Imaginary Entanglement as Cost of Unitarity
  This report is about contradiction between fidelity needed to determine the entanglement and concomitant noise that always accompanies precise measurement.   Account of quantum properties of field leads to additional noise caused by multiple particle creation through nonunitarity of quantum field representation in embedded sections of space (Unruh noise).   Causes of quantum noise vanish at leaving off assumption about statistical independence of detectors. Smearing of detector leads to elimination of causes of Unruh noise and to emergence of imaginary entanglement of few mode states caused by overlap of detector sections. 
Constantin V. Usenko