  We present an experimental realization of the transmon qubit, an improved
superconducting charge qubit derived from the Cooper pair box. We
experimentally verify the predicted exponential suppression of sensitivity to
1/f charge noise [J. Koch et al., Phys. Rev. A 76, 042319 (2007)]. This removes
the leading source of dephasing in charge qubits, resulting in homogenously
broadened transitions with relaxation and dephasing times in the microsecond
range. Our systematic characterization of the qubit spectrum, anharmonicity,
and charge dispersion shows excellent agreement with theory, rendering the
transmon a promising qubit for future steps towards solid-state quantum
information processing.
