In this dissertation, I introduce a novel method for measuring individual nanoscale two-level systems (TLSs) in amorphous solids based on strong direct coupling between a TLS and a cavity. I describe power- and temperature-dependent analysis of individual TLSs using a theoretical model based on cavity quantum electrodynamics (CQED). This method allows for measuring individual TLSs in different insulators and over a wide range of film thicknesses. For a silicon nitride film at 25 mK and a lumped-element cavity resonance at 6.9 GHz, I find TLSs with coherence times on the order of microseconds which can potentially be used as coherent resources.

Furthermore, I introduce a device which enables spectroscopy of TLSs in insulating films by DC-tuning the TLSs. I present measurement results on 60 TLSs accompanied by theoretical analysis and extraction of distribution statistics of the TLS parameters. I find evidence for at least two TLS dipole sizes.

I also investigate the role of RF-induced DC bias voltage on the growth of titanium nitride films on silicon (100) substrates deposited by DC magnetron reactive sputtering. I present hybrid designs of TiN coplanar resonators which were fabricated with an aluminum transmission line to avoid impedance mismatches due to large kinetic inductance of TiN films. I observe remarkably large kinetic inductance at certain substrate DC bias voltages.

Finally, I describe several trilayer resonators designed to measure TLS ensembles within atomic layer deposition (ALD) grown aluminum oxide. Each resonator is unique in trilayer capacitor perimeter and hence the alumina air-exposed cross section. I compare the measured loss tangents of the resonators and investigate the effect of the capacitor perimeter on TLS defect density at different temperatures.