  In many realizations of electron spin qubits the dominant source of
decoherence is the fluctuating nuclear spin bath of the host material. The
slowness of this bath lends itself to a promising mitigation strategy where the
nuclear spin bath is prepared in a narrowed state with suppressed fluctuations.
Here, this approach is realized for a two-electron spin qubit in a GaAs double
quantum dot and a nearly ten-fold increase in the inhomogeneous dephasing time
$T_2^*$ is demonstrated. Between subsequent measurements, the bath is prepared
by using the qubit as a feedback loop that first measures its nuclear
environment by coherent precession, and then polarizes it depending on the
final state. This procedure results in a stable fixed point at a nonzero
polarization gradient between the two dots, which enables fast universal qubit
control.
