NbN-based superconducting qubit on Si substrate

In the superconducting qubit composed of aluminum-based Josephson junctions (JJs), the decoherence from microscopic two-level systems in amorphous aluminum oxide is concerned. As alternative materials for qubits, fully epitaxial NbN/AlN/NbN JJs are an attractive candidate with the potential to solve the above problems because of its high crystal quality, its chemical stability against oxidization, and relatively high transition temperature (~ 16 K) of NbN . Here we note that AlN is grown in cubic phase to avoid piezoelectricity. Early studies of superconducting qubits using epitaxially grown nitride JJs showed significant potential, but the qubit energy relaxation time was limited due to dielectric loss from the MgO substrate. [Y. Nakamura et al., APL, 99, 212502 (2011)]. In order to solve this problem, we have employed a Si substrate with TiN buffer layer for the epitaxial growth of this nitride JJs [K. Makise et al., IEEE Trans. Appl. Sup., 26, 1100403, (2016)] and fabricated a transmon qubit with a λ/2 coplanar waveguide resonator. We observed clear Rabi oscillations and Ramsey interference patterns. We will discuss the detailed coherence characteristics.