Probing Non-Equilibrium Physics, Bell Correlator Detects Dynamical Quantum Phase Transitions in Long-Range XY Spin Chains

Understanding how systems evolve after a sudden change, and identifying the critical points within that evolution, presents a significant challenge in physics. Abhishek Muhuri, Tanoy Kanti Konar, and Leela Ganesh Chandra Lakkaraju, alongside Aditi Sen(De) and their colleagues at the Harish-Chandra Research Institute and the University of Trento, now demonstrate a new method for detecting these critical points in complex systems. The team reveals that a measurable quantity, originally used to identify quantum entanglement, effectively signals these dynamic transitions in a specific type of magnetic material. Their research shows this method accurately distinguishes between different types of changes to the system, and importantly, works regardless of the material’s strength or size, offering a robust new tool for probing non-equilibrium physics and surpassing the limitations of traditional measurement techniques.

Bell Correlators Probe Quantum Many-Body Dynamics

Researchers investigated non-equilibrium dynamics in complex quantum systems, focusing on the two-body Bell correlator as a means to understand entanglement and correlations. This approach combines analytical calculations with numerical simulations to track the correlator’s behaviour, proving particularly effective for systems with strong interactions. A key achievement of this work lies in demonstrating the sensitivity of the two-body Bell correlator to various non-equilibrium phenomena, including quantum phase transitions and the process of thermalisation. The results reveal that the correlator exhibits distinct features depending on the system’s initial state and the strength of interactions, providing a way to diagnose the system’s dynamics.

Furthermore, the team established a connection between the Bell correlator and the Loschmidt echo, highlighting the correlator’s potential as a probe of both entanglement and the loss of quantum information. Identifying equilibrium criticalities and phases from the dynamics of a system is challenging when relying solely on local measurements. Researchers demonstrate that the experimentally accessible two-body Bell operator effectively identifies dynamical quantum phase transitions in a long-range XY spin chain subjected to a magnetic field. Following a sudden change to the system’s parameters, the Bell operator exhibits characteristic behaviour, revealing the underlying quantum phase transition.

Entanglement Probes Quantum Phase Transitions and Many-Body Systems

This collection of references details research into quantum physics, many-body systems, quantum phase transitions, and entanglement. Key themes include quantum phase transitions and critical phenomena, entanglement and non-locality, and the behaviour of many-body systems. A significant portion of the research focuses on using entanglement as a probe of quantum many-body systems, exploring how to detect and quantify entanglement, particularly in the context of quantum phase transitions. The references cover a wide range of many-body systems, including spin chains and Bose-Einstein condensates, with the goal of understanding the collective behaviour of interacting quantum particles. There is a significant interest in using quantum information concepts to characterise quantum states and detect quantum phase transitions.

Bell Correlator Detects Quantum Critical Points

This research demonstrates that a two-body Bell correlator effectively identifies quantum critical points in a long-range XY spin chain subjected to a magnetic field, even when relying on local measurements. By analysing the behaviour of this correlator following sudden changes to the system’s parameters, the team established a clear distinction between changes that remain within a single phase and those that drive the system across a critical boundary. The saturated value of the Bell correlator serves as a robust indicator of these transitions, maintaining high efficiency regardless of the specific change or system size. Importantly, this method surpasses the capabilities of conventional diagnostics for dynamical quantum phase transitions, including entanglement and classical correlators.

The team quantified this advantage through the development of a critical benchmarking threshold and demonstrated its consistent performance across various changes. Furthermore, the Bell correlator offers a practical advantage as it can be measured directly in experiments without requiring complete knowledge of the system’s quantum state. Future work could investigate the behaviour of the Bell correlator in systems with different interaction ranges and explore its potential for characterizing other types of quantum phase transitions. This research establishes a powerful new tool for identifying magnetic phases and understanding dynamical quantum phenomena in many-body systems.