  The high critical magnetic field and relatively high critical temperature of
niobium nitride (NbN) make it a promising material candidate for applications
in superconducting quantum technology. However, NbN-based devices and circuits
are sensitive to decoherence sources such as two-level system (TLS) defects.
Here, we numerically and experimentally investigate NbN superconducting
microwave coplanar waveguide resonator arrays, with a 100 nm thickness,
capacitively coupled to a common coplanar waveguide on a silicon chip. We
observe that the resonators' internal quality factor (Qi) decreases from Qi ~
1.07*10^6 in a high power regime (< nph > = 27000) to Qi ~ 1.36 *10^5 in single
photon regime at temperature T = 100 mK. Data from this study is consistent
with the TLS theory, which describes the TLS interactions in resonator
substrates and interfaces. Moreover, we study the temperature dependence
internal quality factor and frequency tuning of the coplanar waveguide
resonators to characterise the quasiparticle density of NbN. We observe that
the increase in kinetic inductance at higher temperatures is the main reason
for the frequency shift. Finally, we measure the resonators' resonance
frequency and internal quality factor at single photon regime in response to
in-plane magnetic fields B||. We verify that Qi stays well above 10^4 up to B||
= 240 mT in the photon number < nph > = 1.8 at T = 100 mK. Our results may pave
the way for realising robust microwave superconducting circuits for circuit
quantum electrodynamics (cQED) at high magnetic fields necessary for
fault-tolerant quantum computing, and ultrasensitive quantum sensing.
