Robust quality factor assessment of high-coherence superconducting qubits

Large variations in the energy relaxation time (T 1 ) of superconducting qubits make it difficult to accurately evaluate and compare new qubit materials and fabrication processes, or to perform studies that require precise measurements of energy loss. To address this issue, we present techniques for characterizing qubit quality factors by applying electric fields to TLS in the vicinity of qubits. Introducing low-frequency (<1 Hz) AC fields allows us to stabilize the measured T 1 by averaging over the accessible TLS configurations, producing a robust estimate of T 1 that is difficult to replicate with hundreds of measurements over long periods, without applied fields. In a complementary technique, we apply a randomly selected DC field to the qubit and measure T 1 . Repeated ‘fast-random’ measurements reveal a distribution of T 1 values whose harmonic mean is consistent with that obtained through AC measurements but illustrates a wider range of qubit lifetimes induced by TLS interactions. We implement these TLS control techniques in various ways, including demonstrating a simple T 1 improvement protocol and precise measurements of T 1 vs temperature. These techniques will facilitate our understanding of how to improve qubit coherence by enabling better measurements in shorter times or with fewer devices.