Laser Cooling

Physics: Doppler cooling: atom moving toward laser sees blue-shifted photon closer to resonance → more absorption → recoil decelerates atom. Scattering force F = ℏk·Γ·(I/I_sat)/(1 + I/I_sat + 4δ²/Γ²). Doppler temperature limit k_BT_D = ℏΓ/2 (for ⁸⁷Rb: Γ/2π = 6 MHz → T_D ≈ 146 μK). Sub-Doppler (Sisyphus) cooling: spatially varying polarization creates optical potential hills; atoms climb and lose kinetic energy, re-pump near top. MOT: 6-beam optical molasses + quadrupole magnetic gradient. BEC in alkali atoms first achieved 1995 (Cornell-Wieman, ⁸⁷Rb; Ketterle, Na); Nobel 2001.