  While the fundamental principles of light-matter interaction are
well-understood and drive countless technologies, the world of multiphoton
processes remains a fascinating puzzle, holding the potential to drastically
alter our understanding of how light interacts with matter at its most basic
level. This rich interplay of light and matter unveils novel phenomena that can
be harnessed for sensing with exceptional precision, as exemplified by
multiphoton quantum sensing. This thesis delves into the applications of
multiphoton quantum protocols, particularly in imaging, communication, and
plasmonic sensing, to surpass classical limitations and achieve enhanced
sensitivity. We explore the potential of multiphoton quantum processes,
particularly in the nanoscale regime and within subsystems of macroscopic
systems, where novel and ultra-sensitive sensing methodologies emerge.
Subsequent chapters of this thesis demonstrate the transformative potential of
multiphoton quantum sensing, elucidating the design, implementation, and
experimental results of specific sensing protocols tailored to diverse
applications. Our analysis combines experimental observations and theoretical
predictions to assess the sensitivity and performance of these protocols.
Additionally, the thesis discusses potential future directions and advancements
in the field, envisioning applications in biomolecule detection, environmental
monitoring, and fundamental studies of light-matter interactions at the
nanoscale. Concluding reflections highlight the implications of multiphoton
quantum sensing across scientific disciplines and lay the groundwork for future
research endeavors.
