Coherence properties of a capacitively-shunt flux qubit

Coherence times for typical flux qubits have plateaued at $5-10 ~\mu$s for $T_1$ and $1-3 ~\mu$s for $T_{Ramsey}$. To achieve longer coherence times we study capacitively-shunted flux qubits using high-Q capacitors to individually shunt all four Josephson junctions (JJs). The additional shunt capacitance moves $90+\%$ of the qubit energy from the lossy capacitance of the JJs into the high-Q shunts while preserving an anharmonicity greater than $100\%$ and maintaining $f_{01} < f_{12}$. The band structure is also flattened providing moderately decreased sensitivity to flux noise. Using high-quality MBE aluminum [1] we fabricate a capacitively-shunted flux qubit inductively coupled to a lumped-element readout resonator. The qubit junctions are deposited via aluminum e-beam evaporation using a bridgeless mask. We characterize the influence of qubit design parameters such as capacitance and geometry on the coherence time of the device. \newline [1] Megrant, \textit{et al}. APL \textbf{100}, 113510 (2012)