An identity for T-ordered exponentials with applications to quantum mechanics

Boris Maulén, Sergio M. Davis, Daniel Pons

伸一郎 二口

Ignacio García-Mata, Diego A. Wisniacki

One of the fundamental manifestations of classical chaos is exponential sensitivity to initial conditions that is, two trajectories starting from nearly identical initial states diverge exponentially over time. This behavior is quantified by the Lyapunov exponents. Due to the unitary nature of quantum mechanics, such exponential divergence is elusive in quantum systems. As a result, several alternative quantities have been proposed and studied in recent years to capture analogous behavior. In this article, we present a pedagogical overview of three such quantities that have been the focus of intense research in recent years: the Loschmidt echo, out-of-time-order correlators (OTOCs), and Krylov complexity.

Abdelwahab Amr

We present a novel combinatorial proof of the \textbf{Time-Ordered Exponential} formula for the \textbf{Time-Evolution Operator} based on the \textbf{Heaviside Formulation Of The Time-Ordering Operator}. Unlike conventional derivations that rely on induction our approach decomposes the \textbf{Symmetric Group} into permutations that produce decreasing time sequences and those that do not using a simple discriminant function. This decomposition together with an \textbf{Interchange Rule } between time-ordering and multiple integrals yields the \textbf{Dyson–Chen Identity} directly. The proof is self-contained and avoids recursion entirely. It appears in the literature as a problem in Advanced Quantum Mechanics (Springer, 2012, Problem 13.1).

Bogdan Mielnik

Fan Hong-Yi

In this paper, the completeness relations of the fundamental representations in quantum mechanics, together with the "integration within orderd product" technique are exploited to derive normally ordered and antinormally ordered expansions of some exponential operators in Hilbert space. Applications of the normally ordered exponential operators to evaluating Feynman matrix element in coherent state representation are given, which seems to be a new approach.