Density matrix and purity evolution in dissipative two-level systems: I. Theory and path integral results for...

Area of Science:

• Quantum mechanics
• Condensed matter physics
• Spectroscopy

Background:

• Investigates purity and von Neumann entropy in quantum systems.
• Focuses on a symmetric two-level system coupled to a dissipative harmonic bath.
• Examines tunneling dynamics and decoherence processes.

Purpose of the Study:

• To analytically and numerically investigate the time evolution of purity and von Neumann entropy.
• To understand the contributions to purity from different terms under various conditions.
• To elucidate the role of bath memory effects and decoherence in purity dynamics.

Main Methods:

• Analytical arguments and path integral calculations.
• Modeling simple systems and the singly excited bacteriochlorophyll dimer.
• Analyzing contributions to purity from incoherent terms, population differences, and time derivatives.

Main Results:

• Established bounds and limiting behaviors for purity and entropy.
• Demonstrated purity recovery under low-temperature, weak dissipation due to interplay of purity contributions.
• Identified bath memory effects and spontaneous phonon emission as critical for long-time purity recovery.

Conclusions:

• Loss of quantum coherence does not necessitate purity decay.
• Quantum coherence and purity can evolve on different time scales.
• Spontaneous phonon emission drives long-time purity recovery in dissipative quantum systems.