Superconducting qubits consisting of epitaxially grown NbN/AlN/NbN Josephson junctions

We demonstrate superconducting qubits using epitaxially grown Josephson junctions. A fully epitaxial NbN/AlN/NbN trilayer on MgO (100) substrate is processed by photolithography and dry-etching into transmon qubits with a large Josephson energy. The tunnel barrier made of cubic-phase AlN, rather than the ordinary hexagonal phase, is the key to avoid piezoelectric coupling to the phonon bath. The energy-relaxation time and the spin-echo decay time of similar to 500 ns are observed in the qubits that are coupled to a monolithically made coplanar waveguide resonator. (C) 2011 American Institute of Physics. [doi:10.1063/1.3663539]