Alternative materials for the Josephson junctions (JJs) of the qubits have been less studied, even though it has been pointed out that the coherence times of superconducting quantum circuits made from conventional aluminum-based JJs are limited by energy or phase relaxation due to microscopic two-level systems (TLS) in the amorphous aluminum oxide. In this talk, we introduce superconducting qubits based on fully epitaxial nitride JJs consisting of NbN/AlN/NbN tri-layer. Such epitaxially grown nitride JJs have great potential to solve material-related concerns, including the TLS problem, owing to their high crystal quality and chemical stability against oxidation. Moreover, due to the larger superconducting gap (2Δ ~ 5.2 meV) compared to ~0.3 meV for aluminum and relatively high transition temperature (~ 16 K) of NbN, quasiparticle excitation can be suppressed.

Figure 1. All-nitride superconducting quantum circuits on silicon substrate

We report our recent works on the development of all-nitride superconducting qubit on silicon substrate [1]. As shown in figure 1, we have fabricated an all-nitride C-shunt flux qubit coupled to half-wavelength coplanar waveguide resonator by utilizing the growth technique of a full-epitaxial NbN/AlN/NbN tri-layer, conventional photolithography with i-line stepper, reactive ion etching, and chemical mechanical polishing. Also, by employing a Si substrate instead of a conventional MgO substrate for epitaxial growth of NbN film, our nitride flux qubit has demonstrated a significant improvement in coherence times, such as T₁=16.3 μs and T₂= 21.5 μs as the mean values measured hundred times, which are more than an order of magnitude longer than those reported in the literature using MgO substrates [2]. We explain that this improvement in coherence times is attributed mainly to the reduced dielectric loss when replacing the MgO substrate with the Si substrate. These results shed light on the role of alternative material technologies in improving superconducting quantum circuits.

This work was supported by Japan Science and Technology Agency Core Research for Evolutionary Science and Technology (Grant No. JPMJCR1775), JSPS KAKENHI (JP19H05615), JST ERATO (JPMJER1601) and partially by MEXT Quantum Leap Flagship Programs (JPMXS0120319794 and JPMXS0118068682). T.Y. acknowledges the Program for Promoting the Enhancement of Research Universities, Nagoya University.

References
[1] S. Kim et al., “Enhanced coherence of all-nitride superconducting qubits epitaxially grown on silicon substrate”, Commun. Mater. 2, 98 (2021). https://doi.org/10.1038/s43246-021-00204-4
[2] Y. Nakamura et al., “Superconducting qubits consisting of epitaxially grown NbN/AlN/NbN Josephson junctions”, Appl. Phys. Lett. 99, 212502 (2011).

Keywords: superconducting qubit, epitaxial Josephson junction, Nitride superconductor