Decoherent Histories with(out) Objectivity in a (Broken) Apparatus.

Area of Science:

• Quantum physics
• Quantum information theory
• Statistical mechanics

Background:

• Understanding the emergence of classicality from quantum mechanics is a fundamental challenge.
• Quantum Darwinism explains how environmental monitoring leads to classical objectivity.
• Decoherent histories provide a framework for describing quantum evolution over time.

Purpose of the Study:

• To investigate monitored quantum dynamics in a solvable model with a phase transition.
• To distinguish between different concepts of classicality, specifically decoherent histories and environment-induced decoherence.
• To explore the emergence of quantum Darwinism in a measurement apparatus phase.

Main Methods:

• Characterization of monitored quantum dynamics using a solvable model.
• Analysis of phase transitions between measurement apparatus and scrambler regimes.
• Coarse-graining an extensive observable to define histories.
• Investigating the ergodicity and correlations of quantum histories.

Main Results:

• Approximate decoherent histories were observed in both the measurement apparatus and scrambler phases.
• The measurement apparatus phase exhibited nonergodic histories correlated with the measured qubit.
• This nonergodicity identified a preferred ensemble of pointer states in the apparatus phase.
• A clear distinction was established between decoherent histories and environment-induced decoherence.

Conclusions:

• Decoherent histories are a general feature of monitored quantum systems, appearing in different phases.
• Quantum Darwinism is specifically linked to the nonergodic nature of histories in the measurement apparatus phase.
• The study clarifies the relationship between different quantum-to-classical transition mechanisms.