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arXiv:2507.23071 · ACS Photonics

Scalable Ion Fluorescence Collection Using a Trap-Integrated Metalens

Hae Lim, Johannes E. Fröch, Christian M. Pluchar, Arka Majumdar, Sara Mouradian

Published Mar 26, 2026• 0 citations
arXivHTMLPDF

Abstract

A scalable trapped-ion quantum computer demands efficient fluorescence collection from many readout zones distributed over a large chip area. We present a compact, monolithically integrated approach in which a metalens is fabricated on the backside of a surface-electrode ion trap to collect and collimate ion fluorescence. A 40 × 100 μm aperture formed in the trap’s ground electrode yields a simulated point-source solid-angle collection efficiency of 0.91% and a measured point-source detection efficiency of 0.58%. Expanding the aperture to 40 × 600 μm increases the simulated collection efficiency to 3.17%, comparable to a conventional objective with a numerical aperture of 0.35. An engineered undercut beneath the electrodes surrounding the aperture preserves the collection efficiency while increasing the ion-to-dielectric separation. By directly collimating fluorescence at the trap, the metalens eliminates the need for high numerical aperture objectives. Arrays of such readout zones enable scalable, high-fidelity parallel state detection in large-scale trapped-ion quantum computers.

DOI
10.1021/acsphotonics.5c02393

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A scalable trapped-ion quantum computer demands efficient fluorescence collection from many readout zones distributed over a large chip area. We present a compact, monolithically integrated approach in which a metalens is fabricated on the backside of a surface-electrode ion trap to collect and collimate ion fluorescence. A 40 × 100 μm aperture formed in the trap’s ground electrode yields a simulated point-source solid-angle collection efficiency of 0.91% and a measured point-source detection efficiency of 0.58%. Expanding the aperture to 40 × 600 μm increases the simulated collection efficiency to 3.17%, comparable to a conventional objective with a numerical aperture of 0.35. An engineered undercut beneath the electrodes surrounding the aperture preserves the collection efficiency while increasing the ion-to-dielectric separation. By directly collimating fluorescence at the trap, the metalens eliminates the need for high numerical aperture objectives. Arrays of such readout zones enable scalable, high-fidelity parallel state detection in large-scale trapped-ion quantum computers.
In ion trap quantum information processing, efficient fluorescence collection is critical for fast, high-fidelity qubit detection and ion–photon entanglement. The expected size of future many-ion processors requires scalable light collection systems. We report on the development and testing of a microfabricated surface-electrode ion trap with an integrated high-numerical aperture (NA) micromirror for fluorescence collection. When coupled to a low-NA lens, the optical system is inherently scalable to large arrays of mirrors in a single device. We demonstrate the stable trapping and transport of 40 Ca + ions over a 0.63 NA micromirror and observe a factor of 1.9 enhancement of photon collection compared to the planar region of the trap.
Integration of fiber optics may play a critical role in the development of quantum information processors based on trapped ions and atoms by enabling scalable collection and delivery of light and coupling trapped ions to optical microcavities. We trap 24Mg+ ions in a surface-electrode Paul trap that includes an integrated optical fiber for detecting 280-nm fluorescence photons. The collection numerical aperture is 0.37, and total collection efficiency is 2.1%. The ion can be positioned between 80 and 100 μm from the tip of the fiber by use of an adjustable rf pseudopotential.
We demonstrate the first integration of a metalens inside the ultra-high-vacuum (UHV) chamber of an ion trap, enabling near-diffraction-limited imaging of trapped ions. A single trapped Yb + ion is laser-cooled close to the Lamb–Dicke regime to characterize the metalens performance, achieving a full-width at half-maximum (FWHM) of 1.16 ± 0.09 μm, which aligns closely with the Abbe diffraction limit of 1.15 μm and outperforms conventional external-cavity lenses (2.37 ± 0.12 μm) with the same numerical aperture. We further demonstrate near-diffraction-limited imaging of multi-ion systems over the accessible off-axis range. Our results establish the metalens as a superior optical element for ion fluorescence imaging, providing higher resolution for applications in ion position calibration, multi-qubit readout, and precision measurement. Furthermore, this miniaturized and integrable approach overcomes traditional optical constraints, delivering near-diffraction-limited imaging and advancing the development of compact ion trap platforms.
The integration of optics for efficient light delivery and the collection of fluorescence from trapped ions in surface electrode ion traps is a key component to achieving scalability for quantum information processing. Diffractive optical elements (DOEs) present a promising approach as compared to bulk optics because of their small physical profile and their flexibility in tailoring the optical wavefront. The precise alignment of the optics for coupling fluorescence to and from the ions, however, poses a particular challenge. Excitation and manipulation of the ions requires a high degree of optical access, significantly restricting the area available for mounting components. The ion traps, DOEs, and other components are compact, constraining the manipulation of various elements. For efficient fluorescence collection from the ions the DOE must be have a large numerical aperture (NA), which results in greater sensitivity to misalignment. The ion traps are sensitive devices, a mechanical approach to alignment such as contacting the trap and using precision motors to back-off a set distance not only cannot achieve the desired alignment precision, but risks damage to the ion trap. We have developed a non-contact precision optical alignment technique. We use line foci produced by off-axis linear Fresnel zone plates (FZPs) projected on alignment targets etched in the top metal layer of the ion trap and demonstrate micron-level alignment accuracy.
Trapped, laser-cooled atoms and ions produce intense fluorescence of the order 107~108 photons per second. Detection of this fluorescence enables efficient measurement of the quantum state of qubits based on trapped atoms. It is desirable to collect a large fraction of the photons to make the detection faster and more reliable. Additionally, efficient fluorescence collection can improve the speed and fidelity of remote ion entanglement and quantum gates. Refractive and reflective optics, and optical cavities have all been used to collect the trapped ion fluorescence with up to about 10% efficiency. Here we show a novel ion trap design that incorporates a metallic spherical mirror as the integral part of the trap itself, being its RF electrode. The mirror geometry enables up to 35% solid angle collection of trapped ion fluorescence. The movable central pin electrode of this trap allows precise placement of the ion at the focus of the reflector. We characterize the performance of the mirror, and measure 25% collection efficiency, likely limited by the imperfections of the mirror surface. We also study the properties of the images of single ions formed by the spherical mirror and apply aberration correction with an aspherical element placed outside the vacuum system. Owing to the simplicity of its design, this trap structure can be adapted for microfabrication and integration into more complex trap architectures.
The integration of optics for efficient light delivery and the collection of fluorescence from trapped ions in surface electrode ion traps is a key component to achieving scalability for quantum information processing.Diffractive optical elements (DOEs) present a promising approach as compared to bulk optics because of their small physical profile and their flexibility in tailoring the optical wavefront.The precise alignment of the optics for coupling fluorescence to and from the ions, however, poses a particular challenge.Excitation and manipulation of the ions requires a high degree of optical access, significantly restricting the area available for mounting components.The ion traps, DOEs, and other components are compact, constraining the manipulation of various elements.For efficient fluorescence collection from the ions the DOE must be have a large numerical aperture (NA), which results in greater sensitivity to misalignment.The ion traps are sensitive devices, a mechanical approach to alignment such as contacting the trap and using precision motors to back-off a set distance not only cannot achieve the desired alignment precision, but risks damage to the ion trap.We have developed a non-contact precision optical alignment technique.We use line foci produced by off-axis linear Fresnel zone plates (FZPs) projected on alignment targets etched in the top metal layer of the ion trap and demonstrate micron-level alignment accuracy.