We demonstrate a planar, tunable superconducting qubit with energy relaxation times up to $44\text{ }\text{ }\ensuremath{\mu}\mathrm{s}$. This is achieved by using a geometry designed to both minimize radiative loss and reduce coupling to materials-related defects. At these levels of coherence, we find a fine structure in the qubit energy lifetime as a function of frequency, indicating the presence of a sparse population of incoherent, weakly coupled two-level defects. We elucidate this defect physics by experimentally varying the geometry and by a model analysis. Our ``Xmon'' qubit combines facile fabrication, straightforward connectivity, fast control, and long coherence, opening a viable route to constructing a chip-based quantum computer.

Coherent Josephson Qubit Suitable for Scalable Quantum Integrated Circuits