Researchers have developed a method to certify and quantify continuous spatial entanglement in photon pair sources without requiring two-photon coincidence counting. The technique leverages stimulated emission, where a bright classical beam seeds the signal mode of a spontaneous parametric down-conversion (SPDC) source, and the resulting stimulated idler field is measured via classical intensity detection. This approach significantly simplifies the characterization of entangled photon sources, especially under challenging experimental conditions.

Traditionally, verifying quantum entanglement necessitates the simultaneous detection of entangled photons, which involves complex coincidence counting systems and meticulous optical alignment. The new method replaces this coincidence detection with intensity measurements, which are more robust and faster. By applying this strategy to the continuous transverse spatial degrees of freedom of an SPDC source, the scientists demonstrated that spatial entanglement can be certified solely through intensity measurements.

The capability of this method to certify entanglement was demonstrated using variance-based entanglement and steering witnesses, as well as the Fedorov ratio. This innovation is particularly valuable for studying the entanglement properties of photon pair sources in settings where precise alignment and photon counting measurements are difficult and time-consuming. It opens new avenues for the efficient characterization of complex quantum states in quantum computing and communication applications.