Vacuum assembly, atomic source development, and micromotion studies for a trapped Ba+ based quantum processor

Abstract

Trapped-ions are one of the leading platforms for quantum information processing, due to their long coherence times and high-fidelity State Preparation and Measurement (SPAM) and gate operations. However, implementing a trapped-ion system with many desirable features such as Ultra-High-Vacuum (UHV), High-Optical-Access (HOA), and a high trapping probability presents significant technical challenges. These features are critical for enabling complex quantum simulation experiments, including simulations of spin-1/2 systems and beyond, by leveraging the multiple internal states of barium and tunable spin–spin interactions. High optical access is required to deliver tightly focused beams for site-resolved control and inter-ion coupling, while UHV conditions are essential for achieving long ion lifetimes necessary for stable, large-scale simulations. Achieving such optical access, which is vital for both coherent gate operations and high numerical aperture fluorescence collection, whilst maintaining vacuum integrity presents substantial engineering challenges. This thesis presents the construction of a vacuum chamber system intended for an individually addressable 16-qubit quantum processor based on barium-133 ions confined in a microfabricated surface-electrode ion trap. Although the complete processor has not yet been realized, the chamber has been assembled to support its future implementation. The microfabricated surface-electrode ion trap features ∼ 100 electrodes that require independent control of static voltages for precise tailoring of the confining electric potential. The experimental setup was designed to provide high optical access and to reach base pressures in the low 10^(−11) mbar range, thereby minimizing ion–background gas collisions. This is achieved by centrally mounting the trap within the vacuum chamber, a departure from conventional flange-mounted designs. To bring in-vacuum electrical connections to ∼ 100 trap electrodes, a custom implementation of an in-vacuum wire harness is described that enables reliable electrical connectivity to the centrally mounted surface electrode ion trap. The development of this wire harness enabled us to reach UHV with a pressure of ≈ 3 × 10^(−11) mbar. The successful development of this wire harness demonstrates that it is possible to create a UHV compatible wiring solution using commercially available components for a surface-electrode ion trap suspended at the center of the vacuum chamber. This centrally mounted design provides both high optical access and high conductance to vacuum pumps, enabling individual ion addressing and long ion lifetimes, which in turn facilitate more complex quantum simulation experiments. In addition, a multispecies ablation target was developed that incorporates both radioactive barium-133 and metallic barium to enable ion loading at lower trap depths, addressing a common challenge in surface-electrode ion traps. The velocity of the atomic plume generated from barium metal ablation targets was measured and compared to that of BaCl_2 targets. Although we were unable to definitively measure the effective temperature and peak velocity of the atomic plume generated from barium metal, the neutral fluorescence measurements from the barium metal targets indicate a larger number of slower moving atoms compared to BaCl_2 targets. The lower speeds of the atoms generated from metal targets is also plausible due to the low bond energy of metallic bonds compared to covalent bonds in salts. Barium has recently become a popular choice for high-fidelity trapped-ion experiments due to its favorable atomic structure. The larger number of slower moving atoms from barium metal demonstrates that metallic barium could be a much more promising atomic source for barium-based trapped-ion experiments, as the slower plume enables higher loading rates in surface-electrode ion trap systems.

Finally, excess micromotion, caused by stray electric fields in RF traps, can lead to undesirable effects such as heating and frequency modulation of atomic transitions. This work evaluates the micromotion compensation capabilities of the Phoenix ion trap used in our system through numerical simulations of single ion trajectories. Through these simulations, it was determined that stray fields of Ex ≈ 3000 V/m, Ey ≈ 3800 V/m, and Ez ≈ 600 V/m can be effectively compensated for with the Phoenix ion trap. With typical stray electric fields in trapped-ion systems ranging from tens to a few hundred V/m, this demonstrates that the Phoenix trap should be well capable of compensating any stray fields expected in our system.