Fig. 3: Two-photon state and entanglement figures of merit as a function of the main collection angle for a CBR quantum dot.

a Two examples of experimental density matrices for different main collection angles θ. Due to collection from a polarized wavevector region, the two-photon state has a well-defined polarization and entanglement is lost. b Intensity and DOP profiles as a function of θ, extrapolated from the BFP measurements. They are reported with the corresponding fidelity of entanglement to the target state \(\left\vert {\phi }^{+}\right\rangle\) and concurrence, computed as a function of the main collection angle θ and for a small k integration range (selected by a pinhole on the BFP, see the SI). For the experimental data, we average over the azimuthal angle ϕ to obtain an average profile for radiation intensity and DOP as a function of only θ. The same quantities are reported in (c) intensity and DOP resulting from far-field FDTD simulations of the emission together with fidelity and concurrence computed on two-photon states obtained by inserting the simulated fields in Eq. (4). Error bars in the experimental data are computed assuming Poissonian distributions of coincidences and they are remarkably small due to the high number of recorded events.