Superconducting Qubit Control Limits Explained by Microwave Drive Transitions

Superconducting circuits represent a promising architecture for realising practical quantum computation, yet their susceptibility to errors induced by control signals remains a significant challenge. Increasing the power of microwave signals, essential for manipulating and reading the state of these quantum bits or qubits, inevitably triggers unwanted transitions between quantum states, thereby limiting the precision and speed of computations. Researchers at Yale University, led by W. Dai, S. Hazra, D. K. Weiss, and including P. D. Kurilovich, T. Connolly, H. K. Babla, S. Singh, V. R. Joshi, A. Z. Ding, P. D. Parakh, J. Venkatraman, X. Xiao, L. Frunzio, and M. H. Devoret, systematically investigate these detrimental transitions in a fixed-frequency qubit. Their work, entitled ‘Spectroscopy of drive-induced unwanted state transitions in superconducting circuits’, details a classification of these transitions into three distinct categories: resonant energy exchange with parasitic two-level systems, multi-photon transitions within the circuit itself, and inelastic scattering processes transferring energy to spurious electromagnetic modes or material defects known as two-level systems (TLS). Through a combination of Floquet steady-state simulations and electromagnetic modelling, the team accurately predicts these transitions, offering strategies for mitigating them through optimised drive frequencies and improved circuit design.

Superconducting qubits represent a leading platform for realising quantum computation, yet their performance is constrained by unwanted state transitions that limit operational speed and fidelity. Researchers systematically analyse these transitions as drive strength increases, identifying and categorising them into three distinct mechanisms to provide a comprehensive understanding of their physical origins and inform improved qubit designs and control protocols.

The study demonstrates that resonant energy exchange with parasitic two-level systems (TLS) becomes significant when drive-induced ac-Stark shifts match the energy separation of these systems. TLS, originating from material defects and amorphous regions within the superconducting circuit, represent a common source of decoherence, actively degrading qubit performance. These defects create localised energy levels that can interact with the qubit, causing unwanted transitions. Furthermore, the research identifies multi-photon transitions to unintended qubit states as a contributing factor, arising directly from the circuit’s inherent Hamiltonian, the mathematical description of its energy landscape, and limiting achievable control fidelity.

A third category of unwanted transitions involves inelastic scattering processes, where the drive initiates a qubit state transition while simultaneously transferring excess energy to spurious electromagnetic modes or additional TLS defects. Researchers successfully predict transitions not involving TLS using Floquet steady-state simulations, a computational technique for analysing the behaviour of quantum systems under periodic driving forces, complemented by electromagnetic modelling of the physical device. This predictive capability offers a pathway for mitigating these effects through informed choices of drive frequency and improved circuit design, ultimately enhancing qubit coherence and reliability.

To accurately predict these transitions, researchers employ both Floquet steady-state simulations and electromagnetic modelling. The simulations, based on solving the time-dependent Schrödinger equation under periodic driving, capture the quantum dynamics of the qubit. Complementary electromagnetic modelling, which simulates the propagation of microwave fields within the device, provides insights into the spurious modes that contribute to inelastic scattering. This combined approach proves particularly effective at predicting transitions not involving TLS, which are notoriously difficult to model due to their complex and often poorly characterised nature.

Researchers meticulously analyse microwave-driven superconducting qubits, identifying transitions that limit operational speed and fidelity, and systematically categorise these unwanted state changes across a 9 GHz frequency range. They pinpoint three distinct origins for these transitions: resonant energy exchange with parasitic two-level systems, multi-photon transitions to non-ideal states within the circuit, and inelastic scattering processes transferring energy to spurious electromagnetic modes or material defects. They successfully model these transitions using Floquet steady-state simulations, enhancing their understanding with electromagnetic simulations of the physical device, and validate their analytical framework with accurate predictions of qubit behaviour.

Researchers demonstrate a comprehensive classification of these transitions, offering practical mitigation strategies for optimising qubit control and enhancing the performance of superconducting quantum circuits. They advocate for informed choices of drive frequency and improved circuit design to minimise these detrimental effects. This work provides a foundation for optimising control and readout operations in superconducting quantum circuits, and highlights the importance of both accurate theoretical modelling and careful consideration of material properties in the pursuit of robust and scalable quantum computing. These findings are crucial for advancing the field and achieving higher performance in superconducting qubit systems.