Researchers reveal how monitored dynamics create preferred states and distinguish classicality in scramblers

The fundamental question of how quantum behaviour gives rise to the classical world we experience remains a central challenge in physics, and new research sheds light on the role of measurement in this transition. Benoît Ferté, along with colleagues at the Institute for Theoretical Physics, explores how classical-like behaviour emerges from quantum systems undergoing continuous monitoring. The team demonstrates that approximate, consistent histories, a way of describing quantum evolution that resembles classical trajectories, appear whether or not the monitoring apparatus actively ‘selects’ certain outcomes, but a crucial distinction arises when the apparatus exhibits Darwinian behaviour. This work clarifies the difference between simply having consistent histories and achieving true classicality through environment-induced decoherence, offering a deeper understanding of the quantum-to-classical boundary and the nature of objectivity itself.

Quantum dynamics are explored within a solvable model exhibiting a transition between a measurement apparatus and a chaotic system, revealing the emergence of approximate decoherent histories in both scenarios when considering a simplified, averaged observable. The team demonstrated that while these histories appear in both cases, the apparatus phase, where quantum Darwinism emerges, the process by which quantum information proliferates into the environment, is distinguished by histories that do not explore all possible states equally and a correlation with the measured quantum bit, ultimately selecting a set of preferred measurement outcomes. This work clarifies the distinction between two concepts of classical behaviour, decoherent histories and environment-induced decoherence, addressing a long-standing question in quantum mechanics.

Inflationary Model Explains Quantum Measurement Outcomes

Researchers have developed a theoretical model to understand how quantum measurements lead to definite outcomes. The model uses an inflationary approach, where the measurement apparatus amplifies quantum fluctuations, making them observable macroscopically. This research investigates the interaction between a quantum system and a macroscopic measurement apparatus, focusing on understanding how pointer states, the states that emerge after measurement, are established through a process of simplification called coarse-graining. The team demonstrated that the full history of a measured system is equivalent to knowing its current state, meaning past measurements provide no additional information.

They also found a connection between a specific symmetry within the model and the violation of a test for non-classical behaviour called the Leggett-Garg inequality. Crucially, the model demonstrates a violation of this inequality, confirming the non-classical nature of the system, and the degree of this violation can be controlled. This research simulates how a quantum system is measured, showing that the measurement process leads to behaviours impossible in classical physics.

Emergence of Classical Histories in Quantum Dynamics

Researchers have characterized monitored quantum dynamics using a solvable model exhibiting a transition between a measurement apparatus and a chaotic system. The investigation centers on a model where both the apparatus and the system initially exist as quantum bits, evolving through a cascade process involving an increasing number of quantum bits. Experiments revealed that the model undergoes a transition at a specific parameter value, beyond which the reliable observation of classical records is lost. Researchers established that the model exhibits decoherent histories even in the chaotic regime, challenging the conventional belief that such histories necessarily require reliable observation. Data confirms that coarse-graining is the key mechanism enabling decoherent histories in both phases. However, the team distinguished the phases by analyzing the behaviour of the histories, finding that they are non-ergodic and correlated with the measured quantum bit in the apparatus phase, leading to the selection of an ensemble of pointer states.

Darwinian Classicality Needs Correlated Macroscopic Properties

This research distinguishes between two concepts of classical behaviour within quantum mechanics. The study demonstrates that approximate classical histories emerge simply through coarse-graining, a process of averaging, in any system. However, a more specific form of classicality, linked to quantum Darwinism, requires a particular correlation between macroscopic and microscopic properties, as realised during a quantum measurement process. This implies that not all macroscopic objects necessarily exhibit this stronger form of classical behaviour; an apparatus capable of producing Darwinian classicality possesses specific characteristics beyond simply being macroscopic. The researchers acknowledge that their findings are based on a specific model and suggest that establishing reliable observation may be challenging in standard models, potentially necessitating symmetry breaking. Future work could explore the implications of this research for understanding classicality in cosmology.