Neutral atoms trapped in optical tweezer arrays have attracted significant interest as platforms for quantum computing. The implementation of two-qubit gates, which is crucial for universal quantum computation, is achieved through interactions between Rydberg atoms, where the valence electron of the atom is excited to an orbital with a large principal quantum number. Ytterbium (Yb), an alkaline earth-like atom, is a promising avenue for realizing high-fidelity two-qubit gates via single-photon Rydberg excitation from a metastable excited state. This approach addresses the limitations encountered with alkali atoms in earlier studies, such as scattering from the intermediate state during Rydberg excitation. To realize such Rydberg excitation, the development of a high-power ultraviolet (UV) laser is required.

  In this study, we developed a high-power UV laser with a wavelength of 325 nm for the Rydberg excitation of single Yb atoms trapped in optical tweezer arrays. The laser system consists of a 1300-nm seed laser, a Raman fiber amplifier, and two-stage second-harmonic generation to convert the wavelength to 325 nm. A maximum output power exceeding 800 mW was achieved at 325 nm. This capability has the potential to enable 10 MHz-order fast Rydberg excitation, addressing Rydberg state decay, which is a primary limitation in the fidelity of two-qubit gates.

  For high-fidelity quantum operations, it is crucial to manage laser phase noise. To address this, we incorporated a high-finesse Fabry-Pérot cavity between the seed laser and Raman fiber amplifier. The cavity has a finesse of 1.3×10⁴ and a linewidth (full-width at half-maximum) of 230 kHz. We achieved phase noise suppression of approximately 20 dB in the MHz range, anticipating operation fidelities surpassing the threshold required for quantum error correction.

  Using the developed UV laser, we demonstrated coherent excitation from the metastable (6s6p)³P₂ state to the (6s71s)³S₁ Rydberg state of ¹⁷⁴Yb. We successfully observed Rabi oscillations between these states with a Rabi frequency of 2π×2.13(3) MHz, marking an essential first step towards two-qubit operations in Yb atom arrays. This study establishes a solid foundation for the realization of high-fidelity Rydberg-mediated two-qubit gates, advancing the prospects of fault-tolerant quantum computation.

(Written by Y. Nakamura on behalf of all the authors)

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