The split and approximate split property in 2D systems: stability and absence of superselection sectors

TL;DR

This paper investigates the split property in 2D quantum spin systems, showing its failure relates to long-range entanglement and the existence of anyons, and introduces a technique for approximate factorization of automorphisms.

Contribution

It proves that long-range entanglement is necessary for non-trivial superselection sectors and introduces a method to approximately factorize automorphisms under local interactions.

Findings

Split property fails in 2D models like the toric code.

Long-range entanglement is necessary for anyons and non-trivial sectors.

Approximate automorphism factorization is stable and of independent interest.

Abstract

The split property of a pure state for a certain cut of a quantum spin system can be understood as the entanglement between the two subsystems being weak. From this point of view, we may say that if it is not possible to transform a state $\omega$ via sufficiently local automorphisms (in a sense that we will make precise) into a state satisfying the split property, then the state $\omega$ has a long-range entanglement. It is well known that in 1D, gapped ground states have the split property with respect to cutting the system into left and right half-chains. In 2D, however, the split property fails to hold for interesting models such as Kitaev's toric code. In fact, we will show that this failure is the reason that anyons can exist in that model. There is a folklore saying that the existence of anyons, like in the toric code model, implies long-range entanglement of the state. In this paper, we prove this folklore in an infinite dimensional setting. More precisely, we show that long-range entanglement, in a way that we will define precisely, is a necessary condition to have non-trivial superselection sectors. Anyons in particular give rise to such non-trivial sectors. States with the split property for cones, on the other hand, do not admit non-trivial sectors. A key technical ingredient of our proof is that under suitable assumptions on locality, the automorphisms generated by local interactions can be 'approximately factorized': they can be written as the tensor product of automorphisms localized in a cone and its complement respectively, followed by an automorphism acting near the 'boundary' of $\Lambda$, and conjugation with a unitary. This result may be of independent interest. This technique also allows us to prove that the approximate split property, a weaker version of the split property that is satisfied in e.g. the toric code, is stable under applying such automorphisms.