  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.
