Dopplerjäähdytys

Dopplerjäähdytys, also known as Doppler cooling, is a technique used to slow down atoms or ions by using laser light. This process is a cornerstone of laser cooling, a field that has enabled significant advancements in atomic physics, quantum optics, and metrology. The fundamental principle behind Doppler cooling relies on the Doppler effect and the interaction of atoms with photons.

When a beam of laser light is directed at a collection of atoms, and the laser's frequency is tuned slightly below an atomic resonance frequency (red-detuned), the atoms moving towards the laser beam will perceive the light at a higher frequency due to the Doppler shift. If this perceived frequency matches the atomic resonance, the atom will absorb a photon. This absorption process transfers momentum from the photon to the atom, imparting a "kick" that opposes the atom's motion.

Shortly after absorbing a photon, the atom re-emits another photon in a random direction. While this re-emission also involves a momentum transfer, the random nature of the re-emission averages out over many absorption-emission cycles, resulting in a net reduction in the atom's velocity. Atoms moving away from the laser beam will perceive the red-detuned light at an even lower frequency, making absorption less likely. Therefore, atoms moving towards the laser beam are preferentially slowed down.

By using laser beams from multiple directions, typically six, each red-detuned to a specific atomic transition, a cloud of atoms can be effectively trapped and cooled in all three dimensions. This creates a sample of ultracold atoms, with temperatures reaching microkelvin or even nanokelvin. Doppler cooling is the first step in many laser cooling schemes and is essential for preparing atoms for further cooling techniques like Sisyphus cooling, leading to even lower temperatures. The ability to create such cold atomic samples has opened doors to experiments with Bose-Einstein condensates, atom interferometry, and highly precise atomic clocks.