  Experiments on ultracold gases offer unparalleled opportunities to explore
quantum many-body physics, with excellent control over key parameters including
temperature, density, interactions and even dimensionality. In some systems,
atomic interactions can be adjusted by means of magnetic Feshbach resonances,
which have played a crucial role in realizing new many-body phenomena. However,
suitable Feshbach resonances are not always available, and they offer limited
freedom since the magnetic field strength is the only control parameter. Here
we show a new way to tune interactions in one-dimensional quantum gases using
state-dependent dressed potentials, enabling control over non-equilibrium spin
motion in a two-component gas of 87Rb. The accessible range includes the point
of spin-independent interactions where exact quantum many-body solutions are
available and the point where spin motion is frozen. This versatility opens a
new route to experiments on spin waves, spin-"charge" separation and the
relation between superfluidity and magnetism in low-dimensional quantum gases.
