Demonstration of a multi-particle collective measurement for optimal quantum state estimation

We experimentally demonstrate a two-particle collective measurement proposed as the optimal solution to a quantum state estimation game. Our results suggest that, in practice, the collective measurement strategy is at least as good as the best local approach, and it achieves a higher average fidelity when accounting for systematic errors. This photonic implementation uses a recently developed universal two-photon projective measurement based on Hong-Ou-Mandel interference, polarization-dependent loss, and unitary operations. We compare the performance to the case where the entangling component of the measurement is suppressed. We further apply the collective measurement to quantum state tomography, observing a near-optimal scaling of the infidelity with the total number of samples.