  Interfacial superconductors have the potential to revolutionize electronics,
quantum computing, and fundamental physics due to their enhanced
superconducting properties and ability to create new types of superconductors.
The emergence of superconductivity at the interface of
La$_{2-x}$Sr$_{x}$CuO$_{4}$/La$_{2}$CuO$_{4}$ (LSCO/LCO), with a T$_c$
enhancement of $\sim$ 10 K compared to the La$_{2-x}$Sr$_{x}$CuO$_{4}$ bulk
single crystals, provides an exciting opportunity to study quantum phenomena in
reduced dimensions. To investigate the carrier distribution and excitations in
interfacial superconductors, we combine O K-edge resonant inelastic X-ray
scattering and atomic-resolved scanning transmission electron microscopy
measurements to study La$_{2-x}$Sr$_{x}$CuO$_{4}$/La$_{2}$CuO$_{4}$
superlattices (x=0.15, 0.45) and bulk La$_{1.55}$Sr$_{0.45}$CuO$_{4}$ films. We
find direct evidence of charge redistribution, charge order and plasmon in
LSCO/LCO superlattices. Notably, the observed behaviors of charge order and
plasmon deviate from the anticipated properties of individual constituents or
the average doping level of the superlattice. Instead, they conform
harmoniously to the effective doping, a critical parameter governed by the
T$_c$ of interfacial superconductors.
