Alignment of cold Rb atoms by optical pumping*

Atoms in a gas have random direction, speed, and orientation.  We slow Rb atoms by laser cooling, then use them to study quantum wave reflection. In this, it is advantageous for the atoms’ dipole moments to all point in the same direction. To align these, we apply circularly polarized laser light in a uniform magnetic field.  Absorption and scattering of light changes the atomic angular momentum and dipole orientation, a process known as optical pumping (OP). Controlling the orientation will allow us to investigate its effect on the reflection of rubidium atoms. To get a smooth intensity profile, we built a spatial filter combined with a beam expander.  In a typical experiment, we first slow down and collect a cloud of atoms, then turn on a uniform magnetic field and OP light beam for 5 msec, and finally measure the reflection of atoms from a magnetized surface. We vary the polarizer used for the OP light, and observe a change in the reflected atom signal. We see a maximum when the atoms’ dipole moments are aligned antiparallel with the magnetic field, and minimum at polarizer angle 90 degrees after, when the dipole moments are aligned parallel to the field, as expected from quantum theory.