Method for universal detection of two-photon polarization entanglement

TL;DR

This paper introduces a universal, direct experimental method for detecting and quantifying entanglement in any two-qubit mixed state using linear optics, eliminating the need for quantum tomography.

Contribution

It presents the first universal, necessary and sufficient entanglement witness test for two-qubit states that is experimentally feasible with current quantum optics technology.

Findings

Detects entanglement in all two-qubit mixed states

Establishes tight bounds on entanglement amount

Uses minimal measurements for optimal detection

Abstract

Detecting and quantifying quantum entanglement of a given unknown state poses problems that are fundamentally important for quantum information processing. Surprisingly, no direct (i.e., without quantum tomography) universal experimental implementation of a necessary and sufficient test of entanglement has been designed even for a general two-qubit state. Here we propose an experimental method for detecting a collective universal witness, which is a necessary and sufficient test of two-photon polarization entanglement. It allows us to detect entanglement for any two-qubit mixed state and to establish tight upper and lower bounds on its amount. A different element of this method is the sequential character of its main components, which allows us to obtain relatively complicated information about quantum correlations with the help of simple linear-optical elements. As such, this proposal realizes a universal two-qubit entanglement test within the present state of the art of quantum optics. We show the optimality of our setup with respect to the minimal number of measured quantities.