Integrated optical multi-ion quantum logic (2002.02258v2)

Abstract: Practical and useful quantum information processing (QIP) requires significant improvements with respect to current systems, both in error rates of basic operations and in scale. Individual trapped-ion qubits' fundamental qualities are promising for long-term systems, but the optics involved in their precise control are a barrier to scaling. Planar-fabricated optics integrated within ion trap devices can make such systems simultaneously more robust and parallelizable, as suggested by previous work with single ions. Here we use scalable optics co-fabricated with a surface-electrode ion trap to achieve high-fidelity multi-ion quantum logic gates, often the limiting elements in building up the precise, large-scale entanglement essential to quantum computation. Light is efficiently delivered to a trap chip in a cryogenic environment via direct fibre coupling on multiple channels, eliminating the need for beam alignment into vacuum systems and cryostats and lending robustness to vibrations and beam pointing drifts. This allows us to perform ground-state laser cooling of ion motion, and to implement gates generating two-ion entangled states with fidelities $>99.3(2)\%$. This work demonstrates hardware that reduces noise and drifts in sensitive quantum logic, and simultaneously offers a route to practical parallelization for high-fidelity quantum processors. Similar devices may also find applications in neutral atom and ion-based quantum-sensing and timekeeping.

• The paper presents an integrated optical system for ion traps that simplifies multi-ion quantum logic and reduces alignment challenges.
• It achieves high-fidelity entanglement (>99.3%) with 40Ca+ ions using a cryogenic surface-electrode Paul trap.
• The study details the design of SiN-based waveguides and effective error mitigation strategies addressing motional heating and laser frequency noise.

Integrated Optical Multi-Ion Quantum Logic

The paper presents significant advancements in the field of quantum information processing (QIP) by integrating optical capabilities directly into ion trap devices, thereby addressing crucial scalability and fidelity challenges. Traditional ion trap systems rely heavily on external optics for qubit manipulation. This approach, albeit effective for small-scale systems, faces limitations in precision and scalability due to susceptibility to beam drifts and alignment issues over extended distances. This research endeavors to overcome these limitations by embedding scalable optics within surface-electrode ion traps.

Key Contributions and Findings

1. Integrated Optical System: The research rigorously details the development of a trap device incorporating planar-fabricated optics, enabling efficient multi-ion quantum logic gate operations. This integration alleviates the need for complex optical paths typical in conventional setups, which are prone to alignment issues and environmental noise.
2. Experimental Setup and Results: The paper achieved high-fidelity entangled operations using 40Ca+ ions with fidelities exceeding 99.3%. The ions were confined using a surface-electrode Paul trap with a cryogenic chip environment, supporting multiple optical channels directly coupled via fiber. This direct integration minimizes beam drifts and vibrations, hence enhancing system stability and gate fidelity.
3. Device Design and Architecture: Detailed descriptions of the photonic and electromagnetic aspects of the ion trap are provided, highlighting the use of SiN-based waveguides and gratings. These components are meticulously designed to optimize power usage, beam focusing, and minimize cross-talk between trapping zones. The system also demonstrates a two-qubit entangling gate, pivotal for executing quantum logic operations, with considerable success.
4. Challenges and Error Analysis: The error sources contributing to gate infidelities were critically analyzed. Dominant factors involved motional heating, mode frequency drifts, and laser frequency noise. The authors propose strategies for error mitigation, emphasizing improvements in device fabrication, materials, and coherence-enhancing techniques.

Implications for Quantum Computing

The demonstrated integration of optics within ion traps represents a valuable step towards scalable and reliable quantum processors. By eliminating extensive optical paths, this work significantly reduces noise susceptibility and facilitates potential parallel operations, critical for the execution of complex quantum algorithms. Furthermore, their approach supports addressing circuits within 2D trap architectures, a necessity for realizing large-scale quantum networks.

Future Prospects

The implications of this work extend beyond trapped-ion QIP. The authors suggest potential applications in atomic clocks and neutral atom quantum systems, which may benefit from the integration of such optical and mechanical stability improvements. Additionally, the research sets the groundwork for exploring hybrid qubit encoding, combining optical and microwave controls to optimize qubit performance concerning coherence and resource efficiency.

In summary, this paper makes notable contributions to decreasing the operational error rates in quantum systems and provides a scalable platform for future quantum computing developments. The work not only addresses immediate technical challenges but also lays the groundwork for advancements in a variety of atom-based technologs.