Spectroscopy and Coherent Control of Two-Level System Defect Ensembles Using a Broadband 3D Waveguide: Theory and Model

Two-level system (TLS) defects in amorphous materials critically influence coherence, stability, and performance in quantum technologies. While individual TLSs can be probed with narrowband devices such as qubits or resonators, their collective dynamics remain difficult to access. We address this challenge with our recently developed Broadband Cryogenic Transient Dielectric Spectroscopy (BCTDS) technique, which enables modular measurements of TLS ensembles without requiring full device fabrication. BCTDS reveals interference effects, memory-dependent dynamics, and dressed-state evolution within the TLS bath. In Part II, we present a driven minimal spin model with dipole–dipole interactions, developed to interpret the results. The model qualitatively reproduces the key experimental features, including amplitude- and phase-controlled relaxation dynamics and the corresponding spectral signatures of the TLS defects ensembles.