Geometry of Krylov Complexity

TL;DR

This paper introduces a geometric framework for understanding operator growth and Krylov complexity in quantum systems, linking classical phase space trajectories with quantum operator evolution and providing new computational insights.

Contribution

It develops a geometric approach connecting operator growth to classical geodesics and volumes in phase space, offering novel methods to compute Lanczos coefficients and analyze complexity.

Findings

Operator growth corresponds to geodesics in phase space.

Krylov complexity is proportional to phase space volume.

Reproduces known results in SYK model and extends to conformal field theories.

Abstract

We develop a geometric approach to operator growth and Krylov complexity in many-body quantum systems governed by symmetries. We start by showing a direct link between a unitary evolution with the Liouvillian and the displacement operator of appropriate generalized coherent states. This connection maps operator growth to a purely classical motion in phase space. The phase spaces are endowed with a natural information metric. We show that, in this geometry, operator growth is represented by geodesics and Krylov complexity is proportional to a volume. This geometric perspective also provides two novel avenues towards computation of Lanczos coefficients and sheds new light on the origin of their maximal growth. We describe the general idea and analyze it in explicit examples among which we reproduce known results from the Sachdev-Ye-Kitaev model, derive operator growth based on SU(2) and Heisenberg-Weyl symmetries, and generalize the discussion to conformal field theories. Finally, we use techniques from quantum optics to study operator evolution with quantum information tools such as entanglement and Renyi entropies, negativity, fidelity, relative entropy and capacity of entanglement.