Observation of bosonic coalescence of photon pairs

Quantum theory predicts that two indistinguishable photons incident on a beam-splitter interferometer stick together as they exit the device (the pair emerges randomly from one port or the other). We use a special photon-number-resolving energy detector for a direct loophole-free observation of this quantum-interference phenomenon. Simultaneous measurements from two such detectors, one at each beam-splitter output port, confirm the absence of cross-coincidences.