Divisibility classes of qubit maps and singular Gaussian channels

TL;DR

This paper investigates the divisibility properties of qubit quantum channels and characterizes non-Gaussian one-mode Gaussian channels, revealing new insights into their structure, implementation, and the existence of non-Gaussian functional forms.

Contribution

It introduces tools to test divisibility of quantum channels, characterizes the space of qubit channels, and fully characterizes non-Gaussian Gaussian channels including singular cases.

Findings

Infinitely divisible channels are equivalent to those implementable by one-parameter semigroups.

Existence of non-Gaussian functional forms in Gaussian channels.

Characterization of singular Gaussian channels and their transformations.

Abstract

We present two projects concerning the main part of my PhD work. In the first one we study quantum channels, which are the most general operations mapping quantum states into quantum states, from the point of view of their divisibility properties. We introduced tools to test if a given quantum channel can be implemented by a process described by a Lindblad master equation. This in turn defines channels that can be divided in such a way that they form a one-parameter semigroup, thus introducing the most restricted studied divisibility type of this work. Using our results, together with the study of other types of divisibility that can be found in the literature, we characterized the space of qubit quantum channels. We found interesting results connecting the concept of entanglement-breaking channel and infinitesimal divisibility. Additionally we proved that infinitely divisible channels are equivalent to the ones that are implementable by one-parameter semigroups, opening this question for more general channel spaces. In the second project we study the functional forms of one-mode Gaussian quantum channels in the position state representation, beyond Gaussian functional forms. We perform a black-box characterization using complete positivity and trace preserving conditions, and report the existence of two subsets that do not have a functional Gaussian form. The study covers as particular limit the case of singular channels, thus connecting our results with the known classification scheme based on canonical forms. Our full characterization of Gaussian channels without Gaussian functional form is completed by showing how Gaussian states are transformed under these operations, and by deriving the conditions for the existence of master equations for the non-singular cases.