In quantum optics the Hong-Ou-Mandel (HOM) effect is seen as a mechanism for creating NOON States from Bell States. Has this been demonstrated experimentally, how efficient is the process and how difficult is it to achieve?

The HOM effect of quantum optics demonstrates how given an option (by eg. a beamsplitter) photons of similar 'character' (polarisation, spatial location and spectral purity) prefer to bunch together, the exact opposite of fermions.

The Bell States can be created by the nonlinear optics effect of spontaneous parametric down-conversion (SPDC), so the HOM effect from a beamsplitter might then be capable of generating NOON states from these, for use in eg metrology. The SPDC in itself is very inefficient, so together with the efficiency of the HOM effect, is this a good mechanism to create NOON states?

How else may NOON States be generated in quantum optics?

many thanks, Neil