All-Nitride Superconducting Qubit with Epitaxial Josephson Junctions on Si Substrate

Abstract

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 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. To improve this, we have employed a Si substrate with TiN buffer layer for the epitaxial growth of the nitride JJs and fabricated an all-nitride capacitively-shunted flux qubit coupled to a half-wavelength coplanar waveguide resonator. As the results, this nitride qubit has demonstrated a significant improvement in coherence times, such as T₁=16.3 μs and T₂=21.5 μs as the mean values of a hundred measurements, which are more than an order of magnitude longer than those reported in the literature using MgO substrates. These results are an important step towards constructing a new platform for superconducting quantum hardware.