Picture of a white bowl holding several hand painted easter eggs.
Enlarge / The atoms and bowl are exactly like this. Except the bowl is red and the atoms have a softer shell.

Lasers? Individual atoms trapped next to hot surfaces? Lots of dense text about degrees of freedom and stuff? I have found the after life, and it is a big quantum optics experiment.

Researchers have shown how to trap and cool atoms right next to a hot surface. This sounds a bit abstract. But if we ever want to make use of ultra cold atoms in sensors, this is exactly what we need to be able to do. 

The basic idea behind these sorts of studies is that single atoms make highly sensitive probes for magnetic fields. They can be used to model complicated quantum systems to perform calculations for us. They can be used to help with quantum key distribution. There is almost nothing that cannot be done given a laser and some ultra cold atoms.

Keeping cool next to a hot plate

The down side is that the ultra cold atoms usually sit inside a large vacuum chamber, isolated from everything. A cloud of atoms in the middle of the vacuum chamber is easy to detect; a single atom is really difficult. However, single atoms are what we would ideally want to use in a calculating device or sensor. 

To communicate with single atoms, we want them up close and personal with light delivery systems. That puts the atoms in a predictable position, and it exposes them to higher intensity light so that they respond more efficiently. It also puts an atom that might have a temperature of a few nanoKelvin in close proximity to a surface that’s at room temperature. An experience like this does not end happily for the atom.

Picture the atom as sitting in the bottom of a bowl made of laser light. The atom never sits still. Instead, it rolls back and forth on the bottom of the bowl. 

The surface nearby (I think we’d all agree that ~300nm is nearby) doesn’t leave the atom alone. Surfaces, even if mounted as rigidly as possible, move and vibrate. These tiny fluctuations give the atom little shoves and kicks. As a result, the atom starts rolling further and further up the sides of the bowl. This being quantum mechanics, the atom has to take on specific rolling motions (or energy). So as the atom gets hotter, it climbs up a ladder of energetic states.

Eventually, the atom reaches the top of the bowl and flies away, only to plow into the hard stainless steel wall of the vacuum chamber. It never ends well for the atom.

Even if the sides of the bowl were continuously raised so that the atom was held in the trap, the high temperature prevents its use. Heating the atom leaves it in an unknown motional state, while we need to know its motional state to manipulate it if we’re going to use it for sensing and calculation.

Snakes and ladders with laser-powered dice

The alternative is to continuously cool the atom, which comes with its own challenges. Yet this is exactly what researchers from Austria and Germany have done.

The researchers’ high-temperature surface was an optical fiber stretched out until it was just 250nm in diameter. Cooled cesium atoms were dropped onto the fiber and held in place by light along it. That works because part of the light field leaks out of the fiber, and it holds the atoms about 300nm above the surface.

At this stage, if nothing is done, the fiber heats the atoms, and they are blown out of the trap in about 75 milliseconds.

To cool the atoms, we have to play with the atom and trap. The atom trap is a bowl, but the shape of the bowl depends on the internal state of the atom and an applied magnetic field. Effectively, you have two traps. The first trap corresponds to the atom in one internal state, which I’ll call the addressable state. The second trap corresponds to the atom in a second internal state, which I’ll call the hidden state. 

The applied magnetic field reshapes the two traps so that their energy ladders align, except the alignment is offset by one step. Meaning the second rung of the hidden state ladder aligns with the bottom rung of the addressable state ladder.

The light in the fiber only excites our atom if it’s on the excitable state ladder. A laser flips the atom back into hidden state, causing it to climb down one rung on the ladder as it leaves. After a bit of poking to change the atom’s internal state, it returns to the addressable state ladder, but with a lower energy (see picture).

Diagram of how the atom changes energy. The laser (red arrow) drives the atom out of the right hand (addressable) trap. The atom falls back into the hidden trap at a lower energy level. The atom eventually flips back to the addressable trap (green arrow), allowing the process to repeat.
Enlarge / Diagram of how the atom changes energy. The laser (red arrow) drives the atom out of the right hand (addressable) trap. The atom falls back into the hidden trap at a lower energy level. The atom eventually flips back to the addressable trap (green arrow), allowing the process to repeat.

By repeatedly doing this, the atom falls down to the bottom rung of the hidden state ladder. At the bottom rung, there is no equivalent rung in the addressable state, so the atom is trapped in the lowest energy level of the hidden state. Hence, the atom is as cold as possible, which is exactly what we want.

In practice, it’s not quite there. The fiber always heats the atom. However, the researchers demonstrated that they could cool it efficiently enough that it never gets warm enough to leave. Well, not quite indefinitely: the vacuum around the atom is not perfect. Every now and again, a high-velocity water molecule punts the atom out of the trap. Nevertheless, a trap time of nearly two seconds is pretty cool.

Beating the heat

The researchers haven’t yet managed to get atoms into the lowest possible state in the trap. Their cooling rate is sufficient: they suck energy out at about 20 times the rate that the nearby fiber adds it. The problem is the optimization of the experiment. When an atom is in the lowest possible state, it is not completely blind to the lasers that do the trapping and cooling. The power of these lasers is such that they occasionally kick the atom out of the ground state themselves. The result is that, on average, the atom is never quite in the ground state.

The researchers note that their setup is such that they can optimize the power of the laser. When that happens, they should have atoms that stay in the ground state for considerably longer periods of time. From there, then, the real fun can begin: quantum information processing with little arrays of atoms. I’m looking forward to it. 

Physical Review X, 2018, DOI: 10.1103/PhysRevX.8.031054. (About DOIs)



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