This summer I am working at the University of Rochester assembling a magneto-optical trap (MOT) for the advanced laboratory course. Once completed, the MOT will be used to cool rubidium atoms down to fractions of degree above absolute zero to observe the strange physical phenomena that only occur at such low temperatures.
A MOT (graphic above) consists of six intersecting laser beams along three axes (the three red lines above) and a magnetic field produced by two coils (the gold rings). These beams intersect on a vacuum chamber that contains a small amount of rubidium atoms. Due to an effect called Doppler cooling, these beams cause the atoms to lose energy, thus cooling them.
Doppler cooling relies on the Doppler effect, which is the shifting of frequencies by objects in motion. The classic example of this phenomenon is a train passing a station: the frequency of the train is higher as it comes towards you, then drops lower as it passes you. The train is making the same sound continuously, but from your point of view the tone seems to change. Similarly, from the perspective of the rubidium atom, the frequency of the laser light in the MOT shifts depending on whether the rubidium atoms are moving towards or away from the source of the beam.
If a rubidium atom absorbs a photon from the laser beam, conservation of momentum dictates that the atom will feel a force in the direction the photon was previously traveling. However, quantum mechanics tells us that atoms can only absorb photons at very specific frequencies. This fact, combined with the Doppler shifting of the laser light by the moving atoms, is the key to the MOT’s operation. The laser frequency is selected precisely such that atoms can absorb the photons when they are traveling into the beam. However, since this laser light will appear to shift in frequency when the atoms move in a different direction, atoms moving away from the beam will not be able to absorb photons! Thus, the atoms only feel the force of the laser beam when they are moving against the beam, so each collision cancels out some of the original momentum of the atom. Over time and acting in all three dimensions, this effect vastly reduces the kinetic energy of the atoms.
The vacuum cell and magnet for the MOT I will be building this summer have been built by a company called ColdQuanta (link). However, in order to save money, we will be building the lasers ourselves. Look out for more details on the design, construction, and testing of these lasers coming soon!