Fig. 4

Truth table and heralded generation of Bell states. a Truth table showing the probability of each computational-basis output for each computational-basis input. The ideal gate behavior is indicated by the wireframe. The target qubit cross-talk is due to imperfect two-photon interference between partially distinguishable photons from our two sources. The overlap with the ideal truthtable is (83.8 ± 2.6)%. b The real (large) and imaginary (small) parts of the density matrices for the four states produced by the CNOT, depending on which ancilla measurement outcome occurs. The fidelity of these matrices with their respective target Bell states (indicated by the wireframe) are 76% \(\left( {\left| {{\mathrm{\Phi }}^ + } \right\rangle } \right)\), 76% \(\left( {\left| {{\mathrm{\Phi }}^ - } \right\rangle } \right)\),74% \(\left( {\left| {{\mathrm{\Psi }}^ + } \right\rangle } \right)\), and 76% \(\left( {\left| {{\mathrm{\Phi }}^ - } \right\rangle } \right)\) all with errors of ±5%