Quantifying the momentum correlation between two light beams by detecting one

Significance Entanglement as a fundamental concept of quantum physics is manifested in correlations between particles. The correlation between two particles is usually measured by detecting both of them. Here, we present the results of an experiment based on a unique concept, where the momentum correlation between two photons is measured by detecting only one of them. This measurement is possible by exploiting the quantum mechanical complementarity between path-distinguishability and interference. Our approach can potentially be generalized to measure other higher-order correlations in lower order, which would enable experimental access to a broader class of correlated quantum systems, particularly in situations in which technical limitations make it impossible to efficiently detect one of the correlated particles.