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IEEE MTT-S International Microwave Symposium digest

Predicting the Fidelity of Multiplexed Superconducting Qubit Readout with Multiphysics Numerical Methods

Samuel T. Elkin, Michael Haider, Thomas E. Roth

Published Jun 15, 2025• 3 citations
Publisher

Abstract

Josephson traveling-wave parametric amplifiers are a critical component for enabling multiplexed qubit readout. However, these devices introduce nonideal effects which make optimizing the readout circuitry highly challenging. Here, we introduce an approach to model a readout chain which incorporates these effects to calculate readout fidelity, and demonstrate how it can be used to evaluate trade-offs between competing factors in a readout chain design.

DOI
10.1109/ims40360.2025.11103830

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Josephson traveling-wave parametric amplifiers are a critical component for enabling multiplexed qubit readout. However, these devices introduce nonideal effects which make optimizing the readout circuitry highly challenging. Here, we introduce an approach to model a readout chain which incorporates these effects to calculate readout fidelity, and demonstrate how it can be used to evaluate trade-offs between competing factors in a readout chain design.
Superconducting qubits require rapid and high-fidelity multiplexed readout to enable effective quantum computers. However, predicting the fidelity of the physical implementation of a readout chain is challenging, limiting the success of experimental devices. While analytical tools are currently used to support these design efforts, mapping their outputs into a physical design is difficult and prone to error. In this work, we use multiphysics numerical methods to model the effect of individual component values on fidelity, which can more easily guide the design of a physical device geometry. This procedure is utilized here to optimize a pair of key capacitance values, but it is applicable to broader parameterization of a readout chain.
Superconducting qubits require rapid and high-fidelity multiplexed readout to enable effective quantum computers. However, predicting the fidelity of the physical implementation of a readout chain is challenging, limiting the success of experimental devices. While analytical tools are currently used to support these design efforts, mapping their outputs into a physical design is difficult and prone to error. In this work, we use multiphysics numerical methods to model the effect of individual component values on fidelity, which can more easily guide the design of a physical device geometry. This procedure is utilized here to optimize a pair of key capacitance values, but it is applicable to broader parameterization of a readout chain.
Josephson traveling wave parametric amplifiers are a leading tool for enabling readout of superconducting qubits. However, existing design tools for these devices struggle to effectively model variations in amplifier parameters, leading to inconsistencies between theoretical predictions and manufactured devices. This work develops a 1D hybrid numerical modeling approach for these devices that can easily incorporate amplifier property variations, allowing their impact to be considered in the design process. Here, the formulation of the method is introduced and validated against analytical results for a simple architecture. Future work will extend the technique to more advanced amplifier architectures, as well as develop similar methods for 3D full-wave modeling of these systems.
Superconducting quantum computing hardware relies on classical radio-frequency (RF) electronic circuits to control and measure quantum states. As the complexity of quantum computers increases, the significance of the classical circuitry component grows, necessitating the development of accurate models for its design and optimization. This study presents an analytical model that quantifies the influence of circuit nonidealities on readout fidelity in superconducting quantum computing systems. This model considers a wide range of nonidealities typically encountered in the readout chain, encompassing frequency, amplitude, phase inaccuracies, impedance mismatch, quantum noise, and amplifier noise. It further predicts the combined impact of these nonidealities on fidelity. The model’s precision and efficacy are confirmed through numerical quantum-classical co-simulation. The highest observed relative mismatch is roughly 3.5%, but most errors remain below 1%. Such a model can significantly aid in designing and optimizing practical quantum computers.
Josephson traveling-wave parametric amplifiers (JTWPAs) are wideband, ultralow-noise amplifiers used to enable the readout of superconducting qubits. While individual JTWPAs have achieved high performance, behavior between devices is inconsistent due to wide manufacturing tolerances. Amplifier designs could be modified to improve resilience towards variations in amplifier components; however, existing device models often rely on analytical techniques that typically fail to incorporate component variations. To begin addressing this issue, a 1D numerical method for modeling JTWPAs is introduced in this work. The method treats the Josephson junctions and transmission lines in an amplifier as coupled subsystems and can easily incorporate arbitrary parameter variations. We discretize the transmission line subsystem with a finite element time domain method and the Josephson junction subsystem with a finite difference method, with leap-frog time marching used to evolve the system in time. We validate our method by comparing the computed gain to an analytical model for a traditional JTWPA architecture and one with resonant phase matching. We then use our method to demonstrate the impact of variations in Josephson junctions and phase-matching resonators on amplification. In future work, the method will be adjusted to incorporate additional amplifier architectures and extended to a 3D full-wave approach.