Robust entanglement detection in arbitrary two-mode Gaussian state: a Stokes-like operator-based approach

TL;DR

This paper introduces a robust, Stokes-like operator-based method for detecting entanglement in arbitrary two-mode Gaussian states, addressing a key open problem with practical measurement schemes and analyzing their robustness.

Contribution

It proposes a new interferometric scheme using Stokes-like operators for entanglement detection in two-mode Gaussian states, extending the applicability of separability criteria.

Findings

The scheme reliably detects entanglement under moderate detection inefficiencies.

It uses single-copy measurements for full state tomography.

A resource-efficient two-copy measurement scheme is discussed.

Abstract

Detection of entanglement in quantum states is one of the most important problems in quantum information processing. However, it is one of the most challenging tasks to find a universal scheme which is also desired to be optimal to detect entanglement for all states of a specific class--as always preferred by experimentalists. Although, the topic is well studied at least in case of lower dimensional compound systems, e.g., two-qubit systems, but in the case of continuous variable systems, this remains as an open problem. Even in the case of two-mode Gaussian states, the problem is not fully solved. In our work, we have tried to address this issue. At first, a limited number of Hermitian operators is given to test the necessary and sufficient criterion on the covariance matrix of separable two-mode Gaussian states. Thereafter, we present an interferometric scheme to test the same separability criterion in which the measurements are being done via Stokes-like operators. In such case, we consider only single-copy measurements on a two-mode Gaussian state at a time and the scheme amounts to the full state tomography. We further analyze the robustness of the proposed detection method against experimentally relevant imperfections and demonstrate that the separability test remains reliable under moderate levels of detection inefficiency. Although this latter approach is a linear optics-based one, nevertheless it is not an economic scheme. Resource-wise a more economical scheme than the full state tomography is obtained if we consider measurements on two copies of the state at a time. However, optimality of the scheme is not yet known.