How many invariant polynomials are needed to decide local unitary equivalence of qubit states?

TL;DR

This paper derives a formula for the minimal number of invariant polynomials needed to determine local unitary equivalence of L-qubit states with fixed spectra, revealing spectrum-dependent variations.

Contribution

It introduces a geometric approach to compute the minimal invariants required for LU equivalence, showing the number varies with the spectra of reduced states.

Findings

Number of invariants depends on the spectra of reduced states

Geometric methods used to compute dimensions of reduced spaces

Spectrum-specific differences in invariant requirements

Abstract

Given L-qubit states with the fixed spectra of reduced one-qubit density matrices, we find a formula for the minimal number of invariant polynomials needed for solving local unitary (LU) equivalence problem, that is, problem of deciding if two states can be connected by local unitary operations. Interestingly, this number is not the same for every collection of the spectra. Some spectra require less polynomials to solve LU equivalence problem than others. The result is obtained using geometric methods, i.e. by calculating the dimensions of reduced spaces, stemming from the symplectic reduction procedure.