Fig. 2: Schematic overview of the experiment.
a The experimental setup comprises state preparation (blue area) and a broadcast channel with a photon measurement stage (red area). We constructed two single-photon pair sources by pumping two identical periodically poled potassium titanyl phosphate crystals (ppKTP) with a modelocked laser centered at 775 nm. Each source produced frequency-degenerate single photons at 1550 nm via type-II spontaneous parametric downconversion (SPDC). One source (solid white rim) generated a maximally entangled state, which was controllably depolarized (solid white background) to tune the parameter α and prepare the state Wα in equation (4). A second source (striped white background) produced a heralded ancilla photon, initialized to \(\left\vert H\right\rangle\). After the broadcast channel (dashed black box), the resulting state was transmitted to three spatially separated parties for projective polarization measurements. Single photons were detected with superconducting nanowire single-photon detectors (SNSPD), and a time-to-digital converter identified fourfold coincidences within a 1 ns window. b Quantum circuit for activation. The broadcast channel, highlighted by the dashed box, consisted of a C-NOT, Hadamard (\({{{{{{{\mathcal{H}}}}}}}}\)), and \({{{{{\mathcal{S}}}}}}=({i \atop 0} \\ {0 \atop 1})\) gates, where i represents an additional π/2 phase. A, B, and C indicate local projective measurements performed by parties Alice, Bob, and Charlie. QWP, quarter-wave plate; HWP, half-wave plate; PBS, polarizing beamsplitter; NPBS, non-polarizing beamsplitter; PPBS, partially polarizing beamsplitter.
