Experimental measurement of the collectibility of two-qubit states

TL;DR

This paper demonstrates an experimental method to measure the collectibility, a nonlinear entanglement witness, for various two-qubit states, showing its feasibility and robustness in photonic systems.

Contribution

The first experimental implementation of measuring collectibility as an entanglement witness for two-qubit states, applicable to both pure and mixed states.

Findings

Measurement is feasible for different two-qubit states.

The procedure is robust against experimental imperfections.

Collectibility correlates with entanglement measures like negativity.

Abstract

We present a proof-of-principle experiment demonstrating measurement of the collectibility, a nonlinear entanglement witness proposed by Rudnicki et al. [Phys. Rev. Lett. 107, 150502 (2011)]. This entanglement witness works for both mixed and pure two-qubit states. In the later case it can be used to measure entanglement in terms of the negativity. We measured the collectibility for three distinct classes of photonic polarization-encoded two-qubit states, i.e., maximally entangled, separable and maximally mixed states. We demonstrate that the measurement procedure is feasible and robust against typical experimental shortcomings such as imperfect two-photon indistinguishability.