I often get enthusiastic about Bose Einstein condensates, fascinating materials where large groups of atoms show collective quantum behavior. The thing that really gets me going is the process used to make 'em. The main step is something called optical cooling. It may sound very simple, but in reality it is difficult and mostly doesn't work.

A recent paper in Physical Review Letters now adds a new optical cooling method to the physicist's range of tools. In doing so, this opens up a whole lot of new and exciting possibilities.

Like, just cool off dude

The typical optical cooling method is an exceptionally neat bit of physics. Think about a gas of atoms. They are having a fantastic time in life's mosh pit, flying in all directions and bouncing off one another with vim and vigor. But as with all good things in life, some old dude will turn up, complain about the noise, and generally suck all the entertainment out of life—everything just slows down. Slowing everything down is the easiest way to think of cooling.

In physics, the old dudes take the form of lasers. If you choose the color of a laser correctly, then the atoms in the gas will absorb the light and, in doing so, go from their ground state to an excited state. But consider this: if an atom is flying away from the laser, it will see the color as slightly redder than we would. And since the color has to be right, it won't absorb it. Likewise, an atom flying toward the laser will observe the light as having a slightly bluer color and won't absorb it.

This is a standard Doppler shift, and it's essential for the cooling process. To cool a bunch of atoms, a laser with a color that is just slight too red is chosen. Now, atoms that are moving very slowly will not absorb any light. But atoms that are moving toward the laser a bit too fast will absorb a photon. In doing so, they get a momentum kick and slow down. Then, they get rid of the energy by emitting a photon. In emitting a photon, they get a second kick in some random direction.

Assuming you arrange a bunch of lasers correctly, on average, each atom slowly cools down until it is moving slowly enough to not absorb any more light from any of the lasers. But this only works if you have spontaneous emission. If the atom stays in the excited state too long, it will drift out of the cooling zone before emitting. Even worse, if the light fields stimulate it to emit, it will be accelerated out of the cooling zone.

Hiding from the laser

In a perfect world, that would be the whole story. But to continue the metaphor with teenagers who ignore old dudes complaining about the noise, atoms often don't pay any attention to the laser either. The laser excites the atoms and gets some cooling, but there is nothing that says the atom has to return to the ground state. Atoms have many more states to choose from. If the atom never returns to the ground state, it cannot absorb more laser light for further cooling. These atoms will drift out of the cooling zone and are lost.

For atoms, this problem is solved by putting in a second (or third, or fourth) laser to chase the atoms out of these intermediate states and back to the ground state for cooling. This is already difficult with atoms, however. Molecules are even worse, because they have many more energy levels, so it becomes impossible to chase the molecules back to the ground state. Hence, molecular cooling only works on a few very artificial molecules.

Physicists are then limited to just a few atoms that have the right energy level structure to allow optical cooling. And molecules are just totally out. Recent work might change that, however.

If you thought that was complicated...

This new cooling scheme takes a slightly different approach. Unfortunately, explaining it is going to get complicated. Let's start with a single atom. It has a set of electrons that are arranged around the positively charged center in an energy level structure. For an electron to move from one energy level to another, it either has to absorb or give up a fixed amount of energy. Hence, atoms absorb particular colors of light because the frequency of the light field corresponds to the energy required for electrons to make the transition between two different levels.

But what happens when the atom is bathed in light that doesn't have the right color? The electrons still experience the force exerted by the light field, and this distorts the whole energy level system. Because the light field oscillates, the distortion to the energy levels also oscillates.

From our perspective, it looks like the whole structure has doubled. Every energy level splits into two: one slightly higher and one slightly lower than the original. The splitting gets larger and larger as the intensity of the light field grows until, like magic, either the lower or the upper branch comes into resonance with the light field. Suddenly, the atom can absorb the light, and all sorts of really cool effects start to happen.

That is the basis of this new cooling technique. Instead of a single laser with a single color, the researchers use two lasers, emitting two slightly different colors. One has a frequency just slightly higher than required for absorption and the other has a frequency just slightly lower than required for absorption.

Now let's apply the logic above: the first light field splits the energy levels, and the second light field splits the energy levels that have already been split. And these new levels can also be split by the first light field. As a result, each individual energy level breaks up into a cascade of levels.

A key part of turning this into a cooling process is the physical arrangement of the light beams: they point at each other. The light fields mix together to form what is called a standing wave pattern. In a standing wave pattern, some areas have no light field at all (called a node), while others have lots of field (called anti-nodes). At the nodes, some of the level splitting due to the two fields vanishes.

An atom sitting in the upper branch of a split level can emit a photon to transition to a lower branch. However, to do that, it has to emit its light into the light field with the higher frequency. And it can only do this by giving up some motional energy.

To be slightly more accurate: because the atom has to undergo emission stimulated by the light field, it is accelerated to a constant velocity in one direction, while giving up velocity in all other directions. This is still cooling because temperature is defined by the range of speeds in a gas, which has reduced to a narrow distribution, centered on the constant velocity imparted by the two light fields.

Does it work

Yes.

The researchers demonstrated their cooling technique on a helium beam. They chose conditions such that spontaneous emission could not contribute to cooling. Under their conditions, each atom could, on average, emit only 2 to 3 photons through spontaneous emission, while the cooling they observed would have required about 35 photons to be emitted. It's clear the system caused them to lose energy.

The bigger question is whether this technique will be more general than other cooling techniques. That is hard to say. In their calculations, they used a very clean system, so the cascade of energy levels involved here is clear and regular. In a molecule, each electronic energy level is actually a mess of sub-levels. The induced splitting will be highly irregular because it will be applied to each sub-level—the resulting picture, to me, is not so clear.

I would guess that the fundamentals remain the same: the molecule wants to reduce its energy, so it will still spontaneously emit. But the path through the level system might be rather complicated and the cooling time kind of unpredictable. Even worse, you could end up with a molecule that is standing still while vibrating like crazy, making it actually very hot.

Nevertheless, I am looking forward to the first demonstration on molecules.

Physical Review Letters, 2015, DOI: 10.1103/PhysRevLett.114.043002