  Superpositions of rotational states in polar molecules induce strong,
long-range dipolar interactions. Here we extend the rotational coherence by
nearly one order of magnitude to 8.7(6) ms in a dilute gas of polar
$^{23}$Na$^{40}$K molecules in an optical trap. We demonstrate spin-decoupled
magic trapping, which cancels first-order and reduces second-order differential
light shifts. The latter is achieved with a dc electric field that decouples
nuclear spin, rotation and trapping light field. We observe density-dependent
coherence times, which can be explained by dipolar interactions in the bulk
gas.
