Ion trap on borosilicate substrate with integrated femtosecond-laser-written waveguide

AI summary

Overview

Research area: Trapped-ion quantum computing hardware — specifically integrated photonics for on-chip laser light delivery to surface-electrode ion traps.

Technical level: Intermediate. The paper combines femtosecond-laser-written (FLW) integrated optics, surface-trap electrode design, FEM electrostatic simulation, and cryogenic ion-trap operation; readers should be comfortable with waveguide mode concepts and ion-trap operating principles, but the paper is written with substantial process and design detail that makes it accessible to graduate students entering the field.

Scope (one sentence): The paper demonstrates a borosilicate-glass ion-trap platform in which a femtosecond-laser-written waveguide, inscribed in a separate glass block bonded on top of a single-metal-layer surface trap, delivers 729 nm qubit light to a shuttled $^{40}\mathrm{Ca}^{+}$ ion in a cryogenic system.

What This Paper Is About

Scaling trapped-ion quantum computers to thousands of qubits requires delivering many tightly focused laser beams to individual ions, which free-space optics cannot do without a dedicated objective for every gate zone. The standard solution — planar photonic circuits buried under the trap electrodes — needs out-of-plane couplers such as grating couplers or micromirrors, which demand tens-of-nanometer lithographic and etch tolerances and have limited optical bandwidth. This paper instead builds the light-delivery layer as a separate structured borosilicate glass block, bonded onto the trap surface, containing femtosecond-laser-written waveguides that are routed in three dimensions and emit directly at the ion's height — no out-of-plane coupler required.

Key Contributions

1. A decoupled optical/electrical architecture. The optical layer is physically separated from the electrode substrate: a borosilicate glass optics block with an inscribed FLW waveguide is bonded on top of a surface-electrode trap built on a silicon wafer with a borosilicate layer. The authors argue this remains compatible with silicon-based integration and with integrated ASICs connected via through-substrate vias, because borosilicate's coefficient of thermal expansion is closely matched to silicon and allows wafer-level anodic bonding.

2. Waveguide engineering with tunable mode size. By varying pulse energy and the lateral offset between overscans, the mode-field diameter is tuned from approximately 3 to 8 µm, with typical relative transmission on the order of 80% and, for the mode-size series shown, transmission of order 70% or higher when mode-matching losses are accounted for. Low-loss curved guiding is achieved down to a radius of curvature of 6 mm, and single-mode operation is extended to 405 nm.

3. Trap design analysis and a fabrication flow meant for industry. FEM simulations quantify how the bonded dielectric block displaces the RF null, informing a geometry with a 1 mm separation that shifts the trap position by about 0.8 µm in z and 0.2 µm in y. The process is described as compatible with industrial fabrication of a single-metal-layer surface-electrode trap, including active fiber alignment and bonding.

4. End-to-end cryogenic validation. A $^{40}\mathrm{Ca}^{+}$ ion is trapped, shuttled to a zone in front of the waveguide, and driven coherently with 729 nm light delivered through the integrated waveguide, with stray electric fields characterized.

Main Findings

• Guiding occurs away from the written track: Under a broad range of writing conditions, light is guided in regions spatially displaced above and/or below the laser-modified volume, giving one or two distinct guiding regions ("upper" and "lower") separated by a distance on the order of 10 µm. The authors interpret this as thermally driven material redistribution producing densified guiding cores outside the central, comparatively depleted track, following a temperature-gradient-assisted mechanism proposed by Tan et al.
• Deliberate aberration helps: Writing at zero focus offset (nominally compensated spherical aberration) markedly degrades both transmission and mode confinement; optimal performance occurs at moderate negative focus offsets, typically within 170 to 240 µm across writing conditions, and a focus offset of 80 µm is used for subsequent experiments.
• Small mode-field diameters: The achieved MFDs (down to roughly 3 µm) are smaller than those typically reported for FLW waveguides in borosilicate glasses.
• Non-monotonic mode-size control via lateral offset: Increasing the lateral span between overscans first reduces the MFD to approximately 3 µm for offsets in the range 1–1.5 µm, before it increases again — a trend the authors explain with weak-guidance waveguide theory, where the tightest mode occurs when the effective guiding region is comparable to the intrinsic modal extent set by the index contrast.
• Divergence–confinement trade-off quantified: Measured far-field divergence ranges from 84–103 mrad for the smallest modes (between 2.5 and 5 µm MFD) down to about 40–60 mrad at roughly 10 µm MFD, beyond which no further notable reduction occurs; the saturation coincides with the near-field profile deviating from an ideal Gaussian.
• Bend tolerance depends on mode size: Losses stay below 1 dB/cm at large bending radii for all investigated MFDs, but waveguides with 5.3 µm MFD show pronounced bending loss at radii around 17 mm, whereas 3.9 µm MFD structures stay below 1 dB/cm down to approximately 6 mm — smaller radii than the several tens of millimeters commonly reported for FLW single-mode glass waveguides.
• Blue single-mode guiding: Stable single-mode propagation at 405 nm is demonstrated (pulse energy 180 nJ, writing speed 35 mm/s, one overscan, Zeiss Epiplan-Neofluar 20x 0.5 objective), supported by the strong confinement achievable with this writing strategy.
• Dielectric-induced trap shift is bounded and characterized: For the chosen 1 mm glass-to-ion distance, FEM predicts a trap-position shift of about 0.8 µm in z and 0.2 µm in y; the paper also maps RF-null displacement versus glass-to-ion distance and along the trap axis.
• Ion height chosen for voltage headroom: The 170 µm ion height matches the waveguide depth and was selected to prevent the waveguide output from clipping the trap surface while keeping required trapping voltages below 40 V, the limit of the experiment control system.
• Cryogenic operation with dielectric present: Trapping, shuttling to the addressing zone, and coherent carrier Rabi oscillations driven through the integrated waveguide are demonstrated; stray electric fields from the exposed dielectric show slow drift on a timescale of hours. Quantitative stray-field magnitudes and heating-rate values fall beyond the truncated text supplied here and are therefore not reported in this summary.

Methodology in Plain English

The waveguides are written by focusing femtosecond laser pulses (520 nm second harmonic of a 1040 nm source, 209 fs pulses, 1 MHz repetition rate) into 400 µm thick SCHOTT BF33 borosilicate glass mounted on a high-precision translation stage with 0.05 µm repeatability, at a constant depth of 170 µm — the intended ion height. The focused pulses locally and permanently change the glass's refractive index, so a waveguide is "drawn" by moving the sample along a programmed path, with no masks or cleanroom steps. Because focusing deep into glass distorts the focal spot, the team deliberately mis-sets the objective's aberration-correction collar (the "focus offset") as a tuning knob. Clean optical facets are produced by laser-ablating shallow grooves (about 80 µm deep) that guide a manual cleave. Mode profiles are imaged onto a CMOS camera and reduced to a D4σ mode-field diameter, and divergence is extracted from beam profiles taken at two distances. On the trap side, they simulate the electrode-plus-glass geometry electrostatically to see how the bonded dielectric pulls the RF null, then fabricate a single-metal-layer trap with segmented DC electrodes outside the RF rails, bond on a small optics block, and actively align and bond a fiber. The finished device is cooled down and tested by trapping a single calcium ion, shuttling it in front of the waveguide, and driving Rabi oscillations with waveguide-delivered 729 nm light.

Why This Matters

The paper offers a route around two of the hardest packaging problems in trapped-ion scaling: optical access and the fabrication tolerance of out-of-plane couplers. By routing light in three dimensions inside a bonded glass block and emitting parallel to the trap surface at ion height, it removes the grating-coupler bandwidth and etch-tolerance constraints, and by sharing a substrate material system with silicon it keeps a path open to ASIC integration and wafer-level processing.

Real-world applications:

• Scalable quantum computers: many addressing zones fed by waveguides rather than one objective per zone, with the option of pick-and-place micro-lenses for single-ion addressing.
• Compact and portable ion clocks and quantum sensors: on-chip light delivery reduces the optical bench footprint and, because optics and electrodes share a substrate, mechanical vibrations become common-mode and intrinsically stabilize laser–ion alignment.
• Photonic packaging and co-packaged optics: the glass-substrate, laser-written-waveguide, actively-aligned-fiber process flow is transferable to other chip-scale photonic devices requiring low-loss multi-wavelength routing.
• Cryogenic photonics: the demonstration that FLW waveguides and bonded glass survive cooldown and operate in a cryogenic vacuum environment is directly relevant to cryo-CMOS and superconducting-adjacent photonic systems.

Industry relevance: The work is a collaboration between Universität Innsbruck, Infineon Technologies Austria AG, JOANNEUM RESEARCH, and Alpine Quantum Technologies, and the fabrication was carried out at Infineon's semiconductor facilities; the explicit emphasis on single-metal-layer electrodes, anodic bonding, through-substrate vias, and active fiber alignment frames the platform as manufacturable rather than laboratory-only.

Future Directions

• Focused addressing: the authors state the architecture is compatible with hybrid micro-optics such as pick-and-place lenses to form a tightly focused spot for single-ion addressing, but this work characterizes only the bare waveguides.
• Multi-zone and multi-wavelength delivery: extending from a single waveguide to arrays serving many addressing zones, and exploiting the demonstrated 405 nm single-mode operation for cooling, state preparation, and readout alongside 729 nm qubit light.
• ASIC integration: realizing the proposed through-substrate-via connection to application-specific integrated circuits on a separate substrate to solve the electrical routing bottleneck.
• Stray-field and drift management: the observed slow drift of stray electric fields on an hours timescale, and the heating rates measured for the platform, motivate compensation strategies or electrode designs that mitigate the exposed dielectric's charge behavior.
• Compact routing: exploiting the 6 mm bending radius and the smaller radii reported for multi-pass waveguide morphologies to fit denser photonic layouts.

Target Audience

Trapped-ion and neutral-atom experimentalists building integrated-optics hardware; photonics process engineers working on femtosecond-laser-written waveguides in glass; quantum-hardware architects at companies evaluating glass-substrate packaging and ASIC co-integration; and graduate students or advanced undergraduates looking for a concrete, end-to-end example of how an integrated photonic layer is designed, simulated, fabricated, and validated inside a working cryogenic ion trap.

Authors’ abstract

We present an ion-trap platform on borosilicate glass with an integrated femtosecond-laser-written waveguide for on-chip light delivery. The optical layer is physically separated from the electrode substrate and bonded atop the trap, remaining compatible with silicon-based integration. We engineer single-mode low-loss guidance at 729 nm with tunable mode-field diameter and achieve low-loss curved waveguides down to a radius of curvature of 6 mm. We also extend single-mode operation to a wavelength of 405 nm. The fabrication process is compatible with the industrial fabrication of a single-metal-layer surface-electrode trap, including active fiber alignment and bonding. We validate the platform in a cryogenic trapped-ion system with $^{40}$Ca$^+$, demonstrating trapping, shuttling the ion to a zone in front of the waveguide, and coherent operations driven by 729 nm light delivered through the integrated waveguide. We characterize the effect of the exposed dielectric on the ion and measure stray electric fields that show slow drift at a timescale of hours. The architecture is compatible with hybrid micro-optics (e.g. pick-and-place lenses) to realize single ion addressing and provides a robust, scalable route to integrated light delivery for trapped-ion devices.