  One of the most fundamental phenomena and a reminder of the electron's
relativistic nature is the Rashba spin splitting for broken inversion symmetry.
Usually this splitting is a tiny relativistic correction, hardly discernible in
experiment. Interfacing a ferroelectric, BaTiO$_3$, and a heavy 5$d$ metal with
a large spin-orbit coupling, Ba(Os,Ir)O$_3$, we show that giant Rashba spin
splittings are indeed possible and even fully controllable by an external
electric field. Based on density functional theory and a microscopic tight
binding understanding, we conclude that the electric field is amplified and
stored as a ferroelectric Ti-O distortion which, through the network of oxygen
octahedra, also induces a large Os-O distortion. The BaTiO$_3$/BaOsO$_3$
heterostructure is hence the ideal test station for studying the fundamentals
of the Rashba effect. It allows intriguing application such as the Datta-Das
transistor to operate at room temperature.
