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German Microwave Conference

Design of a Microwave Frontend for a Trapped Ion Quantum Computer

Marvin Jaeger, Georg Frederik Riemschneider, Nico Weiß, Bartosz Tegowski, Alexander Koelpin

Published Mar 17, 2025• 3 citations
Publisher

Abstract

Unlike classical computers, which represent information as binary ‘1’ and ‘0’, quantum computers use qubits that exist in a superposition of both, enabling parallel computation. This capability allows them to solve special kinds of problems faster than classical computers. To process information, qubits are manipulated by applying energy, such as via a microwave signal, which is provided by a microwave frontend that ensures an appropriate frequency spectrum and power level. This paper defines the key parameters and implements such a frontend for trapped ion quantum computers which follow the magnetic gradient induced coupling (MAGIC) principle. The frontend is manufactured on a printed circuit board and measurements prove its functionality. It achieves an idle gate fidelity of 99.9% (99.99%) at an output power of 6.32 dBm (2.4 dBm).

DOI
10.23919/gemic64734.2025.10979039

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Unlike classical computers, which represent information as binary ‘1’ and ‘0’, quantum computers use qubits that exist in a superposition of both, enabling parallel computation. This capability allows them to solve special kinds of problems faster than classical computers. To process information, qubits are manipulated by applying energy, such as via a microwave signal, which is provided by a microwave frontend that ensures an appropriate frequency spectrum and power level. This paper defines the key parameters and implements such a frontend for trapped ion quantum computers which follow the magnetic gradient induced coupling (MAGIC) principle. The frontend is manufactured on a printed circuit board and measurements prove its functionality. It achieves an idle gate fidelity of 99.9% (99.99%) at an output power of 6.32 dBm (2.4 dBm).
This paper presents an analog microwave frontend designed to downconvert and feed back the control signal for trapped ion qubits, which are frequency-multiplexed by the magnetic gradient induced coupling (MAGIC) principle. Shorter gate durations require a higher excitation signal power, increasing distortion caused by nonlinear components. The proposed feedback receiver serves the purpose of coupling out a portion of the generated qubit control signal to feed it back into the signal generation hardware. This enables closed-loop signal processing techniques to compensate for nonlinearities in signal generation, thus improving the idle fidelity of untargeted qubits. The characterization of a manufactured prototype is performed, followed by an active cancellation of spurious tones based on the downconverted qubit control signal to demonstrate the capability for qubit fidelity improvement.
For quantum computers to become a reality, high-fidelity logic gates are required to be implemented on a scalable architecture. The IQT group follows an approach based on using a magnetic field gradient and microwave radiation to implement the required operations. To demonstrate their scalability to many quantum bit systems, these operations need to be implemented on micro-fabricated surface ion traps. In this thesis I describe the technologies I developed for surface ion traps with magnetic field gradients. This entailed the design, assembly and operation of two surface ion trap systems with permanent magnets for generating large magnetic field gradients of 140T=m, novel in-vacuum radio-frequency and microwave emitters for implementing high-speed quantum state manipulation and development of robust and reliable laser control systems required for stable long-term experimental operation.
This thesis describes the experimental work towards the development of a scalable quantum computer based on microfabricated ion trap using long-wavelength radiation and magnetic field gradient. There are three key elements in implementing such a quantum computer: the junction trap to shuttle ions, the structure to generate high gradient magnetic field and the structure to induce strong microwave coupling to the ions. A new dynamic simulation tool was developed addressing the problems faced by static solvers. This tool was used to aid the design and optimisation of an X junction geometry allowing the ions to be shuttled with minimised motional heating gain. The design and fabrication techniques were reported on the structure to generate a high gradient magnetic field. A discussion was given on the design of producing microwave and maximise the coupling to the cold ions. A review was given on the far-field methods and near-field methods. A novel design was reported where the single-qubit gate is predicted to be 45 times faster than a conventional setup. Two essential topics on the microfabrication of a reliable scalable quantum computer unit were discussed: breakdown and RF loss. Investigation using numerical simulations showed that dielectric can breakdown due to high voltage and local heating which are results of impedance mismatch. Microfabrication processes were improved, high-quality films were reported to have twice as much breakdown voltages. The mechanisms of RF loss were reviewed. Novel structures and smooth electroplating technique were developed to minimise the loss. A low loss ion trap was produced and tested. An experimentally observed anomalous glow discharge phenomenon was reported and investigated.
Trapped ions, together with superconducting qubits, are one of the two leading hardware platforms for scalable quantum information processing. The development of quantum computers represents a major technological breakthrough comparable to the introduction of classical computing. The benefits of this technology are currently limited by the technical capability to perform high fidelity entangling operations on the qubits. When gate fidelities surpass the fault-tolerance threshold it becomes possible, through error correction, to increase the system size to an arbitrary number of qubits. In this cumulative thesis we address some of the issues in the scalability of the trapped-ion quantum computer based on microwave near-fields. In this approach, gate operations on one or multiple ions are driven by an oscillating magnetic field generated by a current flowing through a conductor. In the first part of this work we discuss the design of traps toward the implementation of large scale systems. We introduce the basic design of a surface-electrode ion trap with embedded microwave conductors. The oscillating magnetic field required to perform the operations is generated by a single optimized conductor. We discuss the simulation and characterization of the magnetic field pattern, which is fixed by the microwave conductor design. In addition, we demonstrate the capability to simulate, fabricate and characterize a multilayer surface ion trap. Multilayer traps are a key aspect for scalability since they are necessary to achieve large system sizes, with many `ion registers', where the registers are interconnected by physically transporting ions between them. In the second part of this thesis we demonstrate the implementation of a two-qubit entangling gate and we explore the possibilities offered by quantum control methods to improve its fidelity. We perform an entangling gate on two 9Be+ ions and measure a Bell state fidelity of 98.2(1.2)%. Error characterization shows that the gate result is limited by technical issues connected to the ions' motional states. To reduce these errors we apply amplitude modulation of the gate microwave drive. After stabilization of the ions' radial modes, we obtain an amplitude modulated gate with infidelity in the 10^-3 range. The result is confirmed by analyzing the data using three different methods. Using additional dynamic decoupling techniques, these results could bring microwave near-field gates past the fault-tolerance threshold.
Scalability is a challenging yet key aspect required for large scale quantum computing and simulation using ions trapped in radio-frequency (rf) Paul traps. In this thesis 171Yb+ ions are used to demonstrate a magnetic field insensitive qubit which has a measured coherence time of 1.5 s, making it an ideal candidate to use for storing quantum information. A magnetic field sensitive qubit is also characterised which can be used for the implementation of multi-qubit gates using a potentially very scalable scheme based on microwaves in conjunction with a static magnetic field gradient instead of using lasers. However, the measured coherence time is limited by magnetic field fluctuations and will prohibit high fidelity gate operations from being performed. To address this issue, the preparation of a dressed-state qubit using a microwave based stimulated rapid adiabatic passage (STIRAP) pulse sequence will be presented. This qubit is protected against the noisy environment making it less sensitive to magnetic field fluctuations. The lifetime of this qubit is measured to demonstrate its suitability for storing quantum information. A powerful method for manipulating the dressed-state qubit will be presented and is used to measure a coherence time of the qubit of 500 ms which is two orders of magnitude longer compared to the magnetic field sensitive qubit. It will also be shown that our method allows for the implementation of arbitrary rotations of the dressed-state qubit on the Bloch sphere using only a single rf field. This substantially simplifies the experimental setup for single and multi-qubit gates. Furthermore, this thesis will present a experimental setup capable of successfully operating microfabricated surface ion traps. This setup is then used to operate and characterise the first two-dimensional (2D) lattice of ion traps on a microchip. A unique feature of the microfabrication technique used for this device is the extremely large voltage that can be applied which allows long ion lifetimes along with large secular frequencies to be measured, demonstrating the robustness of this device. Rudimentary shuttling between neighbouring lattice sites will be shown which could be used as part of a efficient scheme to load a large lattice of ions. One of the many applications of a 2D lattice of ions lies in the field of quantum simulations where many-body systems such as quantum magnetism, high temperature superconductivity, the fractional quantum hall effect and synthetic gauge fields can be simulated. It will be shown how making only minor modifications to the microchip the ion-ion separation can be reduced sufficiently to offer an exciting platform for the successful implementation of 2D quantum simulations. A theoretical investigation on the optimal 2D ion trap lattice geometry will also be presented with the aim to maximise the ratio of ion-ion coupling strength to decoherence from motional heating of the ions and to laser induced off-resonant coupling.
This thesis describes the development of technology and experimental techniques for scalable trapped-ion quantum computing. An architecture for scalable quantum computing is presented in which 171Yb+ ions are trapped in linear R.F. Paul traps and their hyper-fine states are used to form qubits. Quantum logical operations can be carried out by applying R.F. and microwave fields. This thesis presents significant improvements in the experimental setup used for quantum computing, as well as new theoretical developments and experimental demonstrations of new high- fidelity quantum control methods. A new procedure for carrying out high- delity two qubit gates using static magnetic eld gradients and resonant microwave elds is presented. It is shown that this has the potential to enable signi cantly improved delities compared to current methods. Improved methods for measurement and statistical analysis in trapped-ion experiments are also shown. These should enable more accurate measurement of two-qubit gate delities, which becomes increasingly important as delities increase. A new system for generating arbitrary R.F. and microwave waveforms is presented, including a bespoke software control system allowing the user to de ne custom pulse sequences, which represents a signi cant improvement in the scalability of the quantum control system. Experimental developments towards a system for carrying out on-chip trapped-ion quantum logic in a static magnetic eld gradient are also presented. A new technique for generating multi-level control methods is introduced, based on reducing a multi-level system to an e ective two-level system. High- delity quantum control methods generated using this technique, which form a will form a key part of the scalable quantum computing architecture, are implemented experimentally.