In a recent article published in the journal Light Science & Applications, researchers presented a novel approach for collecting fluorescence photons emitted from a trapped ion by integrating photonic components directly into a microfabricated ion-trap chip.
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Quantum Interfaces and the Photon Collection Challenge
Atoms and ions isolated and confined in traps serve as exemplary quantum systems for storing and processing quantum information. Their numerous optical transitions provide natural interfaces between quantum states of matter and light.
Spontaneous emission of photons from these atoms can be harnessed to entangle separate qubits via photon interference, but this requires high-efficiency and mode-matched photon collection. Existing techniques utilize bulk lenses or cavities to enhance photon collection, often limited in scalability and stable operation due to free-space optical alignment complexity and large footprints.
Integrated photonics offers an attractive alternative by embedding optical components within the ion trap substrate itself. Since ions can be trapped tens of microns above the chip surface, integrating diffraction gratings beneath the trap electrodes can achieve a high numerical aperture collection with a compact lateral footprint.
This approach enhances stability and reproducibility by reducing path-length variations and enabling fixed, scalable optics arrays suitable for multiplexed quantum networking or sensing applications.
On-Chip Photonics Design and Fabrication Strategy
The authors designed and fabricated an integrated photonic device on a 200 mm wafer that combines an ion trap with a focusing diffraction grating. The integrated grating is engineered to convert ion fluorescence into a single-mode silicon nitride waveguide mode.
Key to efficient collection is optimizing the grating parameters, namely pitch, curvature, and apodization profile, to tightly focus light into the waveguide while matching the ion’s dipole emission pattern. To maximize upward emission, vertical symmetry was broken using two waveguide layers with a lateral offset, resulting in high directionality.
A novel “phase-shift apodization” technique was introduced, modulating local scattering strength through sub-wavelength zone displacements to finely tailor the emitted beam profile without requiring feature sizes beyond standard lithographic capabilities.
Device Architecture and Experimental Implementation
The device was integrated into a multi-layer photonics platform with silicon nitride, alumina, and silica layers for low-loss photon routing. Electrodes for an 88Sr+ ion trap were fabricated above the photonic layers and covered with a transparent conductive film for electrical shielding.
The collection grating subtended roughly 2.18% of the solid angle at an ion height of 50 μm and collected about 1.97% of emitted photons incident on its surface, resulting in an overall collection efficiency into the waveguide of approximately 0.043%.
The authors characterized the device using finite-difference time-domain simulations, optical imaging of emitted light, and ion fluorescence scans across varying ion positions and polarizations. Additional analysis explored polarization-dependent collection and crosstalk, emphasizing the role of dipole alignment and mode selectivity.
Collected photons were routed through waveguides to fibers at the chip edge and detected using photomultiplier tubes.
Performance Limits and Efficiency Bottlenecks
The integrated grating successfully concentrated ion fluorescence into a single guided mode, demonstrating initial mode matching with the ion’s radiation pattern. Simulations and experimental results confirmed the grating’s focusing behavior and upward directivity.
The phase-shift apodization approach reduced unwanted longitudinal beam structure and improved spatial overlap with the ion’s emission. However, overall photon collection efficiency remained limited by several factors, including:
- The small solid angle of the 30 × 30 μm² grating
- Fabrication imperfections (e.g., surface divots and layer thickness deviations)
- Transmission losses in waveguides and fiber interfaces
- Detector quantum efficiency
The authors measured a signal-to-background ratio of 36 and a photon detection rate of approximately 297 photons per second
Pathways to Higher Collection Efficiency
Detailed analysis attributed nearly 17 dB of efficiency loss to mode mismatch and fabrication defects, highlighting significant room for improvement.
The study suggests that improved fabrication fidelity alone could yield efficiency gains of up to 12 dB. Further enhancements could be achieved through optimized waveguide-to-fiber coupling and the use of higher-efficiency detectors, such as single-photon avalanche diodes instead of photomultiplier tubes.
Combined, these improvements could increase detection efficiency by more than 19 dB, enabling high-fidelity quantum state readout on sub-millisecond timescales.
The study also demonstrated quantum state detection using the integrated system, achieving high, but not yet optimal, fidelities over millisecond detection windows, reinforcing its proof-of-principle status
Toward Scalable, Integrated Quantum Architectures
This work demonstrates a proof-of-concept approach for collecting fluorescence photons from a single trapped ion into an on-chip, single-mode waveguide integrated within the ion trap structure.
Experimental validation through simulations, emission imaging, and fluorescence measurements shows good agreement with theoretical models. While current performance is limited by fabrication-related losses, projected improvements point toward substantial gains in photon detection efficiency.
The scalable and stable nature of photonic integration directly on ion-trap chips opens a pathway to compact, robust, and reproducible quantum information processing systems, reducing reliance on bulky free-space optics and advancing integrated quantum technologies.
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Journal Reference
Knollmann F.W., Corsetti S.M., et al. (2026). Collection of fluorescence from an ion using trap-integrated photonics. Light Science & Applications 15, 95. DOI: 10.1038/s41377-025-02138-9, https://www.nature.com/articles/s41377-025-02138-9