  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.
