Fig. 1: Interferometric scheme.
From: Quantum-enhanced interferometry with large heralded photon-number states
a Two type-II parametric down-conversion sources each produce orthogonally-polarized pairs of beams that are separated using polarizing beam splitters. By measuring one of the beams from each source with a photon-number-resolving detector, we herald a pair of photon-number states \(\left|{h}_{1},{h}_{2}\right\rangle\). We inject this probe into an interferometer and perform photon counting at the output to estimate the unknown phase difference ϕ. b Quantum Fisher information \({\mathcal{Q}}\) calculated for 8-photon (N = h1 + h2 = 8) probes inside the interferometer as a function of the signal transmissivity ηs which is assumed to be equal in both interferometer modes. Coloured curve in the main figure plots \({\mathcal{Q}}\) of the probe with the optimal Δ = ∣h1 − h2∣ for a given ηs, while the inset shows the full curves of each probe for ηs ∈ [0.4, 0.75]. Our probe approximates the performance of the optimal state [black line] and surpasses that of the N00N state [dashed line] for efficiencies below ~90%. The gray filled region indicates performance below the shot-noise limit.
