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.’