Strong Einstein-Podolsky-Rosen steering with unconditional entangled states

Abstract

In 1935 Schr\"odinger introduced the terms entanglement and steering in the context of the famous gedanken experiment discussed by Einstein, Podolsky, and Rosen (EPR). Here, we report on a sixfold increase of the observed EPR-steering effect with regard to previous experiments, as quantified by the Reid criterion. We achieved an unprecedented low conditional variance product of about $0.04<1$, where 1 is the upper bound below which steering is demonstrated. The steering effect was observed on an unconditional two-mode-squeezed entangled state that contained a total vacuum state contribution of less than 8$%$, including detection imperfections. Together with the achieved high interference contrast between the entangled state and a bright coherent laser field, our state is compatible with efficient applications in high-power laser interferometers and fiber-based networks for entanglement distribution.