Increased control over ions’ motions may help improve quantum computers

Adrian Y. Cho

Arjun D. Rao

The technology of linear ion traps are constantly innovating with increasingly varying atomic species being trapped in more scalable systems. In some iterations, multiple species can be confined, offering advanced computing capabilities such as sympathetic cooling. As the ion chains grow in size, the ion operations must scale with it, permitting universal quantum computation in larger qubit arrays. In this thesis, the design, construction and preliminary characterisation of a dual-species trapped-ion system is described. Additionally, an investigation into how entangling gates operate in these systems is performed. Laser setups at 355nm drive the spin-motion coupling in ytterbium ions to produce entanglement at high fidelities in longer chains. First, entangling gates are implemented in the ytterbium trapwith improvements to the control software. Primitive Mølmer-Sørensen gates are introducedand phase modulated gates using Q-CTRL’s graph based optimiser are implemented. Second, entanglement is computationally investigated in multipartite systems. Comparisons aredrawn between quantum control techniques such as laser modulation and Rabi rate imbalances are investigated.

Ming Li, Nhung H. Nguyen, Alaina Green +3 more

Two-qubit gates are a fundamental constituent of a quantum computer and typically its most challenging operation. In a trapped-ion quantum computer, this is typically implemented with laser beams which are modulated in amplitude, frequency, phase, or a combination of these. The required modulation becomes increasingly more complex as the quantum computer becomes larger, complicating the control hardware design. Here we develop a simple method to essentially remove the pulse-modulation complexity by engineering the normal modes of the ion chain. We experimentally test the feasibility of the method in a three-ion chain. This opens up the possibility to trade off complexity between the design of the trapping fields and the optical control system, which will help scale the ion-trap quantum computing platform.

Pak Hong Leung

YingYe Huang, Wentao Chen, Guoyu Zou +3 more

Trapped-ion systems have emerged as a leading platform for scalable quantum information processing owing to their high-fidelity operations and long-range entangling capabilities. As the number of ions in a trap increases, the growing density of collective motional modes makes the synthesis of multimode entangling gates increasingly challenging. Designing large-scale gates requires simultaneously realizing the desired spin-spin interactions, suppressing residual spin-motion entanglement, and limiting experimental control resources, leading to a high-dimensional non-convex optimization problem. Here we develop a numerical framework for multi-tone gate synthesis that directly searches for control fields satisfying these competing requirements. By employing an alternating-minimization strategy, the framework improves numerical stability and remains effective for large systems with many motional modes and target interactions. As representative demonstrations, we synthesize gates implementing all-to-all and nearest-neighbor interaction patterns in ion chains of up to N = 1000, using only global laser control. Across the parameter regimes explored here, the control resources required to maintain high-fidelity interactions do not exhibit rapid growth with system size. We extend the framework to individual addressing using a structured qLDPC target at N = 512 as an example. These results identify multimode gate synthesis as a viable route toward programmable interaction engineering in large-scale trapped-ion quantum processors.

Wolfgang Lange