Quantum Systems Show Hidden Entropy Surge

In the quest to understand how quantum systems behave far from equilibrium, researchers have uncovered surprising dynamics in entropy production during abrupt changes. This matters because such systems, like those in quantum optics, could lead to more efficient lasers and sensors, bridging fundamental physics with practical applications. The study focuses on the Kerr bistability model, a well-known system in quantum optics that exhibits phase transitions under light driving, similar to how water abruptly changes phase under temperature shifts.

The key finding is that when the external pump intensity in a quantum optical cavity is suddenly altered, the entropy production rate—a measure of irreversibility—spikes dramatically. This occurs as the system transitions between different steady states, such as moving from a 'dark' phase with low photon occupation to a 'bright' phase with high occupation. The researchers discovered that this entropy surge is not uniform; it depends heavily on the final pump setting and the system size, revealing distinct patterns in how quantum fluctuations contribute to irreversibility.

To achieve this, the team employed a phase-space based on the Husimi Q-function, a tool that maps quantum states into probability distributions. This approach, detailed in their previous work, allows for splitting entropy production into two parts: one from classical irreversibility and another from quantum effects. They simulated quench protocols where the pump was abruptly changed, using numerical computations on Fock basis states to track entropy dynamics over time. This ensured accuracy without assuming Gaussian states, which are common simplifications but inadequate here due to the non-Gaussian nature of the dynamics.

, Illustrated in figures like Fig. 5 and Fig. 7, show that for quenches within the same phase, entropy production from dissipation and unitary contributions are comparable. However, when transitioning between phases, the dissipative part dominates, with entropy production rates peaking before relaxation. For instance, in quenches to the bright phase within the bistability region, entropy production rates reached values significantly higher than in steady states, indicating high irreversibility during the transition. The data also highlighted that larger system sizes led to more pronounced non-Gaussian behavior, as shown in Fig. 4, where deviations from Gaussianity increased with N in certain quenches.

In practical terms, this research deepens our understanding of non-equilibrium thermodynamics in quantum systems, which could inform the design of optical devices like switches or memory elements in quantum computing. By quantifying how entropy behaves under sudden changes, it provides a framework for predicting system stability and efficiency in real-world applications, such as in advanced laser systems where control over light-matter interactions is crucial.

Despite these insights, the study has limitations. The simulations were computationally intensive, restricting analysis to smaller system sizes and specific parameter ranges. Additionally, the behavior of entropy production in infinitesimal quenches or other models with continuous transitions remains unexplored, leaving questions about universality across different quantum systems. Future work could address these gaps, potentially leading to broader applications in multimode systems.