Long-lived, radiation-suppressed superconducting quantum bit in a planar geometry

We present a superconducting qubit design that is fabricated in a 2D geometry over a super-conducting ground plane to enhance the lifetime. The qubit is coupled to a microstrip resonator for readout. The circuit is fabricated on a silicon substrate using low loss, stoichiometric titanium nitride for capacitor pads and small, shadow-evaporated aluminum/aluminum-oxide junctions. We observe qubit relaxation and coherence times (T$_{\mathrm{1}}$ and T$_{\mathrm{2}})$ of 11.7 $\pm $ 0.2 $\mu $s and 8.7 $\pm $ 0.3 $\mu $s, respectively. Calculations show that the proximity of the superconducting plane suppresses the otherwise high radiation loss of the qubit. A significant increase in T$_{\mathrm{1}}$ is projected for a reduced qubit-to-superconducting plane separation.