Design and Fabrication of Metal-Shielded Fiber-Cavity Mirrors for Ion-Trap Systems

TL;DR

This work tackles the challenge of stable ion–photon interfaces in ion-trap systems integrated with fiber Fabry–Pérot cavities by designing metal-shielded fiber mirrors that suppress surface charging and dielectric-induced heating. The authors combine arc-discharge sealing, HF etching, photolithography, and electroplating to create a central, gold-coated optical region while shielding surrounding areas, achieving an alignment precision near a few micrometers and preserving cavity finesse for practical lengths. Finite-element simulations show that the shielding geometry yields a steep decline in heating with cavity length, quantified by a heating-rate scaling exponent of $\\alpha=5.98$ for a 30 μm exposed region, indicating strong dielectric-noise suppression. Experimental demonstrations with a needle trap show stable single-ion trapping inside a $L=230\ \mu\text{m}$ cavity and a measured heating rate of $57.62 \pm 2.09$ phonons/ms, in good agreement with simulations and significantly lower than unshielded configurations, validating the approach for scalable quantum networks. Overall, this metal-shielded mirror technique provides a robust route to high-efficiency, integrated ion–photon interfaces with minimal cavity degradation, paving the way for practical quantum networking nodes.

Abstract

Trapped ions in micro-cavities constitute a key platform for advancing quantum information processing and quantum networking. By providing an efficient light-matter interface within a compact architecture, they serve as highly efficient quantum nodes with strong potential for scalable quantum network. However, in such systems, ion trapping stability is often compromised by surface charging effects, and nearby dielectric materials are known to cause a dramatic increase in the ion heating rate by several orders of magnitude. These challenges significantly hinder the practical implementation of ion trap systems integrated with micro-cavities. To overcome these limitations, we present the design and fabrication of metal-shielded micro-cavity mirrors, enabling the stable realization of ion trap systems integrated with micro cavities. Using this method, we constructed a needle ion trap integrated with fiber Fabry-Perot cavity and successfully achieved stable trapping of a single ion within the cavity. The measured ion heating rate was reduced by more than an order of magnitude compared with unshielded configurations. This work establishes a key technique toward fully integrated ion-photon interfaces for scalable quantum network.

Figures (7)

• Figure 1: Photoelectric effect. (a) The optical fiber is fully exposed. (b) The optical fiber is enclosed in a metal tube, with gaps filled using thermosetting adhesive. (c) A gold layer is deposited on the fiber surface, exposing only the central $60\:\mu\text{m}$ region of the end face.
• Figure 2: (a) Trap geometry used in the simulations. The optical fiber is aligned along the z-axis within a blade trap. The exposed end-face region has a diameter of $2r$, with $r = 30\:\mu\text{m}$ in this simulation. RF signals of opposite phase are applied to the two pairs of RF electrodes. Only the configuration corresponding to Fig. \ref{['photoelectric']}(c) is shown; cases involving a metal tube enclosure or no surface metal shielding are not depicted. (b) Pseudopotential distributions for the unshielded fiber, the fiber enclosed in a metal tube (d, f), and the fiber with only the central $60\:\mu\text{m}$ of the end face exposed (h). The surface current density is $10^{-12}\,\mathrm{A{\cdot}m^{-2}}$ for (b, d), and $10^{-10}\,\mathrm{A{\cdot}m^{-2}}$ for (f, h). (c, e, g, i) Corresponding pseudopotential profiles along $x = 0$ for (b, d, f, h), respectively. Insets in (e, g, i) show quadratic fitting errors within the central $\pm\,200\:\mu\text{m}$ region.
• Figure 3: Simulation results for the relationship between the heating rate and the cavity length. The uppermost line corresponds to the case without a metal shield, and r decreases progressively from top to bottom.
• Figure 4: Flow chart for fabricating fiber electrodes. (a) Removing the coating of the fiber. (b) Creating concave surface by arc discharge. (c) Etching the fiber head with HF solution. (d) IBS coating. (e) Photolithography. (f) Developing. (g) Sputtering gold. (h) Photoresist strpping. (i) Electroplating thickening.
• Figure 5: Schematic of the fabrication process (scale bar: $30\:\mu\text{m}$) and the corresponding thickness statistics. (a, b) End facet before photolithography and after reversal baking, respectively. (c)End facet after development, showing a circular photoresist region at the center serving as a sputtering mask. (d) After magnetron sputtering, a metal layer covers the entire end facet, with the central circle corresponding to the photoresist. (e) End facet after photoresist removal, revealing the central cavity mirror while retaining a surrounding metal ring. (f) After electroplating, the outer metal ring thickens and slightly extends toward the center. (g) Under identical electroplating parameters, multiple samples were fabricated, and the gold thickness was measured at various positions.