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 $\sim 500$~ns are observed in the qubits that are coupled to a monolithically-made coplanar waveguide resonator.