Fig. 4: Quantifying entanglement of the generated states.

a Schematic illustrating the chosen localizable entanglement metric for a 10 photonic qubit state. Qubits are projected into either the X or the Z bases, as indicated, then localizable entanglement is calculated for the two corner qubits (green edges). b Localizable entanglement as a function of number of generated photonic qubits N = 2 × n, as measured by matrix product operator tomography of the experimentally generated states (exp. generated, blue, solid squares), as estimated from process maps (orange, solid circles), and as obtained by master equation simulations taking into account all errors (green, empty squares) and only decoherence (simul. coh. limit (simulated coherence limit), black, open circles), and the localizable entanglement of the ideal state (gray, dashed line). Shown is the mean over all possible paths of length n.