  We show that the Rashba effect at the polar perovskite surfaces and
interfaces can be tuned by manipulating the two-dimensional electron gas (2DEG)
by an applied electric field, using it to draw the 2DEG out to the surface or
push it deeper into the bulk, thereby controlling the surface-sensitive
phenomenon. These ideas are illustrated by a comprehensive density-functional
study of the recently-discovered polar KTaO$_3$ surface. Analytical results
obtained with a tight-binding model unravel the interplay between the various
factors affecting the Rashba effect such as the strength of the spin-orbit
interaction and the surface-induced asymmetry. Our work helps interpret the
recent experiments on the KTaO$_3$ surface as well as the SrTiO$_3$/LaAlO$_3$
interface.
