Ultra-cold trapped atoms and ions
The laws of quantum mechanics govern the behavior of small objects, such as single electrons or atoms. Atoms are not only an ideal system to study quantum mechanics, they are also core building blocks of many Quantum Technologies . In order to work with atoms in a controlled way, one usually needs to isolate them. This is commonly done by producing a pure vapor or beam of the atom species, which one wants to work with, inside an ultra-high vacuum chamber and then trap a few of these atoms.
There are several ways of trapping atoms: Neutral atoms can be confined with magnetic fields or in (arrays of) laser light beams, ions are trapped using electric fields. In both cases, the atoms need to be cooled to very low temperatures, meaning very low average particle velocity, so that they do not escape the trap. A common cooling method is laser cooling. It may sound counter-intuitive that a laser can be used to cool something, so here is a short explanation how the most common method, Doppler cooling, works:
At room temperature or in a hot vapor, the atoms are moving with high (~hundreds of meters per second), random velocity in random directions, which constantly change due to collisions with other atoms. In a cool cloud, atoms move with much lower velocity (~cm per second), so cooling means nothing else but slowing down the particles. In laser cooling, one shines light from all six directions on the atoms, which is slightly red detuned (lower frequency) with respect to a natural absorption line. The probability of an atom at rest to absorb this light is low. If an atom moves towards a beam, the frequency ‘seen’ by the atoms increases due to Doppler Effect and it absorbs a photon (a light particle) with higher probability. Each absorbed photon gives the atom a little kick in the direction opposite of its movement and thus acts as a break. So, as a result, the atoms feel a constant breaking force, no matter in which direction they move. With this technique one can cool down an ensemble of atoms within a fraction of a second from room temperature to under a mK and with other techniques to a few nK – a few billionth of a degree above absolute zero (where all atoms would be perfectly still).
Several Nobel Prizes have been awarded in this area: In 1989, Hans Georg Demelt and Wolfgang Paul received the Nobel Prize “”for the development of the ion trap technique”. In 1997, Steven Chu, Claude Cohen-Tannoudji and William D. Philips received it “for development of methods to cool and trap atoms with laser light”. In 2012 Serge Haroche and Dave Wineland received the Nobel Prize “for ground-breaking experimental methods that enable measuring and manipulation of individual quantum systems.”
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2nd photo. Please cite: “With kind permission from the Philip Morris Foundation and Prof. Immanuel Bloch.