Spectroscopy and Coherent Control of Two-Level System Defect Ensembles Using a Broadband 3D Waveguide: Experiment and Results*

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 I, we present experimental results showing V-shaped spectral features at bare eigenmode frequencies, together with amplitude- and phase-controlled interference fringes that provide direct evidence of coherent TLS dynamics.