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Vacuum and Surface Science

Current Progress and Remaining Challenges in Development of On-chip Ion Traps

U. Tanaka

Published Mar 9, 2025• 0 citations
Publisher

Abstract

We report on an ion-shuttling experiment which is one of the core technologies in large scale ion-trap quantum computers. By controlling voltages applied to five pairs of electrodes in an on-chip ion trap, we have demonstrated shuttling of a single calcium ion. Besides the experimental result, we introduce discussion of the relation between ion motion and the condition of surface of trap electrodes and discuss the vacuum techniques which are required for scaling up of ion-trap quantum computers.

DOI
10.1380/vss.68.143

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We report on an ion-shuttling experiment which is one of the core technologies in large scale ion-trap quantum computers. By controlling voltages applied to five pairs of electrodes in an on-chip ion trap, we have demonstrated shuttling of a single calcium ion. Besides the experimental result, we introduce discussion of the relation between ion motion and the condition of surface of trap electrodes and discuss the vacuum techniques which are required for scaling up of ion-trap quantum computers.
The major challenges in trapped-ion quantum computation are to scale up few-ion experiments to many qubits and to improve control techniques so that quantum logic gates can be carried out with higher fidelities. This thesis reports experimental progress in both of these areas. In the early part of the thesis we describe the fabrication of a surface-electrode ion trap, the development of the apparatus and techniques required to operate it and the successful trapping of 40Ca+ ions. Notably we developed methods to control the orientation of the principal axes and to minimise ion micromotion. We propose a repumping scheme that simplifies heating rate measurements for ions with low-lying D levels, and use it to characterise the electric field noise in the trap. Surface-electrode traps are important because they offer a route to dense integration of electronic and optical control elements using existing microfabrication technology. We explore this scaling route by testing a series of three traps that were microfabricated at Sandia National Laboratories. Investigations of micromotion and charging of the surface by laser beams were carried out and improvements to future traps are suggested. Using one of these traps we also investigated anomalous electrical noise from the electrode surfaces and discovered that it can be reduced by cleaning with a pulsed laser. A factor of two decrease was observed; this represents the first in situ removal of this noise source, an important step towards higher gate fidelities. In the second half of the thesis we describe the design and construction of an experiment for the purpose of replacing laser-driven multi-qubit quantum logic gates with microwave-driven ones. We investigate magnetic-field-independent hyperfine qubits in 40Ca+ as suitable qubits for this scheme. We make a design study of how best to integrate an ion trap with the microwave conductors required to implement the gate and propose a novel integrated resonant structure. The trap was fabricated and ions were successfully loaded. Single-qubit experiments show that the microwave fields above the trap are in excellent agreement with software simulations. There are good prospects for demonstrating a multi-qubit gate in the near future. We conclude by discussing the possibilities for larger-scale quantum computation by combining microfabricated traps and microwave control.
With all the key elements of quantum computing in ion traps demonstrated by the research community, the focus is now placed on building more sophisticated traps with larger numbers of ions to allow practical scale information processing. One promising avenue is to store ions in and shuttle them between many independent traps which serve as potential interaction sites. The core of the work described in this thesis is the experimental evaluation of a microfabricated segmented ion trap, built by Sandia National Laboratories ("Sandia trap"). These experiments required construction of a wholly new optical setup including laser and detection systems, a vacuum system and control electronics. Among our experimental achievements were: successful loading of single and pairs of ions in the microscale trap, measurement of ion storage lifetime, measurement of the motional heating rate with a time-resolved Doppler-cooling method — which showed above than average heating, and implemented a single-ion shuttling method — which reliably transferred the ion through a distance of 360 μm (two DC electrode widths away) and back. These results have been used to improve the next version of the Sandia trap design. We also used computer modelling to study several aspects of ion traps: a mesoscopic ion trap designed for fast ion separation, simulated ion loading to quantify requirements for successful trapping in small and shallow traps, and analyzed a precise shuttling method — where the time dependence of the trapping potential is engineered such that there is minimal motional heating. The results show that ion trap arrays at the 100 μm distance scale are feasible and suggests that such multiple trap designs merit further study.
Abstract A large-scale ion trap array fabricated using the microelectromechanical systems (MEMS) technology is expected to be a promising device for building a practical quantum computer. Shuttling trapped ions is essential for operating scalable ion trap structures. This paper proposes an ion shuttling method for a MEMS-fabricated surface ion trap. Change of secular frequency of trapping potential can cause heating and subsequent loss of ions. Therefore, direct current voltage sets to form uniform ion trapping potentials around the ions while shuttling are calculated by simulations. A 32-channel digital-to-analog converter system is developed to apply the calculated voltage sets to the electrodes of the MEMS-fabricated surface ion trap. The shuttling process is experimented using trapped 174Yb+ ions. The successful round trip of the ion for 1920 μm is demonstrated using the developed approach.
This paper presents an overview of the latest research results on integrated, cryogenic-compatible electronics for ion shuttling operation in Trapped-Ion Quantum Computers (TIQCs). Integrated circuit (IC) realization is essential to enable system scaling towards a larger number of qubits. Firstly, we review the specific challenges related to the shuttling operation and ion confinement. Consequently, implications on circuit design of an integrated shuttling controller system on chip (SoC) are discussed. Particularly, among other circuit design parameters, we analyze in detail the requirements on power and area consumption, noise performance, and cryogenic-compatibility. Secondly, we provide an overview on reported discrete solutions, eventually highlighting the need for a dedicated integrated shuttling controller SoC. Hence, state of the art integrated shuttling controller approaches are presented and reviewed. Finally, we conclude the paper with an outlook on promising integrated circuit concepts for shuttling controllers.
All of the essential elements for a trapped ion quantum computer have been demonstrated in previous experiments. There are, however, many technical challenges to scaling the number of quantum bits from the current state-of-the-art (about 8) to the number of qubits needed for practical quantum computing. Although there is experimental evidence supporting one possible method (using cw laser pulses and common motional modes as the ion interaction) for deterministic ion entanglement, it is not known if that will ultimately be the most practical method for building a large-scale quantum computer. One model for scalable quantum computing with trapped ions calls for large interconnected arrays of small traps. Several advancements reported in this work include the development of three layer alumina traps as well as MEMS fabricated microtraps. Other models for scalable trapped ion quantum computing do not call for local entanglement or call for relaxed constraints on the motional control of the ions. These proposals require the use of ultrafast laser pulses interacting with the ions. This work reviews several experiments that explore the interaction between the ultrafast laser and single tripped ions including early indication of ultrafast control and entanglement between a single ion and a single photon frequency qubit.
A fault-tolerant quantum computer is expected to require thousands of qubits. Trapped ion architectures provide a modular approach where the quantum register is divided into multiple subregisters connected by physically moving the corresponding ions. Transporting ions at scale comes with several challenges such as the need to connect thousands of control lines to an ion trap chip. Multiplexing the required control voltages from few input signals to multiple electrodes offers a solution to this wiring challenge. Here we demonstrate a quantum processing unit that combines a surface ion trap with a time multiplexer via a sample-and-hold technique that initially charges electrodes to fixed voltages and disconnects them during qubit operations. We characterize the unit's performance by measuring motional heating rates below one phonon per second in both open and closed switch configurations. We further characterize the sample and hold process and find that sampling intervals below 50 ms are sufficient to keep expected gate errors from decaying charges during the hold phase below 10−410^{-4}10−4. Our results indicate that the multiplexing scheme is compatible with high-fidelity operations.