Frontier physics

Optical trapping and laser cooling

Photon scattering provides Doppler cooling, while intensity gradients create dipole potentials that trap atoms.

Photon scattering provides Doppler cooling, while intensity gradients create dipole potentials that trap atoms.

Model and interpretation

In Doppler cooling, a red-detuned beam is Doppler-shifted closer to resonance for an atom moving toward it; spontaneous-emission recoil averages to zero, leaving a mean damping force. A dipole trap uses the AC Stark shift: an atom of polarizability α experiences U(r)=−α⟨E²⟩/2, attracting ground-state atoms to high intensity for red detuning.

Quick check

For a two-level transition of resonance frequency $\omega_0$, a red-detuned beam with $\Delta=\omega_L-\omega_0<0$ appears closer to resonance to an atom moving toward it, increasing scattering; each photon transfers momentum $\hbar k$ opposite the velocity. The one-dimensional Doppler limit is $T_D=\hbar\Gamma/(2k_B)$, but cooling below it requires mechanisms such as Sisyphus cooling or evaporation in a trap.

Doppler cooling and optical trapping serve different roles: repeated scattering removes kinetic energy, whereas an intensity-dependent dipole potential can confine atoms with little scattering. Far from resonance, the potential scales roughly as $U\propto I/\Delta$, while scattering falls rapidly with detuning. Balancing confinement, photon-recoil heating, and collisions determines whether a cold gas or a Bose–Einstein condensate can be reached.

In a magneto-optical trap, cold atoms are confined so elastic collisions redistribute energy. Selective evaporation removes the hottest atoms; rethermalization lowers the temperature of those remaining. When phase-space density $n\lambda_{dB}^3$ approaches unity, quantum statistics becomes important and a Bose–Einstein condensate may form.