Fig. 4: Fidelity measurement of 3-qubit GHZ state and single-photon characterization.

a Illustration of the measurement basis on the Bloch sphere. b, c Histogram of fidelity measurement in the z-basis and x, y-basis, respectively. In c, \(\left\vert+\right\rangle\) and \(\left\vert -\right\rangle\) are eigenstates of the operator \({\hat{{\mathcal{M}}}}_{i}\). The acquisition time for each operator is  ~2 h. The shaded bars in (a–c) indicate the ideal cases. d Measurement of HOM visibility and single-photon purity at different excitation powers (normalized to the π-pulse power). Single-photon indistinguishability V_s is extracted from the y-intercept of the linear extrapolation. g⁽²⁾(0) is measured in the first and third time bins whereas V_HOM is measured in the second time bin, see main text. The error bars represent one standard deviation.