The Complexity of Being Entangled

TL;DR

This paper explores the concept of binding complexity in quantum systems, relating it to geodesics on the manifold of Schmidt coefficients, and establishes connections with entropy, holography, and quantum computation.

Contribution

It introduces a novel analysis of binding complexity using geodesics on Schmidt coefficient manifolds and relates it to entropy and holographic duals, providing new analytic results and bounds.

Findings

Exact relation between binding complexity and minimal Rényi entropy.

Analytic results for the $F_1$ norm in non-Riemannian cases.

Lower bounds for state complexity and circuit complexity with 2-local interactions.

Abstract

Nielsen's approach to quantum state complexity relates the minimal number of quantum gates required to prepare a state to the length of geodesics computed with a certain norm on the manifold of unitary transformations. For a bipartite system, we investigate binding complexity, which corresponds to norms in which gates acting on a single subsystem are free of cost. We reduce the problem to the study of geodesics on the manifold of Schmidt coefficients, equipped with an appropriate metric. Binding complexity is closely related to other quantities such as distributed computing and quantum communication complexity, and has a proposed holographic dual in the context of AdS/CFT. For finite dimensional systems with a Riemannian norm, we find an exact relation between binding complexity and the minimal R\'enyi entropy. We also find analytic results for the most commonly used non-Riemannian norm (the so-called $F_1$ norm) and provide lower bounds for the associated notion of state complexity ubiquitous in quantum computation and holography. We argue that our results are valid for a large class of penalty factors assigned to generators acting across the subsystems. We demonstrate that our results can be borrowed to study the usual complexity (not-binding) for a single spin for the case of the $F_1$ norm which was previously lacking from the literature. Finally, we derive bounds for multi-partite binding complexities and the related (continuous) circuit complexity where the circuit contains at most $2$-local interactions.