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arXiv:2409.05123 · Physical Review Applied

How to integrate a miniature optical cavity in a linear ion trap: Shielding dielectrics and trap symmetry

Ezra Kassa, Shaobo Gao, Soon Teh, Dyon van Dinter, Hiroki Takahashi

Published Feb 18, 2025• 10 citations
arXivHTMLPDF

Abstract

Scaling up qubit numbers remains a major challenge across all quantum computing platforms, including trapped-ion systems. A modular approach seems promising, but hinges on efficient interfaces like miniature optical cavities. Despite decades of effort, a lack of understanding of the relevant challenges has precluded integration. This study identifies key obstacles posed by dielectric optical cavities, and uncovers a critical component that alleviates their negative effects. The findings provide a clear roadmap for improving ion-trap interfacing and suggest that three-dimensional traps are more suitable than planar ones.

DOI
10.1103/physrevapplied.23.024038

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Scaling up qubit numbers remains a major challenge across all quantum computing platforms, including trapped-ion systems. A modular approach seems promising, but hinges on efficient interfaces like miniature optical cavities. Despite decades of effort, a lack of understanding of the relevant challenges has precluded integration. This study identifies key obstacles posed by dielectric optical cavities, and uncovers a critical component that alleviates their negative effects. The findings provide a clear roadmap for improving ion-trap interfacing and suggest that three-dimensional traps are more suitable than planar ones.
The realisation of efficient ion-photon quantum interfaces is an essential step towards developing operational quantum information networks. Individual trapped ions have been shown as promising candidates for quantum information processing nodes, while effective ion manipulation and photonic links between nodes can be realised by deterministic coupling of ions to optical cavities. However, integrating optical cavities into ion traps remains a challenge as the trapping potential is affected by the presence of any dielectric or conductor.
Integrated optical control of trapped-ion qubits leveraging dielectric waveguides photonics within planar-fabricated trap devices offers routes to scaling the optical control critical to trapped-ion quantum computing. The ultraviolet to infrared functionality required for many atomic systems, the multiple metal layers for large-scale trap arrays, and the long-term interest in integration of active functionality and integrated electronics motivates development of foundry-based platforms for such systems, to facilitate accessibility to the complex fabrication involved. Aside from scaling, utilization of such technology in trapped-ion quantum systems further stands to alleviate limitations on basic physical operations employed in both quantum computing and metrology, which experiments are starting to demonstrate. I will summarize our recent results using phase-stable standing-wave delivery practically enabled by integrated photonic addressing to enhance ground-state laser cooling, which we experimentally demonstrate using trap devices with integrated Al2O3 and Si3N4 waveguides for UV-NIR addressing fabricated in a commercial foundry. I will also discuss relevance of similar ideas to alleviating power requirements in laser-based entangling gates, and work in progress towards experimental realization, including in a 300 mm-wafer foundry platform for such devices in development.
We study a range of radio-frequency ion trap geometries and investigate the effect of integrating dielectric cavity mirrors on their trapping potential using numerical modelling. We compare five different ion trap geometries with the aim to identify ion trap and cavity configurations that are best suited for achieving small cavity volumes and thus large ion-photon coupling as required for scalable quantum information networks. In particular, we investigate the trapping potential distortions caused by the dielectric material of the cavity mirrors in all 3 dimensions for different mirror orientations with respect to the trapping electrodes. We also analyze the effect of the mirror material properties such as dielectric constants and surface conductivity, and study the effect of surface charges on the mirrors. As well as perfectly symmetric systems, we also consider traps with optical cavities that are not centrally aligned where we find a spatial displacement of the trap centre and asymmetry of the resulting trap only at certain cavity orientations. The best trapcavity configurations with the smallest trapping potential distortions are those where the cavities are aligned along the major symmetry axis of the electrode geometries. These cavity configurations also appear to be the most stable with respect to any mirror misalignment. Although we consider particular trap sizes in our study, the presented results can be easily generalized and scaled to different trap dimensions.
Atomic ions trapped in microfabricated surface traps can be utilized as a physical platform with which to build a quantum computer. They possess many of the desirable characteristics of such a device, including high fidelity state preparation and readout, universal logic gates, and long coherence times, and can be readily entangled with each other through photonic interconnects. The use of optical cavities integrated with trapped ion qubits as a photonic interface presents the possibility for order of magnitude improvements in performance in several key areas for their use in quantum computation. The first part of this thesis describes the design and fabrication of a novel surface trap for integration with an optical cavity. The trap is custom made on a highly reflective mirror surface and includes the capability of moving the ion trap location along all three trap axes with nanometer scale precision. The second part of this thesis demonstrates the suitability of small microcavities formed from laser ablated, fused silica substrates with radii of curvature in the 300-500 micron range for use with the mirror trap as part of an integrated ion trap cavity system. Quantum computing applications for such a system include dramatic improvements in the photon entanglement rate of up to 10 kHz, the qubit measurement time down to 1 microsecond, and the qubit measurement error rate down to the 1e-5 range. The final part of this thesis describes a performance simulator for exploring the physical resource requirements and performance demands to scale a quantum computer to sizes capable of implementing quantum algorithms beyond the limits of classical computation.
Combining optical cavities with trapped atomic ions is a powerful tool to investigate the interaction of light with matter on the most fundamental scale: a single atom with a single particle of light. Recent advances have not only demonstrated the emission of single photons, ion-photon entanglement and ion-ion entanglement but paved the way to use these systems in modern quantum technologies. Ion-cavity systems are now one of the most promising technologies for realising a scalable quantum computer and quantum networks. While the integration of optical cavities and ion traps is challenging, new techniques and technologies made it possible to demonstrate important building blocks for such systems.
Ion trap systems allow the faithful storage and manipulation of qubits encoded in the energy levels of the ions, and can be interfaced with photonic qubits that can be transmitted to connect remote quantum systems. Single photons transmitted from two remote sites, each entangled with one quantum memory, can be used to entangle distant quantum memories by interfering on a beam splitter. Efficient remote entanglement generation relies upon efficient light collection from single ions into a single mode fiber. This can be realized by integrating an ion trap with an optical cavity and employing the Purcell effect for enhancing the light collection. Remote entanglement can be used as a resource for a quantum repeater for provably secure long-distance communication or as a method for communicating within a distributed quantum information processor. I present the integration of a 1 mm optical cavity with a micro-fabricated surface ion trap. The plano-concave cavity is oriented normal to the chip surface where the planar mirror is attached underneath the trap chip. The cavity is locked using a 780 nm laser which is stabilized to Rubidium and shifted to match the 369 nm Doppler transition in Ytterbium. The linear ion trap allows ions to be shuttled in and out of the cavity mode. The Purcell enhancement of spontaneous emission into the cavity mode would then allow efficient collection of the emitted photons, enabling faster remote entanglement generation.