Integrated optical addressing of an ion qubit (1510.05618v2)

Abstract: The long coherence times and strong Coulomb interactions afforded by trapped ion qubits have enabled realizations of the necessary primitives for quantum information processing (QIP), and indeed the highest-fidelity quantum operations in any qubit to date. But while light delivery to each individual ion in a system is essential for general quantum manipulations and readout, experiments so far have employed optical systems cumbersome to scale to even a few tens of qubits. Here we demonstrate lithographically defined nanophotonic waveguide devices for light routing and ion addressing fully integrated within a surface-electrode ion trap chip. Ion qubits are addressed at multiple locations via focusing grating couplers emitting through openings in the trap electrodes to ions trapped 50 $\mu$m above the chip; using this light we perform quantum coherent operations on the optical qubit transition in individual $^{88}$Sr$^+$ ions. The grating focuses the beam to a diffraction-limited spot near the ion position with a 2 $\mu$m 1/$e^2$-radius along the trap axis, and we measure crosstalk errors between $10^{-2}$ and $4\times10^{-4}$ at distances 7.5-15 $\mu$m from the beam center. Owing to the scalability of the planar fabrication employed, together with the tight focusing and stable alignment afforded by optics integration within the trap chip, this approach presents a path to creating the optical systems required for large-scale trapped-ion QIP.

Integrated Optical Addressing of an Ion Qubit

The paper "Integrated Optical Addressing of an Ion Qubit" introduces an approach for enhancing scalability in quantum information processing (QIP) using trapped ion qubits. The intrinsic high fidelity and coherence of trapped ions make them promising candidates for quantum computations. However, the scalability of optical systems used for ion manipulation and measurement has often been a bottleneck in expanding ion-based quantum systems to more significant numbers of qubits. This research paper lays the foundation for overcoming such limitations by integrating nanophotonic waveguide devices directly into the ion trap chip.

Technical Overview

The authors demonstrate the application of lithographically defined nanophotonic waveguide devices for addressing ion qubits. These devices operate within a surface-electrode ion trap, offering a novel mechanism for light routing on-chip, which is crucial for precise ion addressing. Specifically, the integration involves focusing grating couplers that emit through openings in the electrodes toward ions trapped at a certain height above the chip. This arrangement allows quantum coherent operations on optical qubit transitions in individual $^{88}$Sr$^+$ ions. The tight focusing achieved by these couplers minimizes the optical crosstalk, recorded at error levels between $10^{-2}$ and $4 \times 10^{-4}$ at distances from the beam center.

By employing semiconductor fabrication techniques, the authors have developed a highly scalable architecture. The on-chip waveguide system is adaptable to complex geometries housing potentially thousands of devices. Compared to traditional optical fibers and lenses, the integrated devices offer compactness and scalability, allowing various configurations with flexible qubit arrangements.

Key Results

Notably, the authors present significant results regarding ion addressing accuracy and efficiency. Grating couplers used in the experiment achieved a diffraction-limited focus with a beam width of approximately 2 $\mu$m. Rabi oscillations on the qubit transitions showed high fidelity, with the first $\pi$-rotation achieving 99% accuracy. Moreover, the integrated system demonstrated its potential by addressing individual ions in multi-ion chains with minimal crosstalk. Such precision in ion addressing is essential for executing error-resilient quantum logic operations.

Implications and Future Directions

The implications of this research extend beyond merely improving ion addressing. The integrated approach significantly reduces the system complexity involved in trapped-ion quantum computing. The elimination of external optical components simplifies the setup and improves operational stability, particularly in beam pointing accuracy.

In practical terms, integrating electro-optic modulators and enhancing fiber coupling to the trap chip will further broaden the system's capabilities. The avenues for future research include exploring the integration of devices operating at different wavelengths needed for comprehensive ion control. The scalable optical interface proposed here presents a transformative step towards developing large-scale, robust quantum information processing systems.

Theoretical and Practical Contributions

On a theoretical level, the work underscores the importance of scalable photonic systems in achieving significant advancements in QIP. The novel design allows tighter focusing and improved optical path stability, crucial for ensuring qubit operation fidelity. Practically, the integration of photonic systems into CMOS-compatible ion traps paves the way for a new class of quantum processors capable of civilian and industrial applications.

In conclusion, this paper provides a valuable contribution to the field of trapped ion quantum computing. The integration of optical addressing within ion trap chips offers a promising pathway to scaling quantum systems to larger numbers of qubits—a critical factor in realizing the potential of quantum computing for solving complex problems beyond classical computing capabilities.