Perturbative Complexity of Interacting Theory

TL;DR

This paper develops a perturbative method to compute quantum complexity in interacting theories, using operator transformations and diagrammatic techniques, with applications to coupled oscillators and lattice field theories.

Contribution

It introduces a systematic perturbative approach and diagrammatic rules for calculating quantum complexity in interacting systems, extending to excited states and higher orders.

Findings

Interaction corrections to complexity can be positive or negative.

Diagrammatic methods simplify perturbative calculations.

Complexity depends on reference-state frequency.

Abstract

We present a systematic method to expand the quantum complexity of interacting theory in series of coupling constant. The complexity is evaluated by the operator approach in which the transformation matrix between the second quantization operators of reference state and the target state defines the quantum gate. We start with two coupled oscillators and perturbatively evaluate the geodesic length of the associated group manifold of gate matrix. Next, we generalize the analysis to $N$ coupled oscillators which describes the lattice $\lambda\phi^4$ theory. Especially, we introduce simple diagrams to represent the perturbative series and construct simple rules to efficiently calculate the complexity. General formulae are obtained for the higher-order complexity of excited states. We present several diagrams to illuminate the properties of complexity and show that the interaction correction to complexity may be positive or negative depending on the magnitude of reference-state frequency.