A simple probe of chaos and operator growth in many-body quantum systems is the out of time ordered four point function. In a large class of local systems, the effects of chaos in this correlator build up exponentially fast inside the so called butterfly cone. It has been previously observed that the growth of these effects is organized along rays and can be characterized by a velocity dependent Lyapunov exponent, $λ({\bf v})$. We show that this exponent is bounded inside the butterfly cone as $λ({\bf v})\leq 2πT(1-|{\bf v}|/v_B)$, where $T$ is the temperature and $v_B$ is the butterfly speed. This result generalizes the chaos bound of Maldacena, Shenker and Stanford. We study $λ({\bf v})$ in some examples such as two dimensional SYK models and holographic gauge theories, and observe that in these systems the bound gets saturated at some critical velocity $v_*<v_B$. In this sense, boosting a system enhances chaos. We discuss the connection to conformal Regge theory, where $λ({\bf v})$ is related to the spin of the leading large $N$ Regge trajectory, and controls the four point function in an interpolating regime between the Regge and the light cone limit. Finally, we comment on the generalization of the chaos bound to boosted and rotating ensembles and clarify some recent results on this in the literature.
Quantum chaos, pole-skipping and hydrodynamics in a holographic system with chiral anomaly
众所周知，由于手征磁效应，手征异常可以通过能量和电荷输运来宏观检测。另一方面，在全息多体系统中，混沌模式可能只与能量守恒有关。这表明，也许人们可以通过诊断这种系统中的量子混沌来检测微观异常。为了研究这一想法，我们考虑在AdS时空中的磁化膜与陈-西蒙斯耦合的散装。通过研究这种背景下的激波几何，我们首先计算了在$μ\ll T$和$ B \ll T^2$极限下，存在外磁场$B$时相应的蝶形速度。我们发现，在$B$的方向上的蝴蝶传播有一个不同的速度比在相反的方向。蝴蝶速度的分裂证实了手征反常可以通过量子混沌在宏观上表现出来的观点。然后，我们表明，极点跳跃点的能量密度格林函数的边界理论准确地符合混沌点。这可能被认为是反常系统中量子混沌的流体动力学起源。另外，在上述背景下，通过研究标量场的近视界动力学，我们发现了与对偶边界算子的两点函数相联系的跳极点谱。我们发现，特定松原频率处跳极点对应的波数之和是一个普适量，与对偶边界算子的标度维度无关。然后，我们表明，这个量遵循一个封闭的公式，可以被视为另一个宏观表现的手征异常。
