  In optical dipole traps, the excited rotational states of a molecule may
experience a very different light shift than the ground state. For particles
with two polarizability components (parallel and perpendicular), such as linear
$^1\Sigma$ molecules, the differential shift can be nulled by choice of
elliptical polarization. When one component of the polarization vector is $\pm
i\sqrt{2}$ times the orthogonal component, the light shift for a sublevel of
excited rotational states approaches that of the ground state at high optical
intensity. In this case, fluctuating trap intensity need not limit coherence
between ground and excited rotational states.
