Paired-atom laser beams created via four-wave mixing

A method to create paired-atom laser beams from a metastable helium atom laser via four-wave mixing is demonstrated. Radio-frequency outcoupling is used to extract atoms from a Bose-Einstein condensate near the center of the condensate and initiate scattering between trapped and untrapped atoms. The unequal strengths of the interactions for different internal states allows an energy-momentum resonance which leads to the creation of pairs of atoms scattered from the zero-velocity condensate. The resulting scattered beams are well separated from the main atom laser in the two-dimensional transverse atom laser profile. Numerical simulations of the system are in good agreement with the observed atom laser spatial profiles and indicate that the scattered beams are generated by a four-wave mixing process, suggesting that the beams are correlated.

Figure 1

(Color online) First two rows show experimental atom laser spatial profiles observed on our MCP $4\mathrm{cm}$ below the trap, in a three-dimensional rendering (left) and a two-dimensional image (right). Both sets of data were taken for an outcoupling detuning of $2\mathrm{kHz}$; however, the Rabi frequency is increased by an order of magnitude between the two sets. The upper row shows the usual ${\mathrm{He}}^{*}$-atom laser [15], while the middle row demonstrates the appearance of the resonant scattering peaks. The bottom row is the result of a simulation of the second experiment. The weak axis of the trap is aligned along the vertical axis of the right hand images.

Figure 2

(Color online) Schematic diagram of experimental configuration. The four-wave mixing process generates momentum components along the weak trapping axis, which get outcoupled to two cones. After outcoupling from the condensate, atoms fall $4\mathrm{cm}$ to the MCP detector.

Figure 3

(Color online) A Gross-Pitaevskii simulation of the in-trap condensate density after $3.6\mathrm{ms}$ of outcoupling, magnified to show the central region of the condensate, where the scattering-induced grating has high visibility. The weak and tight trap directions are $x$ and $y$, respectively.