  We have investigated the illumination effect on the magnetotransport
properties of a two-dimensional electron system at the LaAlO$_3$/SrTiO$_3$
interface. The illumination significantly reduces the zero-field sheet
resistance, eliminates the Kondo effect at low-temperature, and switches the
negative magnetoresistance into the positive one. A large increase in the
density of high-mobility carriers after illumination leads to quantum
oscillations in the magnetoresistance originating from the Landau quantization.
The carrier density ($\sim 2 \times 10^{12}$ cm$^{-2}$) and effective mass
($\sim 1.7 ~m_e$) estimated from the oscillations suggest that the
high-mobility electrons occupy the d$_{xz/yz}$ subbands of Ti:t$_{2g}$ orbital
extending deep within the conducting sheet of SrTiO$_3$. Our results
demonstrate that the illumination which induces additional carriers at the
interface can pave the way to control the Kondo-like scattering and study the
quantum transport in the complex oxide heterostructures.
