Observation of universal dynamics in an isolated one-dimensional Bose gas far from equilibrium (1805.12310v1)

Abstract: We provide experimental evidence of universal dynamics far from equilibrium during the relaxation of an isolated one-dimensional Bose gas. Following a rapid cooling quench, the system exhibits universal scaling in time and space, associated with the approach of a non-thermal fixed point. The time evolution within the scaling period is described by a single universal function and scaling exponent, independent of the specifics of the initial state. Our results provide a quantum simulation in a regime, where to date no theoretical predictions are available. This constitutes a crucial step in the verification of universality far from equilibrium. If successful, this may lead to a comprehensive classification of systems based on their universal properties far from equilibrium, relevant for a large variety of systems at different scales.

Universal Dynamics in an Isolated One-Dimensional Bose Gas

This paper addresses the experimental observation of universal dynamics in an isolated one-dimensional Bose gas far from equilibrium, particularly after a rapid cooling quench. By examining the Bose gas in a non-equilibrium state, this work confirms theoretically predicted universal scaling behaviors in temporal and spatial domains as the system evolves towards a non-thermal fixed point.

The authors start with a thermal gas of ultra-cold $^{87}$Rb atoms and employ a cooling quench to drive the system far from equilibrium. Subsequent observations reveal that during the relaxation period, the system exhibits universal scaling dynamics, described by a single universal function and a scaling exponent, regardless of the initial state specifics. Notably, the time evolution of the momentum distribution becomes a key observable, collapsing onto a universal curve when rescaled appropriately with temporal and momentum scale factors.

The experimental results align with theoretical predictions of non-thermal fixed points and non-equilibrium universality classes, suggesting a broader applicability across various systems and scales. The experimental setup allows the identification and quantification of the scaling exponents $\alpha$ and $\beta$, with values found to be $\alpha \approx 0.09$ and $\beta \approx 0.10$, indicating transport towards the infrared regimes. Such transport suggests a mechanism for building a quasicondensate, advancing to thermal equilibrium eventually. In particular, the conservation of particle number within the scaling regime distinguishes this paper from equilibrium phenomena, where thermal dynamics generally involve time-invariant properties.

Furthermore, the experimental setup pioneers quantum simulations in regimes where theoretical models are not fully predictive, emphasizing the complexity inherent in dimensional crossovers, which add layers of intricacy beyond existing quantum field theories. The measurement techniques encompass in-situ density profiles and momentum distributions post time-of-flight, elucidating the microscopic evolution dynamics of the gas. The approach ensures precise control of the parameters influencing the non-equilibrium state, leading to replicable and scalable insights.

The implications of these results are manifold. On a practical level, they validate universal scaling as a framework for understanding complex quantum systems far from equilibrium without necessitating fine-tuning—unlike typical critical phenomena. Theoretically, establishing universal dynamics could contribute significantly to refining the classification of non-equilibrium systems analogous to equilibrium critical phenomena. The results also have potential implications for diverse fields, from condensed matter physics to cosmology, in systems where classical statistical mechanics do not apply.

Looking forward, the paper paves avenues for considering non-thermal fixed points in other quantum many-body systems, suggesting potential parallels in diverse engineered and natural systems. Future work could explore the dimensional effects and explore scaling behaviors in more complex quantum fluids, leveraging advances in computational and experimental methodologies in quantum simulation.

In conclusion, this research substantiates universal phenomena in isolated quantum systems and sets a precedent for future explorations into the intricate nature of non-equilibrium quantum dynamics—a domain promising vast untapped potential for innovation and discovery in quantum physics.