  The two-dimensional metal forming at the interface between an oxide insulator
and SrTiO3 provides new opportunities for oxide electronics. However, the
quantum Hall effect, one of the most fascinating effects of electrons confined
in two dimensions, remains underexplored at these complex oxide
heterointerfaces. Here, we report the experimental observation of quantized
Hall resistance in a SrTiO3 heterointerface based on the modulation-doped
amorphous-LaAlO$_{3}$/SrTiO$_{3}$ heterostructure, which exhibits both high
electron mobility exceeding 10000 cm$^{2}$/Vs and low carrier density on the
order of ~10$^{12}$ cm$^{-2}$. Along with unambiguous Shubnikov-de Haas
oscillations, the spacing of the quantized Hall resistance suggests that the
interface is comprised of a single quantum well with ten parallel conducting
two-dimensional subbands. This provides new insight into the electronic
structure of conducting oxide interfaces and represents an important step
towards designing and understanding advanced oxide devices.
