  We identify a distinct superconducting phase at the interface of a
La$_{1.48}$Nd$_{0.4}$Sr$_{0.12}$CuO$_4$ (LNSCO)/La$_{1.84}$Sr$_{0.16}$CuO$_4$
(LSCO) epitaxial bilayer system using ac screening measurements. A model based
on inter-diffusion of quasiparticles and condensate at the interface yields a
thickness of $\sim$ 25 nm for the interfacial layer. Two-dimensional
superconductivity of the interface layer appears to be governed by
Kosterlitz-Thouless-Berezinskii transition. A parallel magnetic field
suppresses the superconducting transition temperature of this layer with a pair
breaking parameter $\alpha$ varying as $H^2$.
