Efficient qubit quality factor characterization using broadband SWAP spectroscopy

Realizing high-coherence qubits is essential for building large-scale, fault-tolerant quantum computers with superconducting devices. It is common in the literature to report qubit energy relaxation (T1) times using median or best-case values from repeated measurements. However, this approach does not fairly sample the T1 distribution due to the time dynamics of two-level systems (TLS) defects at cryogenic temperatures. As such, repeated measurements on fixed-frequency qubits, or over a narrow frequency band, provides an estimate of the distribution that is not representative over multiple experimental realizations. Here we show that scaling up the bandwidth of SWAP spectroscopy in frequency-tunable qubits can efficiently sample the qubit quality factor (Q) distribution. Our analysis compares Q distributions of identical qubit designs on single dies and across different dies from the same wafer over multiple measurement dates. The results reveal that Q distributions for qubits within single dies remain consistent over time, while showing significant variations between different dies. These findings demonstrate that this sampling method enables more efficient and accurate comparisons of qubit fabrication quality compared to repeated samplings.