  The spin-orbit (SO) interaction couples electron spin and momentum via a
relativistic, effective magnetic field. While conveniently facilitating
coherent spin manipulation in semiconductors, the SO interaction also
inherently causes spin relaxation. A unique situation arises when the Rashba
and Dresselhaus SO fields are matched, strongly protecting spins from
relaxation, as recently demonstrated. Quantum computation and spintronics
devices such as the paradigmatic spin transistor could vastly benefit if such
spin protection could be expanded from a single point into a broad range
accessible with in-situ gate-control, making possible tunable SO rotations
under protection from relaxation. Here, we demonstrate broad, independent
control of all relevant SO fields in GaAs quantum wells, allowing us to tune
the Rashba and Dresselhaus SO fields while keeping both locked to each other
using gate voltages. Thus, we can electrically control and simultaneously
protect the spin. Our experiments employ quantum interference corrections to
electrical conductivity as a sensitive probe of SO coupling. Finally, we
combine transport data with numerical SO simulations to precisely quantify all
SO terms.
