Capacitively shunted flux qubit based on epitaxially grown NbN/AlN/NbN Josephson junctions on Si substrate*

In superconducting qubits composed of aluminum-based Josephson junctions (JJs), the decoherence from microscopic two-level systems in amorphous aluminum oxide has long been a concern. As an alternative material for the 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, chemical stability against oxidization, and relatively high transition temperature (~ 16 K) of NbN. Early studies of superconducting qubits using epitaxially grown nitride JJs have shown significant potential, but their coherence time was limited due to dielectric loss from the MgO substrate [1]. To improve this, we have employed a Si substrate with a TiN buffer layer for the epitaxial growth of this nitride JJs [2] and fabricated a capacitively shunted flux qubit. For the dispersive readout, the qubit is coupled to a half-wavelength coplanar waveguide resonator. We characterize the qubit relaxation time and dephasing time using spin-echo in the tens of microseconds range.
[1] Y. Nakamura et al., Appl. Phys. Lett., 99, 212502 (2011).
[2] K. Makise et al., IEEE Trans. Appl. Sup., 26, 1100403, (2016).