Systematic study of flux noise in transmon qubits

Flux-tunable superconducting qubits enable fast and flexible control in quantum processors; however, magnetic flux noise remains a major source of dephasing. We present a systematic characterization of flux noise in transmon qubits with varying junction-lead geometries (aspect ratios P/W = 5:1–200:1). Using Ramsey and spin-echo interferometry together with direct power spectral density measurements, we extract flux noise amplitudes approaching 1 μΦ₀/√Hz at 1 Hz. The measurements reveal geometric scaling, broadly consistent with surface-spin models. Importantly, we demonstrate that low flux noise can be achieved without compromising energy relaxation times (T₁ ≈ 100–150 μs), showing that high coherence and flux tunability can coexist in optimized designs. These results provide quantitative design guidelines for engineering geometry-dependent flux noise in superconducting qubits.