Dependence of Transmon Coherence on Geometry and Interface Treatment

Superconducting qubits are a leading candidate for realizing a large-scale, fault-tolerant quantum computer. For continued progress, it is necessary to understand and mitigate qubit decoherence, which is largely dominated by lossy amorphous interfaces. Here, we examine the effect of qubit geometry and surface preparation on qubit energy relaxation time. Working with both single-ended and differential transmon qubits, we vary the separation between electrodes forming the qubit shunt capacitance over an order of magnitude in order to change the participation ratio of the various interfaces. Additionally, we study the effect of surface treatments both prior to and following Josephson junction formation. Finally, we examine the dependence of qubit energy relaxation and dephasing from 1/f flux noise on the geometry of the qubit junction leads.