Entanglement Experiments

F.J. Duarte

Otfried Gühne, Michael Reimpell, Reinhard F. Werner

We present a method to estimate entanglement measures in experiments. We show how a lower bound on a generic entanglement measure can be derived from the measured expectation values of any finite collection of entanglement witnesses. Hence witness measurements are given a quantitative meaning without the need of further experimental data. We apply our results to a recent multiphoton experiment [M. Bourennane, Phys. Rev. Lett. 92, 087902 (2004)], giving bounds on the entanglement of formation and the geometric measure of entanglement in this experiment.

F. J. Duarte

Jørn Kløvfjell Mjelva

In recent years, there has been a growing interest in the possibility of temporal nonlocality, mirroring the spatial nonlocality supposedly evidenced by the Bell correlations. In this context, Glick (2019) has argued that timelike entanglement and temporal nonlocality is demonstrated in delayed-choice entanglement swapping (DCES) experiments, like that of Ma et al. (2012), Megidish et al. (2013) and Hensen et al. (2015). I will argue that a careful analysis of these experiments shows that they in fact display nothing more than "ordinary" spacelike entanglement, and that any purported timelike entanglement is an artefact of selection bias. Regardless any other reason one may have for challenging the assumption of temporal locality, timelike entanglement as evidenced by these experiments is not among them. I conclude by discussing what lessons on the nature of entanglement might be drawn from an examination of DCES experiments.

Fuli Li, Muhammad Suhail Zubairy

Masanori Sato

We propose experiments of quantum entanglement for investigating the EPR problem with the polarization directions of photons. These experiments are performed to investigate whether or not the polarization directions in an entangled state are affected through quantum entanglement. EPR-type sequential experiments can be performed using a twin-photon beam and two pairs of linear polarization analyzers. If one of the linear polarization analyzers in the second pair is set at a different angle to the first one, it indicates that one of the entangled state photons has changed its polarization direction. In this experiment, we can predict the following two answers. (1) The first pair of linear polarization analyzers projects the entangled state on a nonentangled state, or (2) the "continuity of entanglement" under a pure Hamiltonian evolution is detected.