  Here, we review some quantum architectures designed for the engineering of
the N00N state, a bipartite maximally entangled state crucial in quantum
metrology applications. The fundamental concept underlying these schemes is the
transformation of the initial state $|N\rangle \otimes |0\rangle$ to the N00N
state $\frac{1}{\sqrt{2}} (|N\rangle \otimes|0\rangle +|0\rangle
\otimes|N\rangle)$, where $|N\rangle$ and $|0\rangle$ are the Fock states with
$N$ and $0$ excitations. We show that this state can be generated as a
superposition of modes of quantum light, a combination of light and motion, or
a superposition of two spin ensembles. The approach discussed here can generate
mesoscopic and macroscopic entangled states, such as entangled coherent and
squeezed states, as well. We show that a large class of maximally entangled
states can be achieved in such an architecture. The extension of these state
engineering methods to the multi-mode setting is also discussed.
