Capacitively Shunted Flux Qubit with High Anharmonicity and Long Coherence Times*

We present experiments on a three Josephson junction flux qubit with all junctions shunted by large capacitive pads. The qubit is capacitively coupled to a coplanar waveguide resonator for dispersive readout and capacitively coupled to a drive line for control. The qubit energy level structure has high anharmonicity, with the ratio of the second to the first transition frequency larger than 3. The dependence of the first two transition frequencies on flux is in excellent agreement with numerical simulations. Close to the symmetry point, the qubit has long relaxation time, in the tens of microseconds range. We characterize qubit dephasing using spin-echo and dynamical decoupling and we analyze the flux dependence of dephasing times to extract the flux noise power spectral density. In addition, we present measurement of multi-level coherence. Finally, we discuss the characterization of single-qubit gates using randomized benchmarking.