We have developed superconducting qubits based on NbN/AlN/NbN epitaxial Josephson junctions on Si substrates which promise to overcome the drawbacks of qubits based on Al/AlO$_{x}$/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 (further reducing energy relaxation and dephasing), and a larger superconducting gap of $sim$5.2 meV for NbN compared to $sim$0.3 meV for Al (suppressing the excitation of quasiparticles). Replacing conventional MgO by a Si substrate with a TiN buffer layer for epitaxial growth of nitride junctions, we demonstrate a qubit energy relaxation time $T$$_{1}$=16.3 $mu$s and a spin-echo dephasing time $T$$_{2}$=21.5 $mu$s. These significant improvements in quantum coherence are explained by the reduced dielectric loss compared to previously reported NbN-based qubits with MgO substrates ($T$$_{1}$$approx$$T$$_{2}$$approx$0.5 $mu$s). These results are an important step towards constructing a new platform for superconducting quantum hardware.
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