Extreme non-negative Wigner functions

TL;DR

This paper characterizes the extreme points of Wigner-positive quantum states using convex geometry, introducing new maps and methods to generate and understand these states, especially in mixed states and low dimensions.

Contribution

It provides a constructive method to identify extreme Wigner-positive states, including a new quantum map and characterization techniques for phase-invariant states.

Findings

Generated all extreme WPS in low dimensions from beam-splitter states.

Introduced the Vertigo map linking WPQS and WPS.

Unveiled the structure of mixed Wigner-positive states.

Abstract

Providing an operational characterization of the Wigner-positive states (WPS), i.e., the set of quantum states with non-negative Wigner function, is a longstanding open problem. For pure states, the only WPS are Gaussian states, but the situation is considerably more subtle for mixed states. Here, we approach the problem using convex geometry, reducing the question to the characterization of the extreme points of the set of WPS. We give a constructive method to generate a large class of such extreme WPS, which combines the following steps: (i) we characterize the phase-invariant extreme points of the superset of Wigner-positive quasi-states (WPQS); (ii) we introduce a new quantum map, named Vertigo map, which maps extreme WPQS to extreme WPS while preserving phase invariance; (iii) we identify families of extremality-preserving maps and use them to obtain extreme WPS while relaxing phase invariance. Our construction generates all extreme WPS of low dimension, starting from a specific kind of WPS known as beam-splitter states. Our results build upon new mathematical properties of the set of WPS derived in a companion paper and unveil the remarkable structure of mixed states with non-negative Wigner functions.