Krylov complexity of modular Hamiltonian evolution

TL;DR

This paper explores the complexity of states and operators under modular Hamiltonian evolution using Krylov basis, revealing universal late-time behavior and connections to entanglement spectrum in various quantum systems.

Contribution

It introduces a Krylov basis approach to analyze modular Hamiltonian complexity, uncovering universal late-time growth governed by a modular Lyapunov exponent and linking complexity to entanglement spectrum.

Findings

Modular Lanczos spectrum offers a new perspective on quantum entanglement.

Late-time spread complexity is universally governed by a modular Lyapunov exponent of 2π.

Entanglement spectrum encodes the same information as complexity in certain quantum systems.

Abstract

We investigate the complexity of states and operators evolved with the modular Hamiltonian by using the Krylov basis. In the first part, we formulate the problem for states and analyse different examples, including quantum mechanics, two-dimensional conformal field theories and random modular Hamiltonians, focusing on relations with the entanglement spectrum. We find that the modular Lanczos spectrum provides a different approach to quantum entanglement, opening new avenues in many-body systems and holography. In the second part, we focus on the modular evolution of operators and states excited by local operators in two-dimensional conformal field theories. We find that, at late modular time, the spread complexity is universally governed by the modular Lyapunov exponent $\lambda^{mod}_L=2\pi$ and is proportional to the local temperature of the modular Hamiltonian. Our analysis provides explicit examples where entanglement entropy is indeed not enough, however the entanglement spectrum is, and encodes the same information as complexity.