  We propose a free falling particle in an AdS space as a holographic model of
local quench. Local quenches are triggered by local excitations in a given
quantum system. We calculate the time-evolution of holographic entanglement
entropy. We confirm a logarithmic time-evolution, which is known to be typical
in two dimensional local quenches. To study the structure of quantum
entanglement in general quantum systems, we introduce a new quantity which we
call entanglement density and apply this analysis to quantum quenches. We show
that this quantity is directly related to the energy density in a small size
limit. Moreover, we find a simple relationship between the amount of quantum
information possessed by a massive object and its total energy based on the
AdS/CFT.
