Efficient entanglement length measurements for photonic cluster state   sources
  We present a procedure for confirming the generation of long strings of photons in an entangled (cluster) state that does not rely on complete state tomography and that works even at low collection efficiencies. The scheme has the added advantage of being passive - it does not require switching of optical elements to perform measurements in different bases for instance. 
Ilai Schwarz and Terry Rudolph
quant-ph