Entanglement generated between a single atom and a laser pulse

We quantify the entanglement generated between an atom and a laser pulse in free space. We find that the entanglement calculated using a simple closed-system Jaynes-Cummings Hamiltonian is in remarkable agreement with a full open-system calculation, even though the free-space geometry is far from the strong-coupling regime of cavity QED. We explain this result using a simple model in which the atom couples weakly to the laser, while coupling strongly to the vacuum. Additionally we place an upper bound on the total entanglement between the atom and all paraxial modes using a quantum trajectories unravelling. This upper bound provides a benchmark for atom-laser coupling.