Investigating saturable two-level-systems in Josephson junction array based fluxoniums using a Szilard engine

With the ongoing advancements in the lifetime and coherence of superconducting qubits, recent research has identified saturable two-level systems as a major sources of decoherence and energy loss. Located on surfaces or embedded within Josephson junctions they impose fundamental limitations on the performance of qubits. Recent experiments with granular aluminum based fluxonium qubits have identified long-lived two-level systems as the main limitation for qubit coherence [1]. For systematically improving superconducting qubits it is crucial to understand the origin, coupling mechanism and frequency distribution of theses defects. Here, we investigate two-level systems of a fluxonium qubit based on Josephson junction arrays by implementing a quantum Szilard engine. Through the analysis of quantum-jump traces, we monitor the qubit dynamics and extract the intrinsic lifetimes and the number of two-level systems. Additionally, we investigate the coupling of two-level systems to two different isolated qubits on the same substrate, analyzing potential correlated errors. Moreover, our architecture allows for fast flux-pulsing the qubit to investigate the defects at different frequencies. These findings open up a broader understanding of two-level system dynamics to develop mitigation strategies for further enhanced lifetime and coherence of superconducting qubits.



[1] Spiecker, M. et al. Two-level system hyperpolarization using a quantum Szilard engine. Nat. Phys. 19, 1320–1325 (2023)