High-Fidelity Individual Addressing of Single Atoms in Quantum Registers at Three-Photon Laser Excitation of Rydberg States
Precise individual addressing of single atoms in quantum registers formed by optical trap arrays is essential to achieve high-fidelity quantum gates in neutral-atom quantum computers and simulators. Two-qubit quantum gates are typically realized using coherent two-photon laser excitation of atoms to strongly interacting Rydberg states. However, two-photon excitation encounters challenges in individual addressing with tightly focused laser beams due to atom position uncertainty and the spatial inhomogeneity in both Rabi frequencies and light shifts. In this work, we theoretically demonstrate that the fidelity of individual addressing can be improved by employing coherent three-photon laser excitation of Rydberg states. For a specific example of $5s_{1/2}\!\xrightarrow{Ω_1}\!5p_{3/2}\!\xrightarrow{Ω_2}\!6s_{1/2}\!\xrightarrow{Ω_3}\!np$ excitation in $^{87}$Rb atoms, we find that upon strong laser coupling in the second step (Rabi frequency $Ω_2$) and moderate coupling in the first and third steps (Rabi frequencies $Ω_1$ and $Ω_3$), the three-photon Rabi frequency is given by $Ω\!=\!Ω_1Ω_3/Ω_2$. If the spatial distributions of $(Ω_1Ω_3)$ and $Ω_2$ are arranged to be identical, $Ω$ becomes independent of atom position, even within very tightly focused laser beams. This approach dramatically improves individual addressing of Rydberg excitation for neighboring atoms in trap arrays compared to conventional two-photon excitation schemes. Our findings are crucial for large-scale quantum registers of neutral atoms, where distances between adjacent atoms should be minimized to ensure stronger Rydberg interactions and compact arrangement of atom arrays.