Sub-Doppler laser cooling using electromagnetically induced transparency

We propose a sub-Doppler laser-cooling mechanism that takes advantage of the unique spectral features and extreme dispersion generated by the phenomenon of electromagnetically induced transparency (EIT). EIT is a destructive quantum interference phenomenon experienced by atoms with multiple internal quantum states when illuminated by laser fields with appropriate frequencies. By detuning the lasers slightly from the ``dark resonance,'' we observe that, within the transparency window, atoms can be subject to a strong viscous force, while being only slightly heated by the diffusion caused by spontaneous photon scattering. In contrast to other laser-cooling schemes, such as polarization gradient cooling or EIT-sideband cooling, no external magnetic field or strong external confining potential is required. Using a semiclassical approximation, we derive analytically quantitative expressions for the steady-state temperature, which is confirmed by full quantum mechanical numerical simulations. We find that the lowest achievable temperatures approach the single-photon recoil energy. In addition to dissipative forces, the atoms are subject to a stationary conservative potential, leading to the possibility of spatial confinement. We find that under typical experimental parameters, this effect is weak and stable trapping is not possible.