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A system of cold atoms is ideal to explore quantum physics owing to its cleanliness, controllability, and isolation from the environment. By observing the behavior of atoms in an optical lattice, for example, the quantum simulation of the Hubbard model that is difficult to trace with a classical computer has been demonstrated successfully.

Recently, a new platform that uses single atoms trapped in individually controllable optical tweezer arrays and strong interactions mediated by Rydberg states has emerged and been attracting attention toward quantum computing as well as quantum simulation. Using this system, various experiments ranging from the observation of quantum phase transition to gate operations on multiple qubits have been performed.

Whereas one-electron atoms, or alkali-atoms, have been used in most previous experiments, two-electron atoms such as ytterbium (Yb) or strontium (Sr) have many advantages over them. The existence of the metastable excited state is a remarkable example. These states enable coherent optical transition from the ground state, in addition to single photon excitation to Rydberg states with minimal decoherence. These features are beneficial toward the realization of quantum computing.

The authors succeeded in constructing a two-dimensional Yb atom array that is detected by fluorescence imaging with the inter-combination ¹S₀-³P₁ transition and preparing the defect-free atom array by the rearrangement with the feedback. They also exploited the metastable excited ³P₂ state with radiative lifetime on the order of seconds. High-resolution ¹S₀-³P₂state spectroscopy for the single atoms was performed, demonstrating the utilities of this ultranarrow transition. They further performed single-photon excitation from the ³P₂ state to Rydberg states and the systematic spectroscopy, in which a complex energy structure of a series of D states was measured, including the newly observed ³D₃ states. This offers new insights into the analysis of Yb Rydberg states.

These achievements will be the foundation toward the quantum computing architecture based on neutral two-electron atoms.

(written by D. Okuno on behalf of all authors.)