On the role of entanglement in two-photon metrology

The role of entanglement in metrology with photon pairs is investigated. We consider the situations where either one photon only interacts with a probe, or where both photons interact with two independent systems. Based on arguments from quantum information theory, we show that the presence of entanglement in the state cannot lead to results, which could not be obtained with separable states. This shows that entanglement can only provide advantages when genuine two-photon interactions are present.