260 μK in a magneto-optical trap (MOT), well below the Doppler limit (TD = 0.8 mK) of the ¹P₁ state. This scheme provides an alternative to the standard Doppler cooling applied to alkaline-earth atoms, based on a sequence of two magneto-optical traps, with the advantages of varying the effective linewidth of the ¹P₁ state, a higher transfer efficiency (close to 100%), and a more straightforward experimental implementation. Finally, we outline the progress towards optical trapping of ground and circular Rydberg states of calcium atoms in optical tweezers.