Multi-partite entanglement detection with non symmetric probing
  We show that spin squeezing criteria commonly used for entanglement detection can be erroneous, if the probe is not symmetric. We then derive a lower bound on squeezing for separable states in spin systems probed asymmetrically. Using this we further develop a procedure that allows us to verify the degree of entanglement of a quantum state in the spin system. Finally, we apply our method for entanglement verification to existing experimental data, and use it to prove the existence of tri-partite entanglement in a spin squeezed atomic ensemble. 
Luca Dellantonio, Sumanta Das, J\"urgen Appel and Anders S{\o}ndberg   S{\o}rensen
quant-ph cond-mat.quant-gas