  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.
