The experiment used two identical sources from which a photon pair could be emitted. An interference pattern was created by only one of the photons using a quantum phenomenon called induced coherence without induced emission. By analyzing this interference pattern, the strength of the momentum correlation between a photon and its partner could be determined, without ever detecting the partner photon.
The results show that information about the correlations in a photon pair can be accessed by measurements on only one of its constituents, given the pair can be created by two identical sources. In the future, the method could lead to new techniques that allow testing entanglement between two particles by measuring only one of them. Apart from being of fundamental interest, such tests would be applicable in many quantum information technologies.
Link to the publication in PNAS:
A. Hochrainer, M.Lahiri, R. Lapkiewicz, G. B. Lemos, A. Zeilinger. „Quantifying the momentum correlation between two light beams by detecting one”.