Enhanced coherence of all-nitride superconducting qubits epitaxially grown on silicon substrate

Abstract Improving the coherence of superconducting qubits is a fundamental step towards the realization of fault-tolerant quantum computation. However, coherence times of quantum circuits made from conventional aluminum-based Josephson junctions are limited by the presence of microscopic two-level systems in the amorphous aluminum oxide tunnel barriers. Here, we have developed superconducting qubits based on NbN/AlN/NbN epitaxial Josephson junctions on silicon substrates which promise to overcome the drawbacks of qubits based on Al/AlOx/Al junctions. The all-nitride qubits have great advantages such as chemical stability against oxidation, resulting in fewer two-level fluctuators, feasibility for epitaxial tunnel barriers that reduce energy relaxation and dephasing, and a larger superconducting gap of ~5. 2 meV for NbN, compared to ~0. 3 meV for aluminum, which suppresses the excitation of quasiparticles. By replacing conventional MgO by a silicon substrate with a TiN buffer layer for epitaxial growth of nitride junctions, we demonstrate a qubit energy relaxation time T₁=16. 3\;{ }{{{{s}}}} T 1 = 16. 3 μ s and a spin-echo dephasing time T₂=21. 5\;{ }{{{{s}}}} T 2 = 21. 5 μ s. These significant improvements in quantum coherence are explained by the reduced dielectric loss compared to the previously reported T₁ T₂ 0. 5\;{ }{{{{s}}}} T 1 ≈ T 2 ≈ 0. 5 μ s of NbN-based qubits on MgO substrates. These results are an important step towards constructing a new platform for superconducting quantum hardware.