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Atomic Nuclei: Types of Nuclear Relaxation01:28

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Nuclear relaxation restores the equilibrium population imbalance and can occur via spin–lattice or spin–spin mechanisms, which are first-order exponential decay processes.
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Density matrix and purity evolution in dissipative two-level systems: II. Relaxation.

Sambarta Chatterjee1, Nancy Makri2

  • 1Department of Chemistry, University of Illinois, Urbana, IL 61801, USA. nmakri@illinois.edu.

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Quantum systems can surprisingly regain purity. This study shows a two-level system coupled to a bath can recover its initial state purity, even rising towards unity under specific conditions.

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Area of Science:

  • Quantum mechanics
  • Condensed matter physics
  • Quantum information

Background:

  • Investigating the dynamics of quantum systems interacting with their environment is crucial for understanding decoherence and quantum information processing.
  • The purity of a quantum system quantifies its deviation from a maximally mixed state.

Purpose of the Study:

  • To analyze the time evolution of purity and the reduced density matrix (RDM) for a two-level system coupled to a dissipative harmonic bath.
  • To understand the mechanisms behind purity recovery and nonmonotonic population dynamics.

Main Methods:

  • Accurate path integral calculations were employed to quantify the system's dynamics.
  • Analysis of system-bath interactions, distinguishing between classical dephasing and quantum decoherence.

Main Results:

  • Purity always reaches a maximally mixed state during relaxation but can recover towards unity under low-temperature, weak dissipation.
  • Nonmonotonic population dynamics observed in the ground state under specific conditions.
  • Classical dephasing causes monotonic purity decay, while quantum decoherence (phonon emission) drives purity recovery.

Conclusions:

  • Coupling to a low-temperature bath can purify a mixed two-level system.
  • Quantum decoherence events are responsible for purity recovery, counteracting classical dephasing.
  • The study reveals mechanisms for purity restoration in open quantum systems.