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German Microwave Conference

Feedback-Receiver-Based Compensation of Nonlinearities in Trapped Ion Qubit Gate Signaling

Moritz Baecker, Marvin Jaeger, Bartosz Tegowski, Georg Frederik Riemschneider, Alexander Koelpin

Published Mar 9, 2026• 0 citations
Publisher

Abstract

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.

DOI
10.1109/gemic71240.2026.11516472

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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.
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).
High-fidelity quantum gates require precise classical control signals, yet the analog hardware delivering these signals introduces nonlinear distortions that degrade gate performance. We demonstrate digital predistortion of an acousto-optic modulator used to generate multi-tone entangling-gate waveforms in a trapped-ion processor based on 88^{88}88Sr+^++. By measuring and inverting the static nonlinear amplitude response of the modulator, we apply a feed-forward correction that extends its linear operating range and suppresses spurious intermodulation products. Spectral analysis of the gate beam shows 3--5 dB suppression of the dominant intermodulation tones, approximately doubling the usable diffraction efficiency at a 10−310^{-3}10−3 estimated gate-error threshold. Direct two-qubit Bell-state fidelity measurements confirm that predistortion consistently improves entangling-gate performance. The calibrate-and-invert methodology is device and platform agnostic, applicable to any nonlinear element in the classical control chain of a quantum processor.
Power amplifiers (PA) are essential for microwavecontrolled trapped-ion and semiconductor spin based quantum computers (QC). They adjust the power level of the control signal and therefore the processing time of the QC. Their nonlinearities and memory effects degrade the signal quality and, thus, the fidelity of qubit gate operations. Driving the PA with a significant input power back-off reduces nonlinear effects but is neither power-efficient nor cost-effective. To overcome this limitation, this letter augments the conventional signal generation system applied in QCs by digital predistortion (DPD) to linearize the radio frequency (RF) channel. Numerical analysis of the qubit behavior based on measured representative control signals indicates that DPD improves its fidelity.
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.
Power amplifiers (PA) are essential for microwave-controlled trapped-ion and semiconductor spin based quantum computers (QC). They adjust the power level of the control signal and therefore the processing time of the QC. Their nonlinearities and memory effects degrade the signal quality and, thus, the fidelity of qubit gate operations. Driving the PA with a significant input power back-off reduces nonlinear effects but is neither power-efficient nor cost-effective. To overcome this limitation, this letter augments the conventional signal generation system applied in QCs by digital predistortion (DPD) to linearize the radio frequency (RF) channel. Numerical analysis of the qubit behavior based on measured representative control signals indicates that DPD improves its fidelity.
The increasing number of control lines connecting quantum processors to external electronics constitutes a major bottleneck in the realization of large-scale quantum computers. Frequency-division multiplexing is expected to enable control of multiple qubits through a single microwave cable; however, interference from off-resonant microwave tones hinders precise qubit control. Here, we propose an active interference suppression method for frequency-division-multiplexed simultaneous gates on microwave-controlled qubits. We demonstrate that the deliberate incorporation of off-resonant microwave tones improves single-qubit gate fidelity. In particular, the gate infidelity scales inversely with the square of the number of microwave tones when off-resonant orthogonal or quasi-orthogonal tones are incorporated. Furthermore, we show that fast oscillations, neglected under the rotating wave approximation, degrade the gate fidelity, and that this degradation can be mitigated through optimized frequency allocation. The proposed approach is simple and effective for improving the performance of frequency-division-multiplexed quantum gates.