2D laser collimation of a cold Cs beam induced by a transverse $B$-field

We describe transverse collimation of a continuous cold cesium beam (longitudinal temperature 75 $\mu$K) induced by a two-dimensional, blue-detuned near-resonant optical lattice. The mechanism described for a lin-$\parallel$-lin configuration is made possible by the application of a transverse magnetic field $B_\perp$. The phenomenon described differs from grey molasses for which any small magnetic field degrades cooling, as well as from magnetically induced laser cooling in red-detuned optical molasses where there are no dark states. The lowest transverse temperature is experimentally found to vary as $B_\perp^2.$ The collimated flux density shows a dip as a function of $B_\perp$, the width of which is proportional to the cube root of the laser intensity, general features predicted by our semi-classical model. This technique provides a sensitive tool for cancelling transverse magnetic fields in situ at the milligauss level.