Ultracold atom quantum simulator sheds light on quantum thermalization and quantum criticality

Researchers at the University of Science and Technology of China (USTC), part of the Chinese Academy of Sciences (CAS), have achieved a significant breakthrough by developing an ultra-cold atom quantum simulator. Their innovative work, led by Pan Jianwei and Yuan Zhensheng in collaboration with Zhai Hui from Tsinghua University and Yao Zhiyuan from Lanzhou University, sheds light on the intriguing relationship between non-equilibrium thermalization processes and quantum criticality within lattice gauge field theories. Their findings, which have been published in Physical Review Letters, offer valuable insights into the behavior of multi-body systems with gauge symmetry as they tend to approach equilibrium states, particularly when situated near quantum phase transition critical regions.

Gauge theory and statistical mechanics are foundational pillars of physics, influencing everything from classical electromagnetism to the Standard Model describing particle interactions. While gauge theory deals with specific symmetries, statistical mechanics connects microscopic particle states to macroscopic statistical behaviors, such as pressure, volume, and temperature. Understanding how a quantum many-body system governed by gauge theory reaches thermodynamic equilibrium from a far-from-equilibrium state is a crucial question that bridges these two theories. Although theoretical physicists have proposed models, conducting experiments on such systems has been a formidable challenge.

The advent of ultracold atomic quantum simulators has provided an ideal platform for simultaneously exploring gauge theories and statistical physics. In 2020, the USTC research team developed an ultracold atomic optical lattice quantum simulator with 71 lattice points, enabling the experimental simulation of the quantum phase transition in U(1) lattice gauge theory, specifically the Schwinger Model.

In 2022, the team extended their research to simulate the thermalization dynamics in lattice gauge field theories, validating, for the first time experimentally, the “loss” of initial state information due to quantum many-body thermalization under gauge symmetry constraints. Collaborators Zhai Hui and Yao Zhiyuan’s theoretical research predicted a correlation between quantum thermalization and quantum phase transitions in these lattice gauge models. They proposed that full thermalization can only occur in the proximity of the quantum phase transition point, starting from the antiferromagnetic Neel state.

Studying the interplay between quantum thermalization and quantum phase transitions in lattice gauge theories presented new experimental challenges, primarily regarding how to control and detect many-body quantum states precisely at single lattice points with distinguishable atomic numbers.

Building upon their ultracold atomic quantum simulator, the team combined various techniques, including quantum gas microscopy, spin-dependent superlattices, and programmable optical potentials. These innovations enabled atomic operations and detection techniques with single-site precision and distinguishable particle numbers.

In their study, the researchers experimentally prepared initial states with specific atomic configurations and employed adiabatic evolution to explore the quantum phase transition process under gauge symmetry constraints. They achieved a significant milestone by accurately identifying the phase transition point through finite-size scaling theory.

Furthermore, they investigated the annealing dynamics of the same initial configuration when far from equilibrium, revealing a fascinating pattern where many-body systems with gauge symmetry tend to stabilize thermally into an equilibrium state when near the quantum phase transition critical point.

The journal Physics recognized their remarkable achievements in an article titled “Watching a Quantum System Thermalize.”

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