  A most fundamental and longstanding goal in spintronics is to electrically
tune highly efficient spin injectors and detectors, preferably compatible with
nanoscale electronics. Here, we demonstrate all these points using
semiconductor quantum dots (QDs), individually spin-polarized by ferromagnetic
split-gates (FSGs). As a proof of principle, we fabricated a double QD spin
valve consisting of two weakly coupled semiconducting QDs in an InAs nanowire
(NW), each with independent FSGs that can be magnetized in parallel or
anti-parallel. In tunneling magnetoresistance (TMR) experiments at zero
external magnetic field, we find a strongly reduced spin valve conductance for
the two anti-parallel configurations, with a single QD polarization of $\sim
27\%$. The TMR can be significantly improved by a small external field and
optimized gate voltages, which results in a continuously electrically tunable
TMR between $+80\%$ and $-90\%$. A simple model quantitatively reproduces all
our findings, suggesting a gate tunable QD polarization of $\pm 80\%$. Such
versatile spin-polarized QDs are suitable for various applications, for example
in spin projection and correlation experiments in a large variety of
nanoelectronics experiments.
