  We demonstrate sum-frequency generation in a nonlinear whispering gallery
mode resonator between a telecom wavelength and the Rb D2 line, achieved
through natural phase matching. Due to the strong optical field confinement and
ultra high Q of the cavity, we achieve a 1000-fold enhancement in the
conversion efficiency compared to existing waveguide-based devices. The
experimental data are in agreement with the nonlinear dynamics and phase
matching theory in the spherical geometry employed. The experimental and
theoretical results point to a new platform to manipulate the color and quantum
states of light waves toward applications such as atomic memory based quantum
networking and logic operations with optical signals.
