Physicists uncover universal non-equilibrium quantum dynamics in randomly interacting spin models

A new study has uncovered the universal dynamics far from equilibrium in randomly interacting spin models, thereby complementing the well-established universality in low-energy equilibrium physics. The study, recently published in Nature Physics, was the result of a collaborative effort involving the research group led by Prof. DU Jiangfeng and Prof. PENG Xinhua at the University of Science and Technology of China (USTC), along with the theoretical groups of Prof. ZHAI Hui from Tsinghua University and Dr. ZHANG Pengfei from Fudan University.

Non-equilibrium dynamics in quantum many-body systems lies at the heart of the current development of modern quantum science and technology, where rich phenomena have been found during recent years, especially in synthetic quantum platforms such as ultracold atoms, superconducting qubit, trapped ions, NV centers, and NMR systems.

However, despite its rich phenomena, simple and universal rules behind quantum none-equilibrium dynamics are still lacking so far. Non-equilibrium dynamics usually involve highly excited states beyond the conventional low-energy universality. Whether universal behavior can also emerge in non-equilibrium dynamics, is a cutting-edge question in the field of quantum many-body physics. The main difficulties arise from the strongly correlating nature and the complexity of non-equilibrium many-body systems, as well as the experimental challenges associated with high-precision quantum control of these systems.

Solid-state nuclear spin systems are naturally complex many-body quantum systems that can be precisely controlled through quantum control technologies, thereby enabling the implementation of various many-body spin models. This provides a natural and adjustable experimental platform for studying the non-equilibrium dynamics of quantum many-body systems.

Leveraging years of expertise in quantum control and quantum simulation of nuclear spin systems, the researchers designed the pulse sequences (as shown in Fig. b) to high-precisely control the ¹H nuclear spins in adamantane (C₁₀H₁₆) powder (each grain containing approximately 10⁹ to 10¹² molecules, as shown in Fig. a), and realized randomly interacting spin models with adjustable anisotropic parameters. The randomness arises from the random orientations between the lattice axes in different grains and the static magnetic field.

Then the researchers observed a new phenomenon that the spin depolarization dynamics shows a clear transition from monotonic to oscillatory decay as the anisotropic parameter changed. They found that the behavior of the spin depolarization dynamics can be universally described by two parameters (Fig. c). By comparing the experimental observations with several different theoretical approaches, the researchers offered a comprehensive theoretical explanation of this non-equilibrium quantum many-body dynamics (Fig. d).

This study has identified a new type of universality in non-equilibrium dynamics of quantum many-body systems that are challenging to simulate on classical computers. It serves as an excellent example of how quantum information technology can be used to discover new physical laws. Additionally, the methods and techniques used in the experiments provide new insights for other physical systems, such as ultracold atoms or molecules and NV center ensembles. The research significantly enhances the understanding of such complex systems, potentially sparking widespread interest across various fields of physics.