Fig. 3: Experimental activation of nonlocality as a function of the quantum state parameter α.

a Results for the broadcast inequality \({{{{{{{{\mathcal{I}}}}}}}}}_{B}\) of equation (3). Diagonal lines represent theoretical predictions for ideal (dashed gray) and experimentally observed (solid blue) two-photon interference. Red and blue data points indicate activatable and non-activatable states, respectively. A red triangle symbolizes certified nonlocality activation. The vertical dashed line shows the current Bell-local upper bound (α ≤ 0.6875) for isotropic states under projective measurements in the two-party scenario. The gray area indicates the classical region above which broadcast nonlocality is observed. b Locality test for the experimental bipartite states. The dashed curve depicts the certificate results for an ideal isotropic state. Values of the certificate η ≥ 1, shown in the gray area, guarantee the existence of an LHV model for the corresponding quantum state for two-outcome POVMs. Error bars are ± 1 standard deviations in total. Uncertainties in \({{\mathcal{I}}}_{{{{{{B}}}}}}\) arise from Poissonian statistics, whereas the uncertainties in η and α are calculated from Monte Carlo simulations of the different \({\rho }_{\exp }\) that include Poissonian photon-counting noise and systematic errors in measurements (see Methods).