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Doppler Cooling
Doppler cooling is a method used to cool atoms to extremely low temperatures by using lasers. Here's how it works:
- The Doppler Effect: When an atom moves towards a laser, it "sees" a slightly higher frequency of light than the laser actually emits. Conversely, when it moves away, it "sees" a lower frequency.
- Tuning the Laser: Lasers are tuned to a frequency slightly below a specific atomic transition frequency. This means that only atoms moving towards the laser will absorb the light because the Doppler effect shifts the light's frequency into resonance with the atom's transition.
- Momentum Transfer: When an atom absorbs a photon, it gains momentum from the photon, slowing the atom down.
- Random Emission: After absorbing the photon, the atom will spontaneously emit a photon in a random direction. This emission doesn't change the atom's average velocity because the emissions are random.
- Net Cooling: Over many absorption-emission cycles, the atom's velocity decreases, resulting in cooling. By applying lasers from multiple directions, atoms can be cooled in three dimensions.
Doppler cooling is essential for creating ultracold atomic gases used in Bose-Einstein condensates, atomic clocks, and quantum computing.
