Role of tunnel barrier crystallinity in the coherence properties of superconducting phase qubits

The standard amorphous AlO$_{x}$ tunnel barriers in superconducting qubits contain many nanoscopic fluctuators. These nanoscopic fluctuators destroy the coherent quantum information stored in the qubit. Recently, we eliminated 80{\%} of these fluctuators using single-crystal Al$_{2}$O$_{3}$ tunnel barriers. This clearly shows that the tunnel barrier crystallinity is important for coherent superconducting qubits. Along this line, we started investigating another well-known crystalline tunnel barrier, MgO. Unlike the aluminum oxide tunnel barrier, which requires $\sim $800 \r{ }C for crystallization, the MgO tunnel barrier grows crystalline even at room temperature. We will compare the Josephson junctions and the superconducting phase qubits made out of amorphous AlO$_{x}$, single-crytal Al$_{2}$O$_{3}$ and single-crystal MgO tunnel barriers, and discuss the effect of barrier crystallinity and electrode/tunnel-barrier interface quality on the performance of the coherent quantum-devices.