Ultrastrong coupling limit to quantum mean force Gibbs state for anharmonic environment

Area of Science:

• Quantum thermodynamics
• Quantum statistical mechanics
• Condensed matter physics

Background:

• Quantum system equilibrium states can differ from Gibbs states when system-environment (SE) coupling is strong.
• The mean force Gibbs state (MFGS) is known for the Caldeira-Leggett (CL) model in the ultrastrong coupling (USC) regime with harmonic environments.

Purpose of the Study:

• To derive analytical expressions for the MFGS in the USC regime for generalized SE models beyond the CL model.
• To investigate how SE coupling affects quantum equilibrium states in more realistic scenarios.

Main Methods:

• Derivation of analytical expressions for MFGS in the USC regime for generalized SE models.
• Analysis of the USC state's properties and its diagonal basis.
• Numerical verification of the derived analytical results.

Main Results:

• The USC state is diagonal in the SE interaction basis for all generalized models studied.
• The USC-MFGS differs from the CL result for generic models.
• A class of models more general than CL exists where the CL-USC result is preserved.

Conclusions:

• The derived analytical expressions extend the study of quantum thermodynamics to more general and realistic SE models.
• These findings are crucial for understanding strong coupling effects in quantum chemistry, biology, and condensed matter physics.
• The study provides essential tools for analyzing quantum systems beyond the limitations of the CL model.