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Quantised motion of an atom in a Gaussian-Laguerre beam

We quantise the centre of mass motion of a neutral Cs atom in the presence of a classical Gaussian-Laguerre$_{10}$ light field in the large detuning limit. This light field possesses orbital angular momentum which is transferred to the atom via spontaneous emissions. We use quantum trajectory and analytic methods to solve the master equation for the 2d centre of mass motion with recoil near the centre of the beam. For appropriate parameters, we observe heating in both the cartesian and polar observables within a few orbits of the atom in the beam. The angular momentum, $\hat{L}$, shows a rapid diffusion which results in $<\hat{L}>$ reaching a maximum and then decreasing to zero. We compare this with analytic results obtained for an atom illuminated by a superposition of Gaussian-Laguerre modes which possess no angular momentum, in the limit of no recoil.