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

Two-level system (TLS) defects in amorphous materials play a critical role in limiting the coherence, stability, and overall performance of quantum devices. While single TLSs can be individually characterized using narrowband probes such as qubits or resonators, accessing their collective behavior remains challenging. We address this problem using Broadband Cryogenic Transient Dielectric Spectroscopy (BCTDS), a modular technique that probes TLS ensembles without the need for full device fabrication. BCTDS reveals clear interference and memory effects, as well as signatures of dressed-state evolution within the TLS bath. In Part II, we introduce a driven minimal spin model incorporating dipole–dipole interactions to interpret these observations. This model captures the key experimental phenomena, including amplitude- and phase-dependent relaxation dynamics, the characteristic spectral features of TLS ensembles, and the influence of temperature and thermal cycling on their collective behavior.