Relativistic entanglement in single-particle quantum states using   Non-Linear entanglement witnesses
  In this study, the spin-momentum correlation of one massive spin-1/2 and spin-1 particle states, which are made based on projection of a relativistic spin operator into timelike direction is investigated. It is shown that by using Non-Linear entanglement witnesses (NLEWs), the effect of Lorentz transformation would decrease both the amount and the region of entanglement. 
M. A. Jafarizadeh, M. Mahdian
quant-ph