Magnetic-field-induced laser cooling below the Doppler limit

We report a new cooling scheme used to actively collimate an atomic beam of ${}_{}{}^{85}{}_{}{}^{}\mathrm{Rb}$ with one-dimensional optical molasses. The atoms’ transverse velocity is damped by a standing wave of circularly polarized light in a magnetic field of ∼20 μT to 2 cm/s (rms), well below the Doppler velocity $v_D$=(7ħγ/20M${)}^{1/2}$∼10 cm/s for one-dimensional optical molasses. The minimum measured velocity is limited only by the experimental geometry. We have developed a model in which the mixing by the field and optical pumping combine to form a cycle that cools below the Doppler limit, in agreement with our measurements.