Are non-vacuum states much relevant for retrieving shock wave memory of spacetime?

Shock wave gives back reaction to spacetime and the information is stored in the memory of background. Quantum memory effect of localised shock wave on Minkowski metric is investigated here. We find that the Wightman function for massless scalar field, on both sides of the wave location, turns out to be the same for usual Minkowski spacetime. Therefore the observables obtained from this, for any kind of observer, do not show the signature of the shock. Moreover, the vacuum states of field on both sides are equivalent. On the contrary, the correlator for the non-vacuum state does memorise the classical shock wave and hence the effect of it is visible in the observables, even for any frame. We argue that rather than vacuum state, the non-vacuum ones are relevant to retrieve the quantum information of classical memory.