Complete criterion for convex-Gaussian state detection

TL;DR

This paper introduces a new, complete criterion based on semidefinite programming to determine if a fermionic state is a convex combination of pure Gaussian states, improving state characterization methods.

Contribution

It provides a complete inside characterization criterion for convex-Gaussian states, complementing existing outside criteria, with explicit proofs and approximation guarantees.

Findings

Criterion is complete and characterizes convex-Gaussian states from the inside.

States passing the criterion can be approximated arbitrarily closely by convex-Gaussian states.

The criterion is formulated as a sequence of solvable semidefinite programs.

Abstract

We present a new criterion that determines whether a fermionic state is a convex combination of pure Gaussian states. This criterion is complete and characterizes the set of convex-Gaussian states from the inside. If a state passes a program it is a convex-Gaussian state and any convex-Gaussian state can be approximated with arbitrary precision by states passing the criterion. The criterion is presented in the form of a sequence of solvable semidefinite programs. It is also complementary to the one developed by de Melo, Cwiklinski and Terhal, which aims at characterizing the set of convex-Gaussian states from the outside. Here we present an explicit proof that criterion by de Melo et al. is complete, by estimating a distance between an n-extendible state, a state that passes the criterion, to the set of convex-Gaussian states.