  Two-photon anti-bunching at a beamsplitter is only possible if the photons
are entangled in a specific state, anti-symmetric in the spatial modes. Thus,
observation of anti-bunching is an indication of entanglement in a degree of
freedom which might not be easily accessible in an experiment. We
experimentally demonstrate this concept in the case of the interference of two
frequency entangled photons with continuous frequency detunings. The principle
of anti-symmetrisation of the spatial part of a wavefunction and subsequent
detection of hidden entanglement via anti-bunching at a beamsplitter may
facilitate the observation of entanglement in other systems, like atomic
ensembles or Bose-Einstein condensates. The analogue for fermionic systems
would be to observe bunching.
