  We demonstrate a planar, tunable superconducting qubit with energy relaxation
times up to 44 microseconds. 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. This is supported
by a model analysis as well as experimental variations in the geometry. Our
`Xmon' qubit combines facile fabrication, straightforward connectivity, fast
control, and long coherence, opening a viable route to constructing a
chip-based quantum computer.
