Multimode-to-monomode guided-atom lasers: An entropic analysis

We have experimentally demonstrated a high level of control of the mode populations of guided-atom lasers (GALs) by showing that the entropies per particle of an optically GAL and the one of the trapped Bose-Einstein condensate (BEC) from which it has been produced are the same. The BEC is prepared in a crossed beam optical dipole trap. We have achieved isentropic outcoupling for both magnetic and optical schemes. We can prepare GAL in a nearly pure monomode regime (85% in the ground state). Furthermore, optical outcoupling enables the production of spinor guided-atom lasers and opens the possibility to tailor their polarization.

Figure 1

(Color online) (a) Absorption image of a BEC of ${}^{87}\text{R}\text{b}$ atoms released from a crossed dipole trap, after 20 ms time of flight. Inset, top right: atom density across a line through the cloud center. Inset, bottom right: crossed dipole trap laser geometry. (b) Absorption image of a guided-atom laser, outcoupled from the same BEC and imaged in the same conditions. The atom laser has a mean excitation number in the transverse modes of $⟨n⟩=4$. Inset: atom density integrated along the guide axis (1). A bimodal density structure is evident, as in (a) above.

Figure 2

(Color online) Measured condensed fraction $N_0/N_b$ (open circles) of the source BEC and mean excitation number $⟨n⟩$ (solid circles) of the guided-atom laser produced from the source BEC by magnetic outcoupling. Both are shown as a function of the temperature of the trapped source BEC immediately prior to the outcoupling.

Figure 3

(Color online) Entropy per atom of the atom laser $\left(S_a/N_a{k}_B\right)$ against the entropy per atom of the source BEC $\left(S_b/N_b{k}_B\right)$. Each point is the average of ten measurements. The different points correspond to different initial temperatures of the BEC. Solid circles are for magnetic outcoupling, while open circles are for optical outcoupling. The solid line is the expected result for equal entropy per particle of both systems.

Figure 4

(Color online) (a) Spinor guided-atom laser obtained by optically outcoupling atoms from a spinor condensate. (b) The three components $m_F=-1,0,+1$ are separated by applying a Stern and Gerlach field during the time of flight.