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German Microwave Conference

Analysis of an Integer-N Frequency Synthesizer for Trapped Ion Quantum Computers

Marvin Jaeger, Simon Schulz, Alexander Koelpin

Published Mar 9, 2026• 0 citations
Publisher

Abstract

Qubit implementations like trapped ions can be controlled by microwave signals in the gigahertz range, which require a local oscillator (LO) for upconversion. The LO superimposes a static frequency offset and phase noise, both leading to errors in qubit gate operation. Both error terms can be reduced by implementing the LO as an integer-N frequency synthesizer, where a phase-locked loop controls and stabilizes the frequency. An exemplary integer-N frequency synthesizer is realized and its performance for quantum computers is characterized in this work. The qubit fidelity calculated based on phase noise measurement results is above 99.943% for primitive π-gates.

DOI
10.1109/gemic71240.2026.11516354

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Qubit implementations like trapped ions can be controlled by microwave signals in the gigahertz range, which require a local oscillator (LO) for upconversion. The LO superimposes a static frequency offset and phase noise, both leading to errors in qubit gate operation. Both error terms can be reduced by implementing the LO as an integer-N frequency synthesizer, where a phase-locked loop controls and stabilizes the frequency. An exemplary integer-N frequency synthesizer is realized and its performance for quantum computers is characterized in this work. The qubit fidelity calculated based on phase noise measurement results is above 99.943% for primitive π-gates.
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.
The central challenge of quantum computing is implementing high-fidelity quantum gates at scale. However, many existing approaches to qubit control suffer from a scale-performance trade-off, impeding progress towards the creation of useful devices. Here, we present a vision for an electronically controlled trapped-ion quantum computer that alleviates this bottleneck. Our architecture utilizes shared current-carrying traces and local tuning electrodes in a microfabricated chip to perform quantum gates with low noise and crosstalk regardless of device size. To verify our approach, we experimentally demonstrate low-noise site-selective single- and two-qubit gates in a seven-zone ion trap. We implement electronic single-qubit gates with fidelities ≥ 99.99912 ( 8 ) % and demonstrate consistent performance with low crosstalk across the device. We also electronically generate two-qubit maximally entangled states with 99.97 ( 1 ) % fidelity and long-term stable performance over continuous system operation. These state-of-the-art results highlight the potential of our all-electronic approach for coherent control of trapped-ion qubits in large-scale quantum computers.
High-fidelity single- and multiqubit operations compose the backbone of quantum information processing. This fidelity is based on the ability to couple single- or two-qubit levels in an extremely coherent and precise manner. A necessary condition for coherent quantum evolution is a highly stable local oscillator driving these transitions. Here we study the effect of fast noise, that is, noise at frequencies much higher than the local oscillator linewidth, on the fidelity of one- and two-qubit gates in a trapped-ion system. We analyze and measure the effect of fast noise on single-qubit operations, including resonant $\ensuremath{\pi}$ rotations and off-resonant sideband transitions. We further numerically analyze the effect of fast phase noise on the M\o{}lmer-S\o{}rensen two-qubit gate. We find a unified and simple way to estimate the performance of all of these operations through a single parameter given by the noise power spectral density at the qubit response frequency. While our analysis focuses on phase noise and on trapped-ion systems, it is relevant for other sources of fast noise as well as for other qubit systems in which spinlike qubits are coupled by a common bosonic field. Our analysis can help in guiding the design of quantum hardware platforms and gates, improving their fidelity towards fault-tolerant quantum computing.
This paper proposes a scalable control device architecture for trapped-ion quantum computers based on microwave quantum control. While microwave control of trapped-ion qubits offers high gate fidelity, it requires high-precision and scalable microwave control devices. In the control devices for microwavebased trapped-ion quantum computers, three types of signals need to be managed: (1) wideband microwaves with low phase noise for quantum gate operations, (2) numerous signals with sub-microsecond voltage changes to implement the quantum charge-coupled device (QCCD) architecture, and (3) triggers to synchronize measurement instruments. The proposed architecture combines heterogeneous control devices tailored to these three types of signals to achieve the required precision and scalability at a practical cost. The system features a shared sequencer that can manage all units uniformly, ensuring scalability and reducing development costs. To correctly execute the control sequences, it is necessary to align the operating timing of each unit. In the proposed system, each unit maintains a common time and independently issues events by referring to this time. High-precision time synchronization protocols based on IEEE1588 can be used for time synchronization. In addition to that, multiple control devices can be expanded by combining fine-grained synchronization mechanisms using time synchronization and shared reference signals. This paper details the specifications required for trappedion quantum computers using microwaves and proposes an architecture that meets these specifications.
Extensive research on quantum mechanics in past decades has successfully paved the way towards the disruptive technology of quantum computing. A further step to the realization of a fault-tolerant scalable quantum computer is presenting challenges in the field of engineering. It necessitates a microsystem operating at 4K in a dilution refrigerator, which would be inevitable to control thousands or millions of qubits sitting at 10mK. There have been IC implementations of pulse modulators generating qubit-controlling microwave signals [1– 4]. The pulse modulation based on direct synthesis demonstrated feasibility of driving multiple qubits with frequency-division multiplexing (FDM) through one RF cable [1 – 3]. The first implementation of a full controller [1], which includes both read and write chains for spin qubits, demonstrated the highest level of integration. However, the previous work used external local oscillator (LO) sources for frequency synthesis. External feeding of LO requires precise matching; otherwise, a failure would cause leakage to the qubit driving output through PCB traces, resulting in AC-Stark shift of qubit state by unwanted tones at near-resonance. It becomes more problematic in FDM-based multi-qubit driving. Since the fidelity of qubit operations strongly relies on the quality of driving frequency, a low-noise LO generation by a phase-locked loop (PLL) is one of the most important factors [5]. The internal generation of LO gives additional benefits in superconducting qubit interface. It necessitates multiple LOs since the superconducting qubit uses microwave pulse driving for both read and write. To detect the state of a qubit through dispersive readout, the phase reflected from the readout resonator should be measured. Therefore, the driving pulses for the read and the write should be synchronized to a single reference timing. Using built-in PLLs sharing a single reference clock naturally solves this problem.