Trapped-ion quantum computing requires precise optical control for individual qubit manipulation. However, conventional free-space optics face challenges in alignment stability and scalability as the number of qubits increases. Integrated photonics offers a promising alternative, providing miniaturized optical systems on a chip. Here, we propose a design for a multimode photonic circuit integrated with a surface-electrode ion trap capable of targeted and reconfigurable light delivery. Three closely positioned ions can be addressed using a focusing grating coupler that emits multimode light through electrode openings to ions trapped 68 μm above the chip. Simulations show that the couplers achieve a diffraction-limited spot with a 4.3 μm beam waist along the trap axis and 2.2 μm perpendicular to the trap axis. Controlled interference of the TE00 and TE10 modes results in crosstalk of –20 dB to –30 dB at ion separations of 5–8 μm when addressing ions individually, and down to –60 dB when two of the three ions are addressed simultaneously. Additionally, the higher-order TE modes can offer a novel mechanism for driving spin-motion coupling transitions, potentially enabling alternative approaches to quantum gates and simulations. The proposed integrated platform offers a viable path for constructing large-scale trapped-ion systems, leveraging the benefits of nanophotonic design for precise and reliable ion manipulation.

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Nature

Researchers propose an integrated photonic circuit design for trapped-ion quantum computing that uses adjoint-optimized multimode grating couplers to individually address calcium ions. The design achieves diffraction-limited focusing with beam waists of 4.3 μm and 2.2 μm, and controlled interference of TE00 and TE10 modes enables ion addressing with crosstalk suppression between -20 dB and -60 dB at ion separations of 5-8 μm. The compact photonic platform offers a scalable alternative to free-space optics for precise qubit manipulation in large-scale trapped-ion systems.

Individual trapped-ion addressing with adjoint-optimized multimode photonic circuits