  Nonclassical states, having no classical analogue, promise advantage in the
performance in several fields of technology, such as computation,
communication, sensing. This led to an escalated interest in the recent years
for the generation and characterization of nonclassical states in various
physical systems of interest. Keeping this in mind, we examined generation of
various lower- and higher-order nonclassical states in both codirectional and
contradirectional asymmetric nonlinear optical couplers operating under second
harmonic generation with the help of moments-based criteria. Using another such
system (a symmetric nonlinear optical coupler), we have also established the
possibility of observing quantum Zeno and anti-Zeno effects and further reduced
the obtained results to yield the corresponding expressions for the asymmetric
coupler. These studies have been accomplished using a complete quantum
description of the system considering all the field modes as weak. Further,
characterization of nonclassicality in spin systems using quasiprobability
distributions has been performed, which has also provided us the dynamics of
the amount of nonclassicality (using nonclassical volume). As the
reconstruction of quasiprobability distributions is possible with the help of
tomograms accessible in the experiments, tomograms are computed here for both
finite and infinite dimensional systems. Finally, a set of controlled quantum
communication (both insecure and secure) schemes with minimum number of
entangled qubits has been proposed, which is also analyzed over Markovian
channels. The optimized controlled secure quantum communication scheme is
observed to be reducible to several cryptographic tasks. Using this interesting
observation, we obtained the performance of a set of cryptographic schemes over
non-Markovian channels, too.
