Spectroscopy and Coherent Control of Two-Level System Defect Ensembles Using Broadband Cryogenic Transient Dielectric Spectroscopy (BCTDS): Experiments and Results

Two-level system (TLS) defects in amorphous materials limit coherence and stability 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 defect ensembles without requiring complex device fabrication. BCTDS reveals clear interference effects, memory-dependent responses, and signatures of dressed-state evolution within the TLS bath. In Part I, we present experimental results showing spectral features marking bare eigenmode frequencies of the driven system, together with amplitude- and phase-controlled interference fringes that provide direct evidence of coherent TLS dynamics, as well as temperature dependence of the interference and memory effects.