  Interfaces between transition-metal oxides are able to host two-dimensional
electron gases (2DEGs) and exhibit exotic quantum phenomena. Here we report the
observation of superconductivity below 230 mK for the heterostructure composed
of SrNbO3 (SNO) and SrTiO3 (STO). Different from some other counterparts with
two insulators, the metallic SNO provides a novel mechanism to form a quasi
2DEG by charge transfer from bulk towards interface under strain. The
superconductivity, residing within the strained SNO layer near the interface,
is contributed by an electron system with record-low carrier density. Notably,
although embedded in a normal metallic layer with a carrier density 4 to 5
orders higher, the electron system is still uniquely well-protected to retain
high mobility and lies deep in extreme quantum regime.
