Molecules cooled to its lowest temperature, which differs from absolute zero by only 50 millionths of a degree.



The installation diagram. Left laser irradiation target from the descent out of the molecule CaF, which are cooled by laser, and then fall into the trap (right), where they are cooled to a final temperature. Below shows the installation dimension in cm

The scheme of “optical molasses”. Located in the center of the cooled atoms, the yellow color shows the laser beams.”

Very cold molecules, which has almost no thermal motion, we need physicists for various studies, starting with the study of the properties of the molecules before the creation of quantum computers. I believe that this needs to slow down and the different reactions that will allow to study the processes. However, in achieving very low temperatures nature has imposed a number of barriers .

The authors record achievement from the Centre for cold matter at Imperial College London used the most common method of laser cooling of molecules – Doppler.

In order to understand its mechanism, let us remember that the atom absorbs and emits photons with energy HV (ν is the light frequency, h –Planck’s constant) equal to the difference between its energy levels. In this method, the frequency of the laser radiation is chosen somewhat less than is needed for absorption. But because of the Doppler effect of moving towards the photon, the atom “sees” that a greater frequency and absorption occurs.

Then an excited atom spontaneously emits a photon, but with more frequency. Therefore, in each cycle of absorption-emission of it loses part of its kinetic energy, and therefore cools. To catch up with the laser beam, the atom does not react, as the Doppler effect for him even more lowers the frequency, making the absorption impossible.

Multiple laser beams going in different directions, slowing down the atoms in any direction of their movement. The impression is that atoms move in a viscous liquid like honey or molasses. This method of cooling has received the name “optical molasses”.

However, this way can’t cooling molecules and atoms to temperatures lower than a few hundred microkelvins. The fact is that due to the uncertainty relation of absorbed photons not a single frequency but a whole frequency range. Physicists say that the spectral line has a width. But then at low speeds of the atoms will absorb photons from both directions, and the method will fail. The temperature at which this occurs, is called the Doppler limit. For used in the calcium monofluoride CaF it above 640 µk (1 µk = 10-6 K– one millionth of a degree).

Therefore, on the study authors used a second stage with so-called Sisyphus cooling. Here, the molecule loses energy due to the interaction with the light changing in the space of polarization, which is created by two propagating towards the laser beams. The electric field distribution in this case resembles a kind of comb, the “teeth” which dropped the atom in the lower energy state. It is for this alternate motion of the atom as like a mountain and dropping method and got the name of Sisyphus, punished by the gods to eternally roll the stone up the mountain.

This method is known with respect to atoms long ago. In 1997 for work on cooling atoms, and in particular, for the explanation of this method the Nobel prize was awarded to French physicist Claude Cohen-Tannoudji. His team then managed to cool atoms to 0.18 IWC. Recall that the absolute zero of the temperature measured in Kelvin, corresponds to -273,15 degrees Celsius. However, as much to cool molecules for a long time failed because of their more complex structure of energy levels in comparison with atoms.

Physicists from the Centre for cold matter at Imperial College London have managed to solve all the problems and achieve a record low temperature of 50 µk. This was reported by the journal Nature Physics. A Preprint of the work can be found on the website arxiv.org.