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I often get excited about Bose Einstein condensates, fascinating materials in which large groups of atoms exhibit collective quantum behavior. What really gets me going is the process used to to make she. The most important step is something called optical cooling. It may sound very simple, but in reality it is difficult and usually does not work.
A recent article in Physical assessment letters now adds a new optical cooling method to the physicist’s range of tools. In addition, this opens up a lot of new and exciting possibilities.
Like, just cool down dude
The typical optical cooling method is an exceptionally beautiful piece of physics. Think of a gas of atoms. They have a blast in the moshpit of life, flying in all directions and crashing into each other with vibrancy and power. But as with all good things in life, an old dude will show up, complain about the noise and generally suck all the fun out of life – everything just slows down. Slowing everything down is the easiest way to think about cooling.
In physics, the old guys take the form of lasers. If you choose the color of a laser correctly, the atoms in the gas will absorb the light and thus go from their ground state to an excited state. But consider this: if an atom flies away from the laser, it will see the color slightly redder than we would. And since the color has to be right, it won’t absorb it. Similarly, an atom flying towards the laser will perceive the light as a slightly bluer color and not absorb it.
This is a standard Doppler shift and is essential for the cooling process. To cool down a bunch of atoms, a laser with a slightly too red color is chosen. Now atoms moving very slowly will not absorb light. But atoms moving a little too fast towards the laser 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. When emitting a photon, they get a second kick in a random direction.
Assuming you arrange a number of lasers correctly, each atom cools slowly on average until it moves slowly enough to stop absorbing light from one of the lasers. But this only works if you to have Spontaneous emission. If the atom stays in the excited state for too long, it will float out of the cooling zone before being emitted. 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 teens ignoring old dudes complaining about the noise, atoms often don’t pay attention to the laser either. The laser excites the atoms and gets some cooling, but nothing tells the atom to return to its 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 float out of the cooling zone and be lost.
For atoms, this problem is solved by deploying a second (or third or fourth) laser to chase the atoms out of these intermediate states and back to the ground state for cooling. However, this is already difficult with atoms. Molecules are even worse, because they have much more energy levels, so it becomes impossible to chase the molecules back to the ground state. Therefore, molecular cooling only works on a few highly artificial molecules.
Physicists are then limited to just a few atoms that have the proper energy level structure to enable optical cooling. And molecules are just completely off. However, recent work could change that.
If you thought that was complicated…
This new cooling scheme takes a slightly different approach. Unfortunately, it gets complicated to explain. Let’s start with a single atom. It has a series of electrons arranged around the positively charged center in an energy level structure. In order for an electron to move from one energy level to another, it must take in or give up a fixed amount of energy. Therefore, atoms absorb certain colors of light because the frequency of the light field corresponds to the energy that electrons need to make the transition between two different levels.
But what happens if the atom is flooded with light that is not the right color? The electrons still experience the force exerted by the light field, and this disrupts the entire energy level system. Because the light field oscillates, the distortion of the energy levels also oscillates.
From our perspective, it looks like the whole structure has been doubled. Each energy level splits in two: one slightly higher and one slightly lower than the original. The split gets bigger and bigger as the intensity of the light field increases until, as if by magic, 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 happening.
That is the basis of this new cooling technology. Instead of a single laser with a single color, the researchers use two lasers that emit two slightly different colours. 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 above logic: 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 falls into a cascade of levels.
An important part of turning this into a cooling process is the physical arrangement of the light rays: they point at each other. The light fields mix into a so-called standing wave pattern. In a standing wave pattern, some regions have no light field at all (called a node), while other regions have a lot of field (called anti-nodes). At the nodes, part of the level split disappears due to the two fields.
An atom that is in the upper branch of a split level can emit a photon to move to a lower branch. To do that, however, it must emit its light go inside the light field with the higher frequency. And it can only do this by giving up some kinetic energy.
To be a little more precise, because the atom has to undergo a light field-stimulated emission, it is accelerated to a constant speed in one direction, while giving up speed in all other directions. This is still cooling because temperature is determined by the range of velocities in a gas, which is reduced to a narrow division centered on the constant velocity imparted by the two fields of light.
Does it work
Yes.
The researchers demonstrated their cooling technique on a helium jet. They chose conditions such that spontaneous emission could not contribute to cooling. Under their conditions, each atom could emit an average of only 2 to 3 photons by spontaneous emission, while the observed cooling would have required about 35 photons. It is clear that the system caused them to lose energy.
The bigger question is whether this technique will be more common than other cooling techniques. That’s 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 sublevels. The splitting induced will be very irregular as it is applied to each sublevel – the resulting image is not so clear to me.
I suspect the basics remain the same: the molecule wants to reduce its energy, so it will still emit spontaneously. But the path through the level system can be quite complicated and the cooldown time quite unpredictable. Even worse, you could end up with a molecule standing still as it vibrates like crazy, actually making it really hot.
Still, I look forward to the first demonstration on molecules.
Physical assessment letters2015, DOI: 10.1103/PhysRevLett.114.043002