A quantum system prepared in an initial state that is not an eigenstate of its Hamiltonian will exhibit out-of-equilibrium dynamics. These dynamics can span multiple timescales, from the transient response of local correlations to the long-time equilibration of the system. The possibilities seem vast, depending on the initial state, the nature of the interactions and the observables being measured. Reporting in Nature Physics, Yuchen Li and colleagues have observed that the out-of-equilibrium dynamics of local magnetization correlations measured using solid-state nuclear magnetic resonance (NMR) follow a universal form¹.

The free induction decay signal records the temporal correlations of the local magnetization following the quench. Previous studies had suggested that the signal should follow a universal form, with the correlations decaying either as a simple exponential (exp(–γt)) or as a damped oscillation (cos(ωt)exp(–γt)) (ref. ^3,4). Careful NMR experiments on a wide range of systems, including polycrystalline xenon lattices and calcium fluoride^5,6, confirmed such behaviour. Additionally, a second oscillatory decay mode was observed in calcium fluoride⁷. However, the connection between these universal decays and the parameters of the underlying Hamiltonian remained unclear, limiting the possibility of predicting these universal modes in more general settings.

在初始状态下准备的量子系统不是其哈密顿量的特征态，将表现出不平衡动力学。这些动态可以跨越多个时间尺度，从局部相关性的瞬态响应到系统的长期平衡。可能性似乎很大，具体取决于初始状态、交互的性质和被测量的 Observables。Yuchen Li 及其同事在《自然物理学》上报告说，使用固态核磁共振 （NMR） 测量的局部磁化相关性的失衡动力学遵循通用形式¹。

自由感应衰减信号记录了淬火后局部磁化的时间相关性。以前的研究表明，信号应该遵循通用形式，相关性衰减为简单的指数 （exp（–γt）） 或阻尼振荡 （cos（ωt）exp（–γt））（参考文献 ^3,4）。对包括多晶氙晶格和氟化钙在内的各种系统进行的仔细核磁共振实验^5,6 证实了这种行为。此外，在氟化钙⁷ 中观察到第二种振荡衰减模式。然而，这些普遍衰变与基础哈密顿量参数之间的联系仍然不清楚，这限制了在更一般的设置中预测这些普遍模式的可能性。