# How do cooling lasers work?

1. Apr 24, 2013

### Pseudo Epsilon

i mean to say how does this method extract energy, ive read the wiki but in my head it seems as though you can only extract as much energy you put in and why can't we use this to get to absolute zero?

2. Apr 25, 2013

### Staff: Mentor

If the laser is slightly red-detuning from an atomic transitions, meaning that the energy of one photon is less than necessary for the transition, you still have a non-zero probability of the atom absorbing that photon. But the emitted photon will be at the energy of the transition, and therefore part of the energy of the atom will be transferred to the emitted photons. By a clever use of the Doppler effect, and laser beams coming from 6 directions (in 3D), you can get a pretty cold gas of atoms.

There are inherent limits to how low this can cool down atoms, the most fundamental being the "recoil temperature", corresponding to the recoil of atom originally at rest will get following the emission of one photon.

3. Apr 26, 2013

### Pseudo Epsilon

why does the doppler efect have to do with it?

4. Apr 26, 2013

### ZapperZ

Staff Emeritus
5. Apr 26, 2013

### Staff: Mentor

Consider one laser propagating along the +z direction. By conservation of momentum, when an atom absorbs a photon of that laser, its momentum along +z increases. After emission, the atom's momentum changes, but because emission is isotropic, the average effect will correspond to a force directed along +z.

The absorption-emission of photons I described is called photon scattering. The rate at which this scattering occurs depends on the detuning of the laser: the closer it is to resonance, the higher the scattering rate. So an atom travelling in the -z direction of that red-detuned laser will see the light as closer to resonance because of the Doppler effect. The force on the atom will be bigger.

Now take two lasers with the same red detuning, one propagation along +z and the other along -z. An atom at rest will scatter photons at the same rate from both lasers. But if it is travelling in the -z direction, it will scatter more photons from the +z laser than from the -z, so it will feel a pressure towards +z, that is, a stopping force. The converse happens if it is travelling in the other direction. Therefore, both lasers act to stop the atom, because of the Doppler effect.