Design and Characterization of Compact Acousto-Optic-Deflector Individual Addressing System for Trapped-Ion Quantum Computing

TL;DR

The paper addresses the challenge of scalable, high-fidelity individual addressing in long trapped-ion chains by developing a compact AOD-based beam-steering system with minimized optomechanical degrees of freedom. The authors design and assemble a sub-1 ft$^2$ footprint optical platform that delivers Gaussian beams at 355 nm, a broad steering range, and very low crosstalk, demonstrated on a 30-ion chain with a beam-switching time of about $240$ ns. Key innovations include a custom anamorphic prism pair beam expander, precision flexure mounts for AOD alignment, and a K-mirror image rotator to align steering with the ion chain, achieving tight focusing (waists near $8.5\,\mu$m × $1.8\,\mu$m) and robust multi-ion addressing. The results indicate the approach enables scalable, high-fidelity trapped-ion operations on long ion chains and supports potential mid-circuit measurements and quantum simulations with enhanced optical stability.

Abstract

We present a compact design for a beam-steering system based on acousto-optic-deflectors (AODs) used as an individual addressing system for trapped-ion quantum computing. The design targets to minimize the optomechanical degrees of freedom and the optical beam paths to improve optical stability, and we successfully implemented a solution with a compact footprint of less than 1 square foot. The system characterization results show that we achieve clean Gaussian beams at 355nm wavelength with a beam steering range of $\sim$50 times the beam diameter, and an intensity crosstalk of $< 9 \times 10^{-4}$ at all neighboring ions in a five-ion chain. Based on these capabilities, we experimentally demonstrate individual addressing of a 30-ion chain. We estimate the beam switching time of the AOD to be $\sim$240 ns. The compact system design is expected to provide high optical stability, providing the potential for high-fidelity trapped-ion quantum computing with long ion chains.

Figures (12)

• Figure 1: Optical design schematic of the compact AOD individual addressing system. A custom-designed anamorphic prism beam expander expands the circular collimated input beam with an aspect ratio of 4.7 along the $x$-axis, generating an elliptical beam with a beam waist of 0.32 mm by 1.5 mm. The generated elliptical beam passes through the AOD (Brimrose CQD-150-100-355), which deflects the beam with a maximum deflection angle of 6 mrad over an RF bandwidth of 100 MHz. A Fourier lens with a $\sim100$ mm focal length is positioned $\sim100$ mm away from the AOD, converting angular deflection from the AOD into parallel positional translation at the Fourier plane. The semi-focused beam on the Fourier plane has a target design waist of 34 $\mu$m x 7.3 $\mu$m and a steering range of 600 $\mu$m. The final focusing at the ion position is achieved with an NA=0.2 projection lens (Photon Gear 18020-ATP), providing a 4X demagnification with minimized optical aberration. The target beam waist at the ion position is around 8.5 $\mu$m x 1.8 $\mu$m, with a maximum steering range of 150 $\mu$m.
• Figure 2: A detailed schematic diagram of the compact AOD individual addressing system. Two individual beams (Ind1 and Ind2) are routed from the upstream optical setup via photonic crystal fibers. After collimation, a compact anamorphic prism pair (APP) beam expander converts each individual beam into an elliptical beam. Each beam is deflected by its corresponding AOD along the $x$ axis. A beam sampler picks off $\sim\!0.2$% of the deflected beam from AOD and directs it to a photodiode to monitor beam power fluctuations. A 50:50 non-polarizing beam splitter (NPBS) combines both individual beams spatially. A Fourier lens focuses the beams and transforms the angular deflection from AOD into parallel displacements at the Fourier plane. A K-mirror image rotator enables alignment of the beam steering direction along the ion chain. A precision optical pinhole is positioned at the Fourier plane to block 0th and 2nd-order diffraction from AODs. A linear P-polarizer filters the polarization of the final beam. Lastly, a piezoelectric kinematic mirror mount (Polaris-K05P2) provides fine adjustment of beam alignment at the ion position. The system features a compact footprint of less than 1 square foot.
• Figure 3: Compact optomechanical components for mounting mirrors and waveplates. (a) Mirrors are mounted on commercially available miniature kinematic mounts (AOSense). (b) Optical waveplates are mounted on custom-designed rotational mounts. A half-wave and quarter-wave plate can be mounted in pairs to provide arbitrary polarization control. Setscrews provide mechanical locking for the waveplates.
• Figure 4: (a) Schematic of the anamorphic prism pair (APP) beam expander with all geometric angles defined. $\alpha, \alpha'$ and $\beta, \beta'$ are major factors for determining the beam expansion factor. (b) Numerical simulation of the APP beam expander beam expansion factor. We used $\beta=\beta'$ of 30$^{\circ}$ in the simulation. The red dotted line represents the condition satisfying the target expansion factor of 4.7. The red star represents the actual system design parameters ($\alpha$, $\alpha'$) = (39.0$^\circ$, 14.75$^\circ$).
• Figure 5: A simplified schematic diagram of the compact anamorphic prism pair (APP) beam expander is presented. Non-polarization-maintaining PCF (NKT Photonics LMA-10-UV-FUD) routes the beam from the upstream into a nitrogen-purgeable collimator (Schäfter + Kirchhoff 60FC-4-S18-49-XV). Nitrogen purging can prevent fiber tip degradation induced by particle deposition at UV wavelengths marciniak2017towards. The collimator is mounted on a kinematic mount for the initial alignment process. A polarizing beam splitter (PBS), together with a quarter-wave plate and half-wave plate, provides initial polarization filtering. The geometry of PBS is set properly so that the transmission of the polarization along the $x$ axis is maximized, at which the anti-reflection coating of the anamorphic prism surfaces is designed to be optimal. The second anamorphic prism is mounted on a flexure mount, allowing for fine adjustment of the relative angles between the prisms. Each setscrew provides clockwise or counterclockwise rotation of the second prism.