Terahertz nanoimaging of sidewall-induced losses in superconducting transmon qubits

Correlating superconducting qubit performance with advanced materials analysis is a key strategy for improving coherence. Existing diagnostics for key properties, such as dielectric loss, structural discontinuity, and interface heterogeneity, often rely on destructive electron microscopy or low-throughput millikelvin measurements. Here, we demonstrate noninvasive terahertz nanoimaging and nanospectroscopy of encapsulated niobium transmon qubits as a high-throughput proxy for performance evaluation. We identify large variations in sidewall near-field signals, implicating sidewall loss and discontinuity as major coherence limiters. We also use terahertz hyperspectral line scans to probe dielectric responses and field participation at aluminum junction interfaces.