Morgan Steven W, Oganesyan Vadim, Boutis Gregory S
Department of Physics, Brooklyn College, CUNY, 2900 Bedford Ave., Brooklyn, NY 11210.
Phys Rev B Condens Matter Mater Phys. 2012 Dec 14;8672(21). doi: 10.1103/PhysRevB.86.214410.
Quantum unitary evolution typically leads to thermalization of generic interacting many-body systems. There are very few known general methods for reversing this process, and we focus on the magic echo, a radio-frequency pulse sequence known to approximately "rewind" the time evolution of dipolar coupled homonuclear spin systems in a large magnetic field. By combining analytic, numerical, and experimental results we systematically investigate factors leading to the degradation of magic echoes, as observed in reduced revival of mean transverse magnetization. Going beyond the conventional analysis based on mean magnetization we use a phase encoding technique to measure the growth of spin correlations in the density matrix at different points in time following magic echoes of varied durations and compare the results to those obtained during a free induction decay (FID). While considerable differences are documented at short times, the long-time behavior of the density matrix appears to be remarkably universal among the types of initial states considered - simple low order multispin correlations are observed to decay exponentially at the same rate, seeding the onset of increasingly complex high order correlations. This manifestly athermal process is constrained by conservation of the second moment of the spectrum of the density matrix and proceeds indefinitely, assuming unitary dynamics.
量子幺正演化通常会导致一般相互作用多体系统的热化。目前已知的能够逆转这一过程的通用方法非常少，我们关注的是魔回波，它是一种射频脉冲序列，已知在大磁场中能近似地“倒回”偶极耦合同核自旋系统的时间演化。通过结合解析、数值和实验结果，我们系统地研究了导致魔回波退化的因素，这在平均横向磁化强度的恢复减弱中有所体现。超越基于平均磁化强度的传统分析方法，我们使用相位编码技术来测量在不同持续时间的魔回波之后，密度矩阵中自旋关联在不同时间点的增长情况，并将结果与自由感应衰减（FID）期间获得的结果进行比较。虽然在短时间内记录到了相当大的差异，但在所考虑的初始态类型中，密度矩阵的长时间行为似乎非常普遍——观察到简单的低阶多自旋关联以相同的速率指数衰减，引发了越来越复杂的高阶关联的出现。假设幺正动力学，这个明显的非热过程受到密度矩阵谱的二阶矩守恒的约束，并会无限持续下去。
