(color online) Principles of detection of coherent evolution through the violation of Leggett–Garg-type inequalities. Two macroscopically separated crystals absorbed H and V polarized photons separately. The two crystals were prepared with two AFCs with a small frequency detuning. The single photons were prepared with an H + V state before the memory crystal. After absorption of single photons, the evolution happened and probed by detecting the echo of the stored photons. K₋ and K₊ were calculated by choosing the appropriate observable M(t).