Operator K-complexity in DSSYK: Krylov complexity equals bulk length

TL;DR

This paper explores Krylov complexity in the double-scaled SYK model, establishing its equivalence to a bulk length operator and analyzing its semiclassical behavior and gravitational interpretation.

Contribution

It demonstrates that Krylov complexity equals a length operator in the holographic dual, providing analytic and numerical evidence for this equivalence and exploring its gravitational implications.

Findings

Krylov complexity corresponds to a length operator in the theory.

Analytic expressions for the semiclassical limit of K-complexity are derived.

K-complexity evolution can be modeled as a particle in a Morse potential.

Abstract

In this paper we study the notion of complexity under time evolution in chaotic quantum systems with holographic duals. Continuing on from our previous work, we turn our attention to the issue of Krylov complexity upon the insertion of a class of single-particle operators in the double-scaled SYK model. Such an operator is described by a matter-chord insertion, which splits the theory into left/right sectors, allowing us, via chord-diagram technology, to compute two different notions of complexity associated to the operator insertion: first a Krylov operator complexity, and second the Krylov complexity of a state obtained by an operator acting on the thermofield double state. We will provide both an analytic proof and detailed numerical evidence, that both Krylov complexities arise from a recursively defined basis of states characterized by a constant total chord number. As a consequence, in all cases we are able to establish that Krylov complexity is given by the expectation value of a length operator acting on the Hilbert space of the theory, expressed in terms of basis states, organized by left and right chord number. We find analytic expressions for the semiclassical limit of K-complexity, and study how the size of the operator encodes the scrambling dynamics upon the matter insertion in Krylov language. We furthermore determine the effective Hamiltonian governing the evolution of K-complexity, showing that evolution on the Krylov chain can equivalently be understood as a particle moving in a Morse potential. A particular type of triple scaling limit allows to access the gravitational sector of the theory, in which the geometrical nature of K-complexity is assured by virtue of being a total chord length, in an analogous fashion to what was found in [1] for the K-complexity of the thermofield double state.