Superresolving collective quantum measurements

We demonstrate a method for superresolving signals encoded as finite mixtures of bosonic modes using collective measurements that exploit permutation symmetry. Specifically, we use multiple copies of the state $\ensuremath{\rho}$ of the finite mixture to extract an estimate for the purity of $\ensuremath{\rho}$ via a spectrum measurement, the weak Schur-sampling measurement. Depending on the outcome, we then further fine-grain the measurement to optimally extract an estimate of the relative intensity between the two incoherent mixtures. Our protocol furnishes simultaneous estimates for both the relative intensity and the separation of incoherent signals saturating the multiparameter Cram\'er-Rao bound and is robust against misalignment errors. We also provide viable experimental avenues for implementing such collective measurements in different setups.