Toward the universality of the Caldeira-Leggett oscillator bath as a model for quantum environments I

Open quantum systems often interact with complex, intractable quantum environments. Establishing a minimal solvable model that captures the essential effects of these environments on central quantum systems has long been of interest in physics, dating back to the origins of quantum mechanics. The Caldeira-Leggett oscillator bath model has become a prominent minimal model that effectively represents many quantum environments. However, it has been argued that this model cannot capture the main effects of spin environments on central two-level systems when the couplings between the spins and the central systems are strong. In contrast, I adopt an appropriate spectral density function for the Caldeira-Leggett model and demonstrate that it can effectively replicate the essential effects of spin environments in the regime of interest. This suggests that the class of the Caldeira-Leggett model is more universal than previously recognized.