  In this paper, we study how quantum correlation between subsystems changes in
time by investigating time evolution of mutual information and logarithmic
negativity in two protocols of mass quench. Hamiltonian in both protocols is
for 2-dimensional free scalar theory with time-dependent mass: the mass in one
case decreases monotonically and vanishes asymptotically (ECP), and that in the
other decreases monotonically before t = 0, but increases monotonically
afterward, and becomes constant asymptotically (CCP). We study the time
evolution of the quantum correlations under those protocols in two different
limits of the mass quench; fast limit and slow limit depending on the speed
with which the mass is changed. We obtain the following two results: (1) For
the ECP, we find that the time evolution of logarithmic negativity is, when the
distance between the two subsystems is large enough, well-interpreted in terms
of the propagation of relativistic particles created at a time determined by
the limit of the quench we take. On the other hand, the evolution of mutual
information in the ECP depends not only on the relativistic particles but also
on slowly-moving particles. (2) For the CCP, both logarithmic negativity and
mutual information oscillate in time after the quench. When the subsystems are
well-separated, the oscillation of the quantum correlations in the fast limit
is suppressed, and the time evolution looks similar to that under the ECP in
the fast limit.
