For example, if one photon is always emitted in the exactly opposite direction of the other, the two photons show perfect correlation. On the other hand, the two photons could be emitted only approximately in opposite directions, corresponding to a weak correlation, or even completely independent of each other, in which case their momenta are uncorrelated. A measurement of the correlation usually requires the detection of both photons. In a recent experiment, scientists at IQOQI Vienna demonstrated a new method to measure the strength of the momentum correlation between two photons without detecting one of them.

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.

„Quantifying the momentum correlation between two light beams by detecting one”; A. Hochrainer, M.Lahiri, R. Lapkiewicz, G. B. Lemos, A. Zeilinger; PNAS Vol. 114, No. 7, 1508-1511