  We present a protocol for increasing the entanglement between two atomic
ensembles based on applying an approximate atom-light beamsplitter
transformation to both ensembles. The effective asymmetric atom-light
beamsplitter is created via a double-pass quantum non-demolition interaction
between polarized light and a spin polarized atomic ensemble, derived from the
linearised dipole interaction. The entanglement concentration protocol itself
uses the procrustean method, similar to that first devised for light by Browne
et al [2003 Phys Rev A 67 062320] and includes photon counting after the
interaction as the required non-Gaussian element. We calculate the output
logarithmic negativity in this scheme and show that entanglement between
macroscopic ensembles can be increased with probabilities comparable with those
for the light scheme even if on-off detectors of low efficiency are used.
