Comments to Asking Photons Where They Have Been (A Danan et al, Phys.   Rev. Lett. 111, 240402 (2013))
  I critize some of the conclusions in the paper by Danan et al, in particular their claim that '(S)ome (of the photons) have been inside the nested interferometer ... but they never entered and never left (it) ...'. A standard quantum-mechanical analysis also casts doubts on their statement that '(o)nly the description with forward and backward evolving quantum states provides a simple and intuitive picture of pre- and postselected quantum particles.' 
Bengt E Y Svensson
quant-ph