A new degree bound for local unitary and $n$-qubit SLOCC Invariants

TL;DR

This paper establishes new upper bounds on the degrees of invariants needed to generate complete sets for local unitary and $n$-qubit SLOCC invariants, simplifying the study of quantum entanglement classifications.

Contribution

It provides explicit degree bounds for generating sets of invariants, improving understanding of the algebraic structure of quantum entanglement invariants.

Findings

Local unitary invariants are generated by invariants of degree at most $d^4$.

$n$-qubit SLOCC invariants are generated by invariants of degree at most $2^{4n}$.

New bounds facilitate the computation and classification of quantum entanglement.

Abstract

Deep connections between invariant theory and entanglement have been known for some time and been the object of intense study. This includes the study of local unitary equivalence of density operators as well as entanglement that can be observed in stochastic local operations assisted by classical communication (SLOCC). An important aspect of both of these areas is the computation of complete sets of invariants polynomials. For local unitary equivalence as well as $n$-qubit SLOCC invariants, complete descriptions of these invariants exist. However, these descriptions give infinite sets; of great interest is finding generating sets of invariants. In this regard, degree bounds are highly sought after to limit the possible sizes of such generating sets. In this paper we give new upper bounds on the degrees of the invariants, both for a certain complete set of local unitary invariants as well as the $n$-qubit SLOCC invariants. We show that there exists a complete set of local unitary invariants of density operators in a Hilbert space $\mathcal{H}$, of dimension $d$, which are generated by invariants of degree at most $d^4$. This in turn allows us to show that the $n$-qubit SLOCC invariants are generated by invariants of degree at most $2^{4n}$.