  Currently, 'time' does not play any essential role in quantum information
theory. In this sense, quantum information theory is underdeveloped similarly
to how quantum physics was underdeveloped before Erwin Schrodinger introduced
his famous equation for the evolution of a quantum wave function. In this
review article, we cope with the problem of time for one of the central
quantities in quantum information theory: entropy. Recently, a replica trick
formalism, the so-called 'multiple parallel world' formalism, has been proposed
that revolutionizes entropy evaluation for quantum systems. This formalism is
one of the first attempts to introduce 'time evolution' in quantum information
theory. With the total entropy being conserved in a closed system, entropy can
flow internally between subsystems; however, we show that this flow is not
limited only to physical correlations as the literature suggest. The nonlinear
dependence of entropy on the density matrix introduces new types of
correlations with no analogue in physical quantities. Evolving a number of
replicas simultaneously makes it possible for them to exchange particles
between different replicas. We will summarize some of the recent news about
entropy in some example quantum devices. Moreover, we take a quick look at a
new correspondence that was recently proposed that provides an interesting link
between quantum information theory and quantum physics. The mere existence of
such a correspondence allows for exploring new physical phenomena as the result
of controlling entanglement in a quantum device.
