Architecture for improved scaling for Ion Trap Quantum Computers

Arxiv – Ball-grid array architecture for microfabricated ion traps

State-of-the-art microfabricated ion traps for quantum information research are approaching nearly one hundred control electrodes. We report here on the development and testing of a new architecture for microfabricated ion traps, built around ball-grid array (BGA) connections, that is suitable for increasingly complex trap designs. In the BGA trap, through-substrate vias bring electrical signals from the back side of the trap die to the surface trap structure on the top side. Gold-ball bump bonds connect the back side of the trap die to an interposer for signal routing from the carrier. Trench capacitors fabricated into the trap die replace area-intensive surface or edge capacitors. Wirebonds in the BGA architecture are moved to the interposer. These last two features allow the trap die to be reduced to only the area required to produce trapping fields. The smaller trap dimensions allow tight focusing of an addressing laser beam for fast single-qubit rotations. Performance of the BGA trap as characterized with 40Ca+ ions is comparable to previous surface-electrode traps in terms of ion heating rate, mode frequency stability, and storage lifetime. We demonstrate two-qubit entanglement operations with 171Yb+ ions in a second BGA trap.

A ball-grid array architecture offers significant improvements in size and scalability for microfabricated ion traps. Trench capacitors fabricated into the trap die replace surface filter capacitors, providing a 30× reduction in trap die area over traps with planar capacitors. Through-substrate vias connect the electrodes to pads on the back side of the trap die, eliminating wirebonds from the trap surface. The trap die is bump-bonded to a separate interposer chip for signal routing to a CPGA carrier. Optical access to a trapped ion is improved by the reduced BGA trap chip area and the absence of wirebond obstructions, allowing tighter focusing of laser beams for qubit operations and addressing.

1. Overview of the BGA design: (a) Die bond region of the CPGA showing the BGA trap and the interposer footprint. (b) Side view. (c) Fully packaged BGA trap. The long bond wire supplies the trap RF signal.

Journal of Applied Physics – Ball-grid array architecture for microfabricated ion traps

Schematic cross section through a BGA trap die and interposer (not to scale).

By all measures, the BGA trap equals or exceeds results from earlier GTRI surface-electrode traps fabricated with aluminum electrodes and standard wirebonding and the heating rate is competitive with most microfabricated traps from other groups30. We demonstrate focusing of an addressing 729 nm laser beam to a waist of ≈ 8 µm, approaching the separation distance for two co-trapped ions, with minimal power loss due to scattering off trap surfaces. Further improvements in ion addressing could be obtained with cylindrical optics to tighten focus along the trap axis while maintaining beam divergence in the vertical direction relevant to scattering loss. In a second BGA trap, we demonstrate a Mølmer-Sørensen gate with 93% entanglement fidelity, enabled by tight focusing of 355 nm Raman beams onto a pair of 171Yb+ ions.

The BGA architecture extends readily to traps of increasing complexity. BGA techniques could simplify fabrication of intricate junction traps, such as the NIST Racetrack and Sandia MUSIQC Circulator , which feature rings of RF rails encircling isolated islands of control electrodes; in such designs it becomes impossible to route leads to all electrodes on a single metal layer. BGA interposer design is flexible and easily modified for signal routing from carriers other than the standard CPGA presented here. The BGA architecture could be integrated with recently demonstrated in-vacuum electronics18 to
produce ion traps with significantly larger numbers of electrodes at manageable physical size.

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